path: root/src/fuse.zig

//! The `/dev/fuse` transport for pardes's acme control filesystem: wire codec,
//! mount and unmount through `fusermount3`, one `poll()` thread, and the park
//! table that turns acme's blocking `event` read into "ask me again later".
//!
//! Raw protocol, no libfuse. libfuse is a thread pool, a request dispatcher and
//! a session lifetime — three things pardes already has and would have to fight.
//! What is left once those are removed is a struct layout and a read/write loop,
//! which is this file. It links nothing; the only external program it runs is
//! the setuid `fusermount3` helper, because an unprivileged process cannot
//! `mount(2)` in the initial user namespace and that helper exists precisely to
//! hand back a `/dev/fuse` descriptor for a mount it made on our behalf.
//!
//! The whole file is one side of a strict division of labour:
//!
//!   - `acmefs.zig` owns the semantics and knows nothing about FUSE. It speaks
//!     `Req`/`Reply` and never blocks.
//!   - this file owns the kernel's opinions and knows nothing about panes. It
//!     answers, in place, every request the core has no business seeing (INIT,
//!     FORGET, INTERRUPT, DESTROY and the whole ENOSYS family), and translates
//!     the eleven that remain.
//!   - the host loop (tty/gui) owns the ordering: `retry()` to null, `next()`
//!     to null, one `update()` per request, effects drained in between.
//!
//! THREADING. The main thread owns the descriptor for read and for write. The
//! poll thread never touches its data, never sees a `Req`, and never calls into
//! the core; it waits for POLLIN, calls the host's wake callback, and then
//! blocks until the main thread has drained. That last handshake is not
//! decoration: `poll()` is level triggered, so a poller that re-polls
//! immediately would spin a core at 100% for as long as one unanswered request
//! sits in the kernel queue. A host with no threads at all skips `wakeThread`
//! and drains from its frame poll; it loses wake latency and nothing else.
//!
//! BLOCKING. A FUSE server blocks a reader by simply not answering, and that is
//! the one and only way (the kernel gives no meaning to an EAGAIN reply). So
//! `Status.again` means "held": the request moves into the park table with its
//! bytes copied out of the read buffer, and `retry()` offers it back once per
//! frame until the core has something to say. Two obligations come with that:
//!
//!   1. a SIGKILLed reader whose request is never answered ends in
//!      *uninterruptible* sleep (`fuse_dev`'s final `wait_event` is not
//!      killable), so it survives its own kill until we reply. FUSE_INTERRUPT
//!      is the escape hatch and is honoured below.
//!   2. teardown must answer everything still parked, and must abort the
//!      connection by closing the descriptor before unmounting, or a reader
//!      that raced the shutdown is stuck in D state with nobody left to wake
//!      it.
//!
//! Linux only, guarded the way `file_watch.zig` guards inotify: every entry
//! point returns the inert answer off Linux, so a macOS or web build compiles
//! and mounts nothing. Only `mount()` can create an `Fs`, so off Linux no other
//! function in this file is ever reached.
//!
//! Verified against `/usr/include/linux/fuse.h` (7.45) and `fs/fuse/{dev,inode,
//! file,dir,readdir}.c`; the comptime size assertions below turn a header drift
//! into a compile error rather than a wedged mount nobody can unmount.
const std = @import("std");
const builtin = @import("builtin");
const libc = std.c;
const linux = std.os.linux;
const acmefs = @import("acmefs.zig");
/// Only for `Transport`, the three-function shape this mount presents to the
/// host loop. This IS a cycle — `fs_service` imports this file back for
/// `Fs.mount`, `sweepStale` and `exportPaneEnv`, and still names `*Fs` in three
/// of its own signatures — and Zig accepts it because imports are analysed
/// lazily. What the seam removed is `drain`'s dependency on the concrete type,
/// not the file's dependency on this one. Do not read it as more than that.
const fs_service = @import("fs_service.zig");

/// Everything below the mount is Linux kernel ABI. Off Linux the module still
/// compiles (it is imported by the shared native shell) and does nothing.
const supported = builtin.os.tag == .linux;

// ---------------------------------------------------------------------------
// wire protocol
// ---------------------------------------------------------------------------

/// The protocol version this server speaks. A mismatch in the *major* aborts
/// the connection outright (`fuse_init_finish`: `arg->major !=
/// FUSE_KERNEL_VERSION` -> `ok = false` -> the mount is dead on arrival), so
/// there is nothing to negotiate there.
const kernel_version: u32 = 7;

/// The highest minor these structs were checked against (see the module
/// header). The INIT reply carries `@min(kernel_minor, what the kernel
/// offered)`: `fuse_init_finish` stores our number as `fc->minor`, and the
/// kernel then sizes the replies it reads back from us by it (the
/// `FUSE_COMPAT_*_SIZE` family in `fs/fuse/`), so echoing a *newer* kernel's
/// minor promises reply fields these structs do not have. Capping costs
/// nothing: with `flags = 0` no feature depends on the number.
const kernel_minor: u32 = 45;

/// `fuse_dev_do_read` refuses to hand over a request when the server's read
/// buffer is smaller than this, and answers the *client* EIO instead: every
/// syscall through the mount fails and nothing says why.
const min_read_buffer: usize = 8192;

/// `FUSE_REC_ALIGN`. A dirent record that is not a multiple of 8 desynchronises
/// the kernel's parse of the rest of the reply, so one bad name turns the whole
/// directory into garbage rather than into an error.
const rec_align: usize = 8;

/// `FUSE_NAME_OFFSET` — the fixed part of a `fuse_dirent`, before the name.
const dirent_name_offset: usize = @sizeOf(fuse_dirent);

fn recAlign(n: usize) usize {
    return (n + rec_align - 1) & ~(rec_align - 1);
}

/// The subset of `enum fuse_opcode` this server can receive. Non-exhaustive on
/// purpose: a newer kernel adds opcodes, and `@enumFromInt` of an unlisted
/// value into an exhaustive enum is undefined behaviour — the one bug in a
/// protocol decoder that cannot be diagnosed from the outside.
const Opcode = enum(u32) {
    lookup = 1,
    forget = 2,
    getattr = 3,
    setattr = 4,
    readlink = 5,
    symlink = 6,
    mknod = 8,
    mkdir = 9,
    unlink = 10,
    rmdir = 11,
    rename = 12,
    link = 13,
    open = 14,
    read = 15,
    write = 16,
    statfs = 17,
    release = 18,
    fsync = 20,
    setxattr = 21,
    getxattr = 22,
    listxattr = 23,
    removexattr = 24,
    flush = 25,
    init = 26,
    opendir = 27,
    readdir = 28,
    releasedir = 29,
    fsyncdir = 30,
    getlk = 31,
    setlk = 32,
    setlkw = 33,
    access = 34,
    create = 35,
    interrupt = 36,
    bmap = 37,
    destroy = 38,
    ioctl = 39,
    poll = 40,
    notify_reply = 41,
    batch_forget = 42,
    fallocate = 43,
    readdirplus = 44,
    rename2 = 45,
    lseek = 46,
    copy_file_range = 47,
    setupmapping = 48,
    removemapping = 49,
    syncfs = 50,
    tmpfile = 51,
    statx = 52,
    copy_file_range_64 = 53,
    _,
};

/// `FATTR_SIZE`. The only setattr bit this filesystem reads: without
/// `FUSE_ATOMIC_O_TRUNC` (which `flags = 0` deliberately does not negotiate)
/// the kernel strips `O_TRUNC` from the OPEN and issues a separate
/// `SETATTR(size = 0)`, so this bit *is* how `> file` reaches the core.
const FATTR_SIZE: u32 = 1 << 3;

/// `FUSE_GETATTR_FH` — says the `fh` field of `fuse_getattr_in` is meaningful.
/// Reading `fh` without checking it hands the core a stale handle from an
/// unrelated open.
const FUSE_GETATTR_FH: u32 = 1 << 0;

/// `FOPEN_DIRECT_IO`. Without it the kernel serves reads out of the page cache
/// and coalesces them, which for this filesystem is wrong in both directions:
/// a second `cat` of `index` would return the first one's bytes, and a blocking
/// `event` read would never reach us at all.
const FOPEN_DIRECT_IO: u32 = 1 << 0;

const fuse_in_header = extern struct {
    len: u32,
    opcode: u32,
    unique: u64,
    nodeid: u64,
    uid: u32,
    gid: u32,
    pid: u32,
    total_extlen: u16,
    padding: u16,
};

const fuse_out_header = extern struct {
    len: u32,
    @"error": i32,
    unique: u64,
};

const fuse_init_in = extern struct {
    major: u32,
    minor: u32,
    max_readahead: u32,
    flags: u32,
    flags2: u32,
    unused: [11]u32,
};

const fuse_init_out = extern struct {
    major: u32,
    minor: u32,
    max_readahead: u32,
    flags: u32,
    max_background: u16,
    congestion_threshold: u16,
    max_write: u32,
    time_gran: u32,
    max_pages: u16,
    map_alignment: u16,
    flags2: u32,
    max_stack_depth: u32,
    request_timeout: u16,
    unused: [11]u16,
};

const fuse_attr = extern struct {
    ino: u64,
    size: u64,
    blocks: u64,
    atime: u64,
    mtime: u64,
    ctime: u64,
    atimensec: u32,
    mtimensec: u32,
    ctimensec: u32,
    mode: u32,
    nlink: u32,
    uid: u32,
    gid: u32,
    rdev: u32,
    blksize: u32,
    flags: u32,
};

const fuse_entry_out = extern struct {
    nodeid: u64,
    generation: u64,
    entry_valid: u64,
    attr_valid: u64,
    entry_valid_nsec: u32,
    attr_valid_nsec: u32,
    attr: fuse_attr,
};

const fuse_attr_out = extern struct {
    attr_valid: u64,
    attr_valid_nsec: u32,
    dummy: u32,
    attr: fuse_attr,
};

const fuse_getattr_in = extern struct {
    getattr_flags: u32,
    dummy: u32,
    fh: u64,
};

const fuse_setattr_in = extern struct {
    valid: u32,
    padding: u32,
    fh: u64,
    size: u64,
    lock_owner: u64,
    atime: u64,
    mtime: u64,
    ctime: u64,
    atimensec: u32,
    mtimensec: u32,
    ctimensec: u32,
    mode: u32,
    unused4: u32,
    uid: u32,
    gid: u32,
    unused5: u32,
};

const fuse_open_in = extern struct {
    flags: u32,
    open_flags: u32,
};

const fuse_open_out = extern struct {
    fh: u64,
    open_flags: u32,
    backing_id: i32,
};

const fuse_read_in = extern struct {
    fh: u64,
    offset: u64,
    size: u32,
    read_flags: u32,
    lock_owner: u64,
    flags: u32,
    padding: u32,
};

const fuse_write_in = extern struct {
    fh: u64,
    offset: u64,
    size: u32,
    write_flags: u32,
    lock_owner: u64,
    flags: u32,
    padding: u32,
};

const fuse_write_out = extern struct {
    size: u32,
    padding: u32,
};

const fuse_release_in = extern struct {
    fh: u64,
    flags: u32,
    release_flags: u32,
    lock_owner: u64,
};

const fuse_flush_in = extern struct {
    fh: u64,
    unused: u32,
    padding: u32,
    lock_owner: u64,
};

const fuse_forget_in = extern struct {
    nlookup: u64,
};

const fuse_batch_forget_in = extern struct {
    count: u32,
    dummy: u32,
};

const fuse_interrupt_in = extern struct {
    unique: u64,
};

const fuse_kstatfs = extern struct {
    blocks: u64,
    bfree: u64,
    bavail: u64,
    files: u64,
    ffree: u64,
    bsize: u32,
    namelen: u32,
    frsize: u32,
    padding: u32,
    spare: [6]u32,
};

const fuse_statfs_out = extern struct {
    st: fuse_kstatfs,
};

/// The `name` array is flexible in C and therefore absent here; this struct IS
/// `FUSE_NAME_OFFSET`, and `dirent_name_offset` is taken from its size so the
/// encoder and the kernel cannot disagree about where a name starts.
const fuse_dirent = extern struct {
    ino: u64,
    off: u64,
    namelen: u32,
    type: u32,
};

/// `DT_*` from `linux/dirent.h`, as `fuse_dirent.type` wants them.
const DT_DIR: u32 = 4;
const DT_REG: u32 = 8;

/// `S_IFMT` bits. `Reply.Attr.mode` carries permissions only, so the format
/// nibble is ours to add; a `fuse_attr.mode` with no format bits is a file of
/// no type and `stat(2)` through the mount returns something no tool expects.
const S_IFDIR: u32 = 0o040000;
const S_IFREG: u32 = 0o100000;

// A drifted header is a mount that hangs with no diagnostic, so every struct
// on the wire asserts its size here. These numbers are `sizeof` from
// /usr/include/linux/fuse.h at FUSE_KERNEL_MINOR_VERSION 45; they are frozen
// ABI and are not allowed to change under us silently.
comptime {
    std.debug.assert(@sizeOf(fuse_in_header) == 40);
    std.debug.assert(@sizeOf(fuse_out_header) == 16);
    std.debug.assert(@sizeOf(fuse_init_in) == 64);
    std.debug.assert(@sizeOf(fuse_init_out) == 64);
    std.debug.assert(@sizeOf(fuse_attr) == 88);
    std.debug.assert(@sizeOf(fuse_entry_out) == 128);
    std.debug.assert(@sizeOf(fuse_attr_out) == 104);
    std.debug.assert(@sizeOf(fuse_getattr_in) == 16);
    std.debug.assert(@sizeOf(fuse_setattr_in) == 88);
    std.debug.assert(@sizeOf(fuse_open_in) == 8);
    std.debug.assert(@sizeOf(fuse_open_out) == 16);
    std.debug.assert(@sizeOf(fuse_read_in) == 40);
    std.debug.assert(@sizeOf(fuse_write_in) == 40);
    std.debug.assert(@sizeOf(fuse_write_out) == 8);
    std.debug.assert(@sizeOf(fuse_release_in) == 24);
    std.debug.assert(@sizeOf(fuse_flush_in) == 24);
    std.debug.assert(@sizeOf(fuse_forget_in) == 8);
    std.debug.assert(@sizeOf(fuse_batch_forget_in) == 8);
    std.debug.assert(@sizeOf(fuse_interrupt_in) == 8);
    std.debug.assert(@sizeOf(fuse_kstatfs) == 80);
    std.debug.assert(@sizeOf(fuse_statfs_out) == 80);
    std.debug.assert(@sizeOf(fuse_dirent) == 24);
    // The one field offset the codec depends on beyond struct sizes: the body
    // of every request starts here, and 40 is a multiple of 8, which is what
    // lets the parse point a struct at the read buffer instead of copying.
    std.debug.assert(@sizeOf(fuse_in_header) % rec_align == 0);
}

// ---------------------------------------------------------------------------
// the neutral readdir staging format
// ---------------------------------------------------------------------------

/// How `acmefs` hands a directory listing to this file. The core is protocol
/// neutral by design, so it must not stage `fuse_dirent`s: those carry an
/// alignment rule, a cookie rule and a `DT_*` table that are the kernel's
/// business, not the editor's. It stages this instead, packed and repeated,
/// little endian, into `State.out`:
///
///     node:    u64   the acmefs node id of the entry, never 0 (see below)
///     kind:    u8    0 = regular file, 1 = directory
///     namelen: u8    1..255, never 0
///     name:    [namelen]u8
///
/// `node` travels so that the `d_ino` a `getdents64` sees is the same number a
/// later `stat` reports. Synthesising one here instead would make `find -inum`
/// and every hardlink-detecting tool lie about this filesystem.
///
/// `node` is never 0. It used to be, for the entries under `new/`: those name
/// panes that do not exist, because acme creates the pane when the name is
/// LOOKED UP. `new/` now stages nothing at all — every name in it is a
/// *creating* lookup, so any tool that stats what a readdir reported (`ls -l`,
/// `find`, tab completion) would make one pane per entry — which is why there
/// is no longer a sentinel `d_ino` for an unresolved name on the wire.
///
/// The core stages entries starting at index `req.off` (the cookie the kernel
/// echoed back) in a stable order. This encoder assigns cookie `off = req.off +
/// n + 1` to the nth entry it emits, and may emit only a *prefix* of what was
/// staged when the kernel's requested `size` runs out — the remainder comes
/// back as another readdir at the higher cookie, so staging has to be
/// idempotent per cookie rather than a stream. Zero staged bytes means EOF; it
/// is not an error, and the kernel stops asking.
///
/// No `.` or `..`: the kernel synthesises neither and needs neither, and a
/// filesystem that emits them has to answer `LOOKUP("..")` too.
pub const dirent_stage_prefix = 10;

/// Encode staged entries into kernel `fuse_dirent` records. Returns the bytes
/// written to `out`. Pure: this is where the alignment and cookie rules live,
/// and it is tested directly.
fn encodeDirents(out: []u8, staged: []const u8, cookie: u64) usize {
    var in: usize = 0;
    var w: usize = 0;
    var n: u64 = 0;
    while (in + dirent_stage_prefix <= staged.len) {
        const node = std.mem.readInt(u64, staged[in..][0..8], .little);
        const kind = staged[in + 8];
        const namelen: usize = staged[in + 9];
        // A zero name length would make the record self-referential (the
        // kernel would parse the padding as the next entry), and a truncated
        // record means the core staged something we cannot read. Stop rather
        // than guess: a short reply is a legal readdir, a malformed one is not.
        if (namelen == 0 or in + dirent_stage_prefix + namelen > staged.len) break;
        const name = staged[in + dirent_stage_prefix ..][0..namelen];
        const record = recAlign(dirent_name_offset + namelen);
        if (w + record > out.len) break;

        // Written field by field rather than through a struct pointer: `out`
        // is a caller's slice of unknown alignment, and one @alignCast that is
        // wrong here is a misaligned store into a kernel-bound buffer.
        std.mem.writeInt(u64, out[w..][0..8], node, .little);
        std.mem.writeInt(u64, out[w + 8 ..][0..8], cookie + n + 1, .little);
        std.mem.writeInt(u32, out[w + 16 ..][0..4], @intCast(namelen), .little);
        std.mem.writeInt(u32, out[w + 20 ..][0..4], if (kind == 1) DT_DIR else DT_REG, .little);
        @memcpy(out[w + dirent_name_offset ..][0..namelen], name);
        // The kernel never shows the padding to anyone, but zeroing it keeps
        // the wire deterministic, which is what the encoder test asserts on.
        @memset(out[w + dirent_name_offset + namelen ..][0 .. record - dirent_name_offset - namelen], 0);

        in += dirent_stage_prefix + namelen;
        w += record;
        n += 1;
    }
    return w;
}

// ---------------------------------------------------------------------------
// fusermount3
// ---------------------------------------------------------------------------

/// The environment variable `fusermount3` reads to find the socket it must send
/// the `/dev/fuse` descriptor back over. Spelled with the leading underscore in
/// libfuse (`FUSE_COMMFD_ENV`); it is a private contract between the two
/// programs, not a user knob.
const commfd_env = "_FUSE_COMMFD";

/// Where the helper might be. Arch puts it in /usr/bin with /usr/sbin a symlink
/// to it, Debian derivatives use /usr/bin, and a machine with only libfuse2
/// installed spells it without the 3 — that binary speaks the same
/// socketpair/SCM_RIGHTS protocol, so it is a real fallback and not a guess.
/// Searched by absolute path rather than through PATH because the thing being
/// executed is setuid root: PATH is attacker-influenced input.
const fusermount_paths = [_][:0]const u8{
    "/usr/bin/fusermount3",
    "/usr/sbin/fusermount3",
    "/bin/fusermount3",
    "/sbin/fusermount3",
    "/usr/local/bin/fusermount3",
    "/usr/bin/fusermount",
    "/usr/sbin/fusermount",
    "/bin/fusermount",
};

/// The `-o` string. Every option here is a deliberate refusal:
///
///   - `fsname`/`subtype` are cosmetic but load bearing: they are what `mount`,
///     `df` and `/proc/self/mountinfo` show, and an unnamed fuse mount in a bug
///     report is indistinguishable from anyone else's.
///   - `nosuid,nodev` are what fusermount3 forces anyway; naming them keeps the
///     intent in the source rather than in someone else's default.
///   - NOT `allow_other`: it needs `user_allow_other` in /etc/fuse.conf, which
///     is commented out on a stock Arch install, and asking for it makes
///     fusermount3 fail the whole mount instead of ignoring the option. It
///     would also be wrong — this filesystem executes text on write.
///   - NOT `default_permissions`: with it the kernel enforces the mode bits we
///     report, which sounds like a free wall but moves access control from the
///     core (which knows that `cons` is write-only) into a mode field, so a
///     wrong nibble in a table becomes an EACCES nobody can explain. Same
///     reason INIT negotiates no flags: fewer kernel behaviours to honour.
fn mountOpts(buf: *[128:0]u8) [:0]const u8 {
    return std.fmt.bufPrintSentinel(buf, "fsname=pardes,subtype=pardes,nosuid,nodev", .{}, 0) catch unreachable;
}

/// `_FUSE_COMMFD=<n>`, the child's end of the socketpair by number. libfuse
/// passes the descriptor this way rather than on the command line because
/// fusermount3 is setuid: its argv is world readable through /proc, its
/// environment is not.
fn commfdEnv(buf: *[32:0]u8, fd: c_int) [:0]const u8 {
    return std.fmt.bufPrintSentinel(buf, commfd_env ++ "={d}", .{fd}, 0) catch unreachable;
}

/// `fusermount3 -o <opts> -- <mountpoint>`. The `--` is not optional: a
/// mountpoint that begins with a dash would otherwise be parsed as a flag by a
/// setuid program.
fn mountArgv(
    argv: *[6:null]?[*:0]const u8,
    prog: [*:0]const u8,
    opts: [*:0]const u8,
    mountpoint: [*:0]const u8,
) void {
    argv.* = .{ prog, "-o", opts, "--", mountpoint, null };
}

/// `fusermount3 -u -q -z -- <mountpoint>`. Lazy (`-z`) because the mount may
/// still have an open descriptor on it — a pane shell that inherited a cwd
/// inside the mount, say — and a non-lazy unmount would fail with EBUSY and
/// leave the mount behind for good. Quiet (`-q`) because the common case at
/// exit is a mount the kernel already tore down, and its complaint would be the
/// last thing on the user's terminal.
fn unmountArgv(argv: *[7:null]?[*:0]const u8, prog: [*:0]const u8, mountpoint: [*:0]const u8) void {
    argv.* = .{ prog, "-u", "-q", "-z", "--", mountpoint, null };
}

/// CMSG_ALIGN/CMSG_LEN/CMSG_SPACE. Only ever evaluated on the Linux path,
/// where the alignment is `sizeof(size_t)`; other platforms align control
/// messages to 4 and would need their own numbers.
fn cmsgAlign(n: usize) usize {
    const a: usize = @alignOf(usize);
    return (n + a - 1) & ~(a - 1);
}
fn cmsgLen(n: usize) usize {
    return cmsgAlign(@sizeOf(libc.cmsghdr)) + n;
}
fn cmsgSpace(n: usize) usize {
    return cmsgAlign(@sizeOf(libc.cmsghdr)) + cmsgAlign(n);
}

/// Build the child's environment: ours, plus `_FUSE_COMMFD`, minus any
/// `_FUSE_COMMFD` we inherited. The subtraction matters — `getenv` returns the
/// *first* match, so an inherited stale entry (pardes launched from inside
/// something that mounts) would win over the one we just appended and
/// fusermount3 would send the descriptor to a closed socket.
fn buildEnv(gpa: std.mem.Allocator, commfd: [:0]const u8) ![]?[*:0]const u8 {
    var count: usize = 0;
    while (libc.environ[count] != null) count += 1;
    const env = try gpa.alloc(?[*:0]const u8, count + 2);
    var n: usize = 0;
    for (0..count) |i| {
        const entry = libc.environ[i].?;
        if (std.mem.startsWith(u8, std.mem.span(entry), commfd_env ++ "=")) continue;
        env[n] = entry;
        n += 1;
    }
    env[n] = commfd.ptr;
    env[n + 1] = null;
    return env[0 .. n + 2];
}

/// Resolve the helper once, by absolute path. Doing it in the parent rather
/// than by chaining execve attempts in the child keeps `argv[0]` honest (it is
/// what `ps` and fusermount3's own diagnostics print) and turns "fuse3 is not
/// installed" into its own error instead of an exit status.
fn findFusermount() ?[:0]const u8 {
    for (fusermount_paths) |candidate| {
        if (libc.access(candidate.ptr, libc.X_OK) == 0) return candidate;
    }
    return null;
}

/// fork + execve the helper and wait for it. Not `std.process.Child`: that has
/// no way to hand a child an arbitrary descriptor, and the entire protocol here
/// is "the child writes to descriptor N". Everything the child does before
/// execve is async-signal-safe (close, execve, _exit) because the parent may
/// well be multithreaded by the time this runs.
fn spawnHelper(
    prog: [*:0]const u8,
    argv: [*:null]const ?[*:0]const u8,
    envp: [*:null]const ?[*:0]const u8,
    close_in_child: c_int,
) !u8 {
    const pid = libc.fork();
    if (pid < 0) return error.ForkFailed;
    if (pid == 0) {
        // The parent's end of the socketpair. Left open, the parent's recvmsg
        // could never see EOF when the helper dies without sending anything,
        // and a refused mount would hang instead of failing.
        if (close_in_child >= 0) _ = libc.close(close_in_child);
        _ = libc.execve(prog, argv, envp);
        // 127 is the shell's convention for "not found". Reachable only when
        // the binary vanished between the access(2) above and now.
        libc._exit(127);
    }
    var status: c_int = 0;
    while (true) {
        const got = libc.waitpid(pid, &status, 0);
        if (got == pid) break;
        if (got < 0 and libc.errno(got) == .INTR) continue;
        // Reaped by somebody else's SIGCHLD handler: the status is gone, and
        // the descriptor either arrived or it did not. Claim success and let
        // the recvmsg be the judge.
        return 0;
    }
    // WIFEXITED/WEXITSTATUS spelled out: std has no portable macro, and a
    // helper killed by a signal is not a helper that refused the mount.
    if (status & 0x7f != 0) return error.FusermountKilled;
    return @intCast((status >> 8) & 0xff);
}

/// Receive the `/dev/fuse` descriptor. fusermount3 sends it as an SCM_RIGHTS
/// control message alongside exactly one byte of ordinary data, and the byte is
/// not padding: a control message with no data attached may be dropped, so both
/// sides are required to send at least one.
///
/// `MSG_CMSG_CLOEXEC` is the important flag. Every pane shell is forked from
/// this process and inherits open descriptors; a bash holding a copy of this
/// one keeps the FUSE connection alive after pardes exits, and the mount stays
/// up, unkillable, answering nothing, until that shell dies.
fn receiveFd(sock: c_int) !c_int {
    var byte: [1]u8 = undefined;
    var iov = [1]std.posix.iovec{.{ .base = &byte, .len = 1 }};
    var control: [cmsgSpace(@sizeOf(c_int))]u8 align(@alignOf(libc.cmsghdr)) = undefined;
    while (true) {
        var msg: libc.msghdr = .{
            .name = null,
            .namelen = 0,
            .iov = &iov,
            .iovlen = 1,
            .control = &control,
            .controllen = @intCast(control.len),
            .flags = 0,
        };
        const n = libc.recvmsg(sock, &msg, linux.MSG.CMSG_CLOEXEC);
        if (n < 0) {
            if (libc.errno(n) == .INTR) continue;
            return error.CommSocketFailed;
        }
        // EOF: the helper exited without sending anything, which is what a
        // refused mount looks like from here.
        if (n == 0) return error.FusermountRefused;
        if (@as(usize, @intCast(msg.controllen)) < cmsgLen(@sizeOf(c_int))) return error.NoDescriptor;
        const cmsg: *const libc.cmsghdr = @ptrCast(&control);
        if (cmsg.level != libc.SOL.SOCKET or cmsg.type != libc.SCM.RIGHTS) return error.NoDescriptor;
        if (@as(usize, @intCast(cmsg.len)) < cmsgLen(@sizeOf(c_int))) return error.NoDescriptor;
        var fd: c_int = -1;
        @memcpy(
            std.mem.asBytes(&fd),
            control[cmsgAlign(@sizeOf(libc.cmsghdr))..][0..@sizeOf(c_int)],
        );
        if (fd < 0) return error.NoDescriptor;
        return fd;
    }
}

/// `mkdir -p` for the mount point, 0700. The leaf is this process's own pid
/// directory and the parent is `.../pardes`, which on a fresh machine does not
/// exist; without the -p the whole feature would switch itself off in silence
/// on exactly the machines that never used it before. Same shape as
/// `nested.zig`'s ensureSocketDir, and 0700 for the same reason: what lives
/// under here takes commands.
fn ensureDir(path: [:0]const u8) void {
    var partial: [4096:0]u8 = undefined;
    if (path.len >= partial.len) return;
    @memcpy(partial[0 .. path.len + 1], path[0 .. path.len + 1]);
    for (1..path.len) |i| {
        if (path[i] != '/') continue;
        partial[i] = 0;
        _ = libc.mkdir(partial[0..i :0], 0o700);
        partial[i] = '/';
    }
    _ = libc.mkdir(path, 0o700);
}

/// Unmount and remove `<dir>/<pid>` for every pid that is gone. A pardes killed
/// with SIGKILL runs no defer, so its mount outlives it as an ENOTCONN stump
/// that `ls` reports as a permission error and that nothing else will ever
/// clean up — the snapshot suite alone would leave one per aborted run.
/// Bounded: one readdir of a directory only we write to, one kill(0) each.
/// Mirrors nested.zig's socket sweep deliberately, including the ESRCH rule:
/// 0 means alive, EPERM means alive and someone else's, only ESRCH is a corpse.
pub fn sweepStale(dir: []const u8) void {
    if (comptime !supported) return;
    var dir_buf: [4096:0]u8 = undefined;
    const dir_z = std.fmt.bufPrintSentinel(&dir_buf, "{s}", .{dir}, 0) catch return;
    const d = libc.opendir(dir_z) orelse return;
    defer _ = libc.closedir(d);
    const me = libc.getpid();
    while (libc.readdir(d)) |ent| {
        const name = std.mem.sliceTo(&ent.name, 0);
        // Strictly digits: parseInt would accept `+7` and `-7`, and this
        // function unmounts and removes whatever it answers about.
        if (name.len == 0) continue;
        for (name) |ch| if (!std.ascii.isDigit(ch)) break;
        if (std.mem.indexOfNone(u8, name, "0123456789") != null) continue;
        const pid = std.fmt.parseInt(libc.pid_t, name, 10) catch continue;
        if (pid == me) continue;
        const rc = libc.kill(pid, @enumFromInt(0));
        if (rc == 0 or libc.errno(rc) != .SRCH) continue;
        var path_buf: [4096:0]u8 = undefined;
        const path = std.fmt.bufPrintSentinel(&path_buf, "{s}/{s}", .{ dir, name }, 0) catch continue;
        // Always ours to remove: the name is a pid under a directory only
        // pardes writes to, and taking the stump away is the point of a sweep.
        unmountPath(path, true);
    }
}

/// Run the helper's unmount and, when the directory is ours, take it away.
/// Best effort in both halves: an already-unmounted point makes fusermount3
/// complain (which -q swallows) and a non-empty one makes rmdir fail, and
/// neither is worth a diagnostic at exit.
///
/// `remove_dir` is not a convenience. The *unmount* is always right — the mount
/// is ours whoever made the directory — but the *rmdir* is only right for a
/// point pardes derived itself (`<parent>/<pid>`, which `ensureDir` created).
/// A `--fs=<dir>` the user named is theirs, and removing it is the same
/// overreach `sweepStale` is already refused under an explicit `--fs` for.
fn unmountPath(path: [:0]const u8, remove_dir: bool) void {
    if (findFusermount()) |prog| {
        var argv: [7:null]?[*:0]const u8 = undefined;
        unmountArgv(&argv, prog.ptr, path.ptr);
        // A minimal environment: the helper wants nothing of ours, and the one
        // variable that WOULD change its behaviour is the comm descriptor it
        // must not find here.
        const envp = [_:null]?[*:0]const u8{null};
        _ = spawnHelper(prog.ptr, &argv, &envp, -1) catch {};
    }
    if (remove_dir) _ = libc.rmdir(path);
}

// ---------------------------------------------------------------------------
// the park table
// ---------------------------------------------------------------------------

/// How many kernel requests may be outstanding at once. Every slot is either in
/// flight (handed to the core, not yet answered) or parked (the core said
/// `.again`). In-flight slots are transient — the host answers each request
/// inside the same drain step — so in practice this counts BLOCKED READERS: one
/// slot per process sitting on `event` or `log`. A session with 32 of those has
/// 32 scripts watching it.
///
/// Overflow is a refusal, not a queue: `take` answers EAGAIN and the descriptor
/// keeps being read. See its comment for why the tempting alternative (stop
/// reading and let the kernel hold the surplus) is a deadlock.
const max_slots = 32;

/// Bytes of request payload a slot can own. A parked request's `data` cannot go
/// on borrowing the read buffer (the next `next()` overwrites it), so it is
/// copied in at parse time when it fits. This covers every payload that can
/// realistically block: a LOOKUP name is at most 255 bytes and a ctl verb line
/// or an event write-back is a few dozen. A WRITE larger than this is left
/// borrowed and answered EAGAIN if the core ever tries to park it — a write is
/// a transaction in this design and is not supposed to block, and growing this
/// table by 64 KiB a slot to make an impossible case zero-copy is the wrong
/// trade.
const park_data_max = 512;

const Slot = struct {
    used: bool = false,
    /// The core answered `.again`; `retry()` will offer it back.
    parked: bool = false,
    /// Already offered in this retry round. Reset when a round finds nothing,
    /// which is what gives every parked request exactly one attempt per frame
    /// instead of letting the oldest one starve the rest.
    retried: bool = false,
    /// `req.data` points into `data` below rather than into the read buffer.
    copied: bool = false,
    /// Arrival order, so retries are FIFO: the reader that blocked first is
    /// offered first.
    seq: u64 = 0,
    op: Opcode = @enumFromInt(0),
    req: acmefs.Req = undefined,
    data: [park_data_max]u8 = undefined,
};

// ---------------------------------------------------------------------------
// Fs
// ---------------------------------------------------------------------------

pub const Fs = struct {
    pub const Options = struct {
        /// Absolute path of the mount point. Absolute because it is handed to a
        /// setuid program that resolves it against its own cwd, and because the
        /// unmount at exit must name the same place after any chdir.
        mount: []const u8,
        /// The largest WRITE payload the kernel may send in one request, and
        /// therefore the size of the read buffer. 64 KiB matches what a `cp`
        /// into `body` will use; smaller only splits the same bytes into more
        /// round trips.
        max_write: u32 = 64 * 1024,
        /// Whether pardes made this directory and may therefore remove it at
        /// exit. True for the derived `<parent>/<pid>`, false for a
        /// `--fs=<dir>` the user named. See `unmountPath`.
        owns_dir: bool = false,
    };

    gpa: std.mem.Allocator,
    /// The `/dev/fuse` descriptor. -1 once torn down; every entry point checks
    /// it, so a double deinit and a post-unmount drain are both no-ops.
    fd: c_int = -1,
    /// Set when the connection is gone (ENODEV/ECONNABORTED, or DESTROY).
    /// `next()` stops reading; replies are still written because a slot may be
    /// mid-flight and the write simply fails.
    dead: bool = false,
    path: [:0]u8,
    /// Mirrors `Options.owns_dir`; gates the rmdir in `deinit`.
    owns_dir: bool = false,
    /// One request per read(2), so this is sized for the largest request that
    /// exists: header + fuse_write_in + max_write. Below FUSE_MIN_READ_BUFFER
    /// the kernel refuses to hand over requests at all and answers the client
    /// EIO. 8-aligned so the parse can point structs at it.
    buf: []align(8) u8,
    /// Encoded `fuse_dirent`s. Separate from `buf` because a readdir reply is
    /// built while its request is still being read from `buf`.
    dirents: [8192]u8 align(8) = undefined,

    uid: u32,
    gid: u32,
    max_write: u32,
    /// The minor the kernel offered, echoed back at INIT. Kept for the record:
    /// it is the one number in this file that a future feature would consult.
    minor: u32 = 0,

    slots: [max_slots]Slot = @splat(.{}),
    seq: u64 = 0,
    thread: ?std.Thread = null,
    /// main -> poller, an `eventfd(2)`. The main thread adds 1 per completed
    /// drain and the poller's blocking read takes the whole counter in one go,
    /// which is the "collapse the acknowledgements that piled up while we were
    /// not waiting" behaviour a pipe needed three functions and a nonblocking
    /// toggle to fake. Not a condition variable, because the poller is blocked
    /// in `poll()` most of the time and an fd is the only thing that both
    /// `poll()` and a blocking read can wait on — which is what lets shutdown
    /// break it out of either state.
    ///
    /// The counter cannot say "stop": a stop and a drain acknowledgement that
    /// race are summed into one indistinguishable number. `stopping` is the
    /// sticky half of the signal, and is re-read after every wake; the eventfd
    /// only ever means "look again". The store/write and read/load pair is a
    /// release/acquire edge over the eventfd's own wait-queue lock, so a poller
    /// that observes the increment observes the flag with it.
    ctl: c_int = -1,
    stopping: std.atomic.Value(bool) = .init(false),
    wake_ctx: ?*anyopaque = null,
    wake_fn: ?*const fn (?*anyopaque) void = null,

    /// Mount, hand out the descriptor, and complete the INIT handshake. On
    /// return the filesystem is live: the kernel will start sending lookups the
    /// moment anything touches the directory.
    pub fn mount(gpa: std.mem.Allocator, opts: Options) !*Fs {
        if (comptime !supported) return error.Unsupported;
        if (opts.mount.len == 0 or opts.mount[0] != '/') return error.MountPathNotAbsolute;

        const path = try gpa.dupeZ(u8, opts.mount);
        errdefer gpa.free(path);
        ensureDir(path);

        const buf_len = @max(
            min_read_buffer,
            @sizeOf(fuse_in_header) + @sizeOf(fuse_write_in) + @as(usize, opts.max_write),
        );
        const buf = try gpa.alignedAlloc(u8, .@"8", buf_len);
        errdefer gpa.free(buf);

        const fd = try mountFusermount(gpa, path);
        errdefer _ = libc.close(fd);

        const fs = try gpa.create(Fs);
        errdefer gpa.destroy(fs);
        fs.* = .{
            .gpa = gpa,
            .fd = fd,
            .path = path,
            .buf = buf,
            .uid = libc.getuid(),
            .gid = libc.getgid(),
            .max_write = opts.max_write,
            .owns_dir = opts.owns_dir,
        };
        // Still blocking here on purpose: INIT is already queued (fusermount3
        // completed mount(2) before it sent us the descriptor), and a
        // non-blocking read would make the handshake a spin loop.
        try fs.handshake();
        try fs.setNonblocking();
        return fs;
    }

    /// socketpair, fork the setuid helper, take the descriptor it sends back.
    fn mountFusermount(gpa: std.mem.Allocator, path: [:0]const u8) !c_int {
        const prog = findFusermount() orelse return error.FusermountMissing;
        var sv: [2]c_int = undefined;
        if (libc.socketpair(libc.AF.UNIX, libc.SOCK.STREAM, 0, &sv) != 0) return error.SocketPairFailed;
        // Both ends close-on-exec first, then the child's end is un-marked just
        // before the fork. The window in between is what any *other* thread's
        // fork would inherit, and pane shells are forked with forkpty and
        // inherit everything open.
        setCloexec(sv[0]);
        setCloexec(sv[1]);
        errdefer _ = libc.close(sv[0]);

        var opts_buf: [128:0]u8 = undefined;
        var commfd_buf: [32:0]u8 = undefined;
        const opts = mountOpts(&opts_buf);
        const commfd = commfdEnv(&commfd_buf, sv[1]);

        const envp = try buildEnv(gpa, commfd);
        defer gpa.free(envp);
        var argv: [6:null]?[*:0]const u8 = undefined;
        mountArgv(&argv, prog.ptr, opts.ptr, path.ptr);

        clearCloexec(sv[1]);
        const code = spawnHelper(prog.ptr, &argv, @ptrCast(envp.ptr), sv[0]) catch |err| {
            _ = libc.close(sv[1]);
            return err;
        };
        // Ours to close either way: the child has its own copy, and while we
        // hold one the recvmsg below can never see EOF when the helper dies.
        _ = libc.close(sv[1]);
        if (code == 127) return error.FusermountMissing;

        const fd = try receiveFd(sv[0]);
        if (code != 0) {
            _ = libc.close(fd);
            return error.FusermountFailed;
        }
        _ = libc.close(sv[0]);
        return fd;
    }

    /// Read the kernel's INIT and answer it. Negotiating nothing is the design:
    /// every flag is a kernel behaviour we would then have to honour forever,
    /// and this filesystem wants none of them — no readdirplus (whose ENOSYS
    /// has no fallback and would fail every getdents), no atomic O_TRUNC (so
    /// `> file` arrives as a plain SETATTR the core already handles), no POSIX
    /// or BSD locks (flags = 0 makes the kernel set `no_lock`/`no_flock` and
    /// answer them itself).
    fn handshake(fs: *Fs) !void {
        const n = readFull(fs.fd, fs.buf);
        if (n < @sizeOf(fuse_in_header) + @sizeOf(fuse_init_in)) return error.InitFailed;
        const h: *const fuse_in_header = @ptrCast(fs.buf.ptr);
        if (@as(Opcode, @enumFromInt(h.opcode)) != .init) return error.InitFailed;
        const in: *const fuse_init_in = @ptrCast(@as([*]align(8) u8, @alignCast(fs.buf.ptr + @sizeOf(fuse_in_header))));
        // A major mismatch is fatal and there is nothing to negotiate: the
        // kernel aborts the connection, and answering anyway just delays the
        // failure to the first syscall through the mount.
        if (in.major != kernel_version) return error.InitVersion;
        fs.minor = in.minor;

        const out: fuse_init_out = .{
            .major = kernel_version,
            // Capped, not echoed: see `kernel_minor`.
            .minor = @min(in.minor, kernel_minor),
            // Zero, not "some readahead": with FOPEN_DIRECT_IO there is no page
            // cache to read ahead into, and a nonzero value here only invites
            // the kernel to ask for bytes nobody wanted.
            .max_readahead = 0,
            .flags = 0,
            // Left at zero so the kernel keeps its own defaults; a nonzero
            // max_background is the one that silently caps concurrency.
            .max_background = 0,
            .congestion_threshold = 0,
            .max_write = fs.max_write,
            // 1 ns. Timestamps on this filesystem are all zero anyway, but a
            // time_gran of 0 is not a legal granularity.
            .time_gran = 1,
            .max_pages = 0,
            .map_alignment = 0,
            .flags2 = 0,
            .max_stack_depth = 0,
            // 0 = no server timeout. A timeout would let the kernel abort the
            // connection while a legitimately parked `event` read waits.
            .request_timeout = 0,
            .unused = @splat(0),
        };
        fs.answer(h.unique, std.mem.asBytes(&out), &.{});
        return;
    }

    fn setNonblocking(fs: *Fs) !void {
        const flags = libc.fcntl(fs.fd, libc.F.GETFL, @as(c_int, 0));
        if (flags < 0) return error.FcntlFailed;
        var o: libc.O = @bitCast(@as(u32, @bitCast(flags)));
        o.NONBLOCK = true;
        if (libc.fcntl(fs.fd, libc.F.SETFL, @as(c_int, @bitCast(@as(u32, @bitCast(o))))) < 0)
            return error.FcntlFailed;
    }

    /// Answer everything still held, abort the connection, unmount, remove the
    /// directory. The order is not interchangeable:
    ///
    ///   1. reply -ENODEV to every slot, so a reader blocked on `event` gets an
    ///      error rather than being left in uninterruptible sleep.
    ///   2. close the descriptor, which aborts the connection — the backstop
    ///      for anything that raced step 1, since the kernel then fails every
    ///      pending request itself.
    ///   3. only then unmount, because a mount whose server is gone is exactly
    ///      what `fusermount3 -u -z` is for.
    ///   4. remove the directory, but only when pardes made it: the derived
    ///      `<parent>/<pid>` is ours, a `--fs=<dir>` the user named is not.
    pub fn deinit(fs: *Fs) void {
        const gpa = fs.gpa;
        fs.stopThread();
        if (fs.fd >= 0) {
            for (&fs.slots) |*s| {
                if (!s.used) continue;
                fs.answerErr(s.req.tag, .NODEV);
                s.* = .{};
            }
            _ = libc.close(fs.fd);
            fs.fd = -1;
        }
        if (comptime supported) unmountPath(fs.path, fs.owns_dir);
        gpa.free(fs.path);
        gpa.free(fs.buf);
        gpa.destroy(fs);
    }

    /// This mount as the three functions `fs_service` actually calls. The
    /// adapter exists so that file needs no `@import("fuse.zig")` to drive a
    /// filesystem: `retry`, `next` and `reply` were always its whole use of an
    /// `Fs`, and naming them lets a second transport answer the same calls.
    ///
    /// The thunks are three lines each because a `*Fs` is not an `*anyopaque`
    /// and a vtable cannot hold the typed function directly. That is the entire
    /// cost of the seam.
    pub fn transport(fs: *Fs) fs_service.Transport {
        return .{ .ctx = fs, .vtable = &transport_vtable };
    }

    const transport_vtable: fs_service.Transport.VTable = .{
        .retry = transportRetry,
        .next = transportNext,
        .reply = transportReply,
    };

    fn transportRetry(ctx: *anyopaque) ?acmefs.Req {
        const fs: *Fs = @ptrCast(@alignCast(ctx));
        return fs.retry();
    }

    fn transportNext(ctx: *anyopaque) ?acmefs.Req {
        const fs: *Fs = @ptrCast(@alignCast(ctx));
        return fs.next();
    }

    fn transportReply(ctx: *anyopaque, r: *const acmefs.Reply, bytes: []const u8) void {
        const fs: *Fs = @ptrCast(@alignCast(ctx));
        fs.reply(r, bytes);
    }

    // -- request pump -------------------------------------------------------

    /// Parse the next pending kernel request, or null when the descriptor is
    /// drained. Call in a loop until null; the loop is the batch, and one wake
    /// serves all of it.
    ///
    /// The returned `Req.data` borrows storage owned by this `Fs` and is valid
    /// until the next `next()` call. The core copies whatever it keeps — the
    /// same rule as `.pty_read`.
    ///
    /// Requests the core has no business seeing are answered here and the loop
    /// continues, so a caller never observes them.
    ///
    /// Running this to null is also what acknowledges the batch to the poll
    /// thread, so a host that stops early keeps the poller waiting and loses
    /// wake latency until the next frame. It is not a correctness bug — the
    /// remaining requests simply wait in the kernel — but the loop is the
    /// contract.
    pub fn next(fs: *Fs) ?acmefs.Req {
        if (comptime !supported) return null;
        // Only the EAGAIN arm below releases the poller, and deliberately so.
        // Every other null return from here implies `dead`, which is write-once
        // and means reads on the descriptor are failing: posting would send the
        // poller back into `poll()` on a still-open fd that reports POLLIN
        // forever, wake the host, drain to this same null, and spin two threads
        // at 100%. Parking the poller in `consume()` is the right resting state
        // for a connection that can never produce work again; `stopThread`
        // releases it. `fd < 0` is unreachable here, since only `deinit` sets it
        // and it joins the poller first.
        if (fs.fd < 0 or fs.dead) return null;
        while (true) {
            const n = libc.read(fs.fd, fs.buf.ptr, fs.buf.len);
            if (n < 0) switch (libc.errno(n)) {
                .INTR => continue,
                .AGAIN => {
                    // Drained: release the poller (see `post`).
                    fs.post();
                    return null;
                },
                // The request was interrupted or aborted between being queued
                // and being read; there is nothing to answer.
                .NOENT => continue,
                // ENODEV (connection aborted, or we were unmounted from under
                // ourselves) and ECONNABORTED are terminal. Anything else here
                // is not a thing /dev/fuse does, and treating the unknown as
                // terminal beats a loop that reads -1 forever.
                else => {
                    fs.dead = true;
                    return null;
                },
            };
            if (n == 0) {
                fs.dead = true;
                return null;
            }
            const total: usize = @intCast(n);
            // Cannot happen (the kernel writes whole requests) but the parse
            // below indexes on it.
            if (total < @sizeOf(fuse_in_header)) continue;
            if (fs.dispatch(total)) |req| return req;
        }
    }

    /// Offer parked requests back, one per call. Call in a loop until null,
    /// once per frame, before `next()`: the null both ends the round and resets
    /// it, so every parked request gets exactly one attempt per frame and a
    /// permanently blocked reader cannot starve the others.
    pub fn retry(fs: *Fs) ?acmefs.Req {
        if (comptime !supported) return null;
        if (fs.fd < 0) return null;
        var best: ?usize = null;
        for (&fs.slots, 0..) |*s, i| {
            if (!s.used or !s.parked or s.retried) continue;
            if (best == null or s.seq < fs.slots[best.?].seq) best = i;
        }
        const i = best orelse {
            for (&fs.slots) |*s| s.retried = false;
            return null;
        };
        fs.slots[i].retried = true;
        // In flight again: `reply()` re-parks it if the core still has nothing.
        fs.slots[i].parked = false;
        return fs.slots[i].req;
    }

    /// Write the core's answer, or park the request when it said `.again`.
    /// Called from the `.fs_reply` effect; `bytes` is the payload resolved by
    /// `pardes.fsPayload` and is borrowed only for the duration of this call.
    pub fn reply(fs: *Fs, r: *const acmefs.Reply, bytes: []const u8) void {
        if (comptime !supported) return;
        const i = fs.findSlot(r.tag) orelse return; // interrupted, or torn down
        const s = &fs.slots[i];

        if (r.status == .again) {
            // The one case a park is refused: a payload too large to have been
            // copied at parse time still borrows the read buffer, so parking it
            // would park a dangling slice. EAGAIN is honest — the writer can
            // retry — and by construction unreachable, since the core answers
            // writes as transactions and only reads ever block.
            if (!s.copied and s.req.data.len != 0) {
                fs.answerErr(s.req.tag, .AGAIN);
                fs.release(i);
                return;
            }
            s.parked = true;
            return;
        }

        if (r.status == .err) {
            fs.answerErr(s.req.tag, @enumFromInt(if (r.errno == 0) @intFromEnum(libc.E.IO) else r.errno));
            fs.release(i);
            return;
        }

        switch (s.req.op) {
            .lookup => {
                const out: fuse_entry_out = .{
                    .nodeid = r.attr.node,
                    // Node ids are never reused in this filesystem (pane
                    // serials are monotonic), which is exactly the condition
                    // for a constant generation to be safe.
                    .generation = 0,
                    // No caching, at all. Every file here changes under the
                    // reader's feet, and a cached negative lookup would make
                    // `new/<name>` (which CREATES a pane) work exactly once.
                    .entry_valid = 0,
                    .attr_valid = 0,
                    .entry_valid_nsec = 0,
                    .attr_valid_nsec = 0,
                    .attr = fs.attr(r.attr, r.attr.node),
                };
                fs.answer(s.req.tag, std.mem.asBytes(&out), &.{});
            },
            .getattr, .setattr => {
                const out: fuse_attr_out = .{
                    .attr_valid = 0,
                    .attr_valid_nsec = 0,
                    .dummy = 0,
                    .attr = fs.attr(r.attr, s.req.node),
                };
                fs.answer(s.req.tag, std.mem.asBytes(&out), &.{});
            },
            .open => {
                const out: fuse_open_out = .{
                    .fh = r.handle,
                    // Direct IO for files; nothing for directories, where the
                    // flag has no meaning and FOPEN_CACHE_DIR (which we do not
                    // set) is the caching knob. An uncached directory is the
                    // point: `new/` and the pane list change constantly.
                    .open_flags = if (s.op == .opendir) 0 else FOPEN_DIRECT_IO,
                    .backing_id = 0,
                };
                fs.answer(s.req.tag, std.mem.asBytes(&out), &.{});
            },
            .read => {
                // Never more than was asked for: a read reply longer than
                // `size` is a protocol error the kernel answers with EIO.
                const len = @min(bytes.len, s.req.size);
                fs.answer(s.req.tag, &.{}, bytes[0..len]);
            },
            .readdir => {
                const room = @min(@as(usize, s.req.size), fs.dirents.len);
                const len = encodeDirents(fs.dirents[0..room], bytes, s.req.off);
                fs.answer(s.req.tag, &.{}, fs.dirents[0..len]);
            },
            .write => {
                // The core's own count, not the request size: `data` refusing a
                // partial grapheme is a real short write, and claiming the
                // whole request would tell the writer its trailing bytes
                // landed when they did not. Clamped anyway, because a count
                // larger than what was offered makes the kernel advance a file
                // offset past bytes that never existed.
                const out: fuse_write_out = .{
                    .size = @min(r.written, s.req.size),
                    .padding = 0,
                };
                fs.answer(s.req.tag, std.mem.asBytes(&out), &.{});
            },
            .release => fs.answer(s.req.tag, &.{}, &.{}),
            .statfs => {
                // Synthetic numbers, but not arbitrary ones: `namelen` is what
                // pathconf(_PC_NAME_MAX) returns and a zero there makes some
                // tools refuse to create any name at all, and `bsize` is what
                // `stat` reports as the IO block size.
                const out: fuse_statfs_out = .{ .st = .{
                    .blocks = 0,
                    .bfree = 0,
                    .bavail = 0,
                    .files = 0,
                    .ffree = 0,
                    .bsize = 4096,
                    .namelen = 255,
                    .frsize = 4096,
                    .padding = 0,
                    .spare = @splat(0),
                } };
                fs.answer(s.req.tag, std.mem.asBytes(&out), &.{});
            },
        }
        fs.release(i);
    }

    /// Translate one request. Null means it was answered here.
    fn dispatch(fs: *Fs, total: usize) ?acmefs.Req {
        const h: *const fuse_in_header = @ptrCast(fs.buf.ptr);
        // Bounded by the header's own length, not just by what the read
        // returned. They agree on /dev/fuse, and taking the smaller of the two
        // is what keeps a WRITE from claiming payload it did not bring even if
        // some future kernel ever pads a request.
        const end = @min(total, @max(@as(usize, h.len), @sizeOf(fuse_in_header)));
        const body: []align(8) const u8 = @alignCast(fs.buf[@sizeOf(fuse_in_header)..end]);
        const op: Opcode = @enumFromInt(h.opcode);
        switch (op) {
            // Already answered in the handshake. A second INIT cannot happen;
            // answering it again is cheaper than a special case that could.
            .init => {
                fs.answerErr(h.unique, .INVAL);
                return null;
            },
            // NEVER replied to. The kernel does not track these as pending
            // requests, so a reply carries a `unique` it will not recognise —
            // -ENOENT at best, and at worst a reply matched against a *live*
            // request that happens to share the number. Ignoring the refcount
            // itself is fine: this filesystem's node table is bounded by the
            // pane count, so nothing grows.
            .forget, .batch_forget => return null,
            // Answer the ORIGINAL with EINTR and drop it. This is the only
            // thing standing between a SIGKILLed reader of `event` and
            // permanent uninterruptible sleep: after the fatal signal the
            // kernel's last wait is not killable, so the process survives its
            // own kill until this reply lands. No reply to the interrupt
            // itself — its unique is `original | 1` and the kernel keeps no
            // pending entry for it, while answering -ENOSYS would switch
            // interrupts off for the whole connection and take the escape
            // hatch away.
            .interrupt => {
                if (body.len >= @sizeOf(fuse_interrupt_in)) {
                    const in: *const fuse_interrupt_in = @ptrCast(body.ptr);
                    if (fs.findSlot(in.unique)) |i| {
                        fs.answerErr(fs.slots[i].req.tag, .INTR);
                        fs.release(i);
                    }
                }
                return null;
            },
            // A missing reply here hangs `umount` outright.
            .destroy => {
                fs.answer(h.unique, &.{}, &.{});
                fs.dead = true;
                return null;
            },
            // -ENOSYS rather than an empty reply: the kernel sets `no_flush`
            // and stops sending them, so this costs one round trip for the
            // whole connection instead of one per close(2). Nothing here has
            // buffered state for a flush to commit.
            .flush => {
                fs.answerErr(h.unique, .NOSYS);
                return null;
            },
            .lookup => {
                // The name is the whole body, NUL terminated. An empty name is
                // not a lookup of anything.
                const name = std.mem.sliceTo(body, 0);
                if (name.len == 0) {
                    fs.answerErr(h.unique, .INVAL);
                    return null;
                }
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = .lookup,
                    .node = h.nodeid,
                    .data = name,
                });
            },
            .getattr => {
                const in = fs.arg(fuse_getattr_in, body) orelse return null;
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = .getattr,
                    .node = h.nodeid,
                    // `fh` is only meaningful with the flag; reading it blind
                    // hands the core a handle from an unrelated open.
                    .handle = if (in.getattr_flags & FUSE_GETATTR_FH != 0) @truncate(in.fh) else 0,
                });
            },
            .setattr => {
                const in = fs.arg(fuse_setattr_in, body) orelse return null;
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = .setattr,
                    .node = h.nodeid,
                    .handle = @truncate(in.fh),
                    // The `> file` path, and the only setattr this filesystem
                    // has an opinion about. A truncate to a nonzero length is
                    // not expressible in the core's ABI and is reported as no
                    // truncate at all: the reply still carries the current
                    // attributes, so ftruncate(fd, n) succeeds and changes
                    // nothing, which is what every synthetic file here wants.
                    .truncate = in.valid & FATTR_SIZE != 0 and in.size == 0,
                });
            },
            .open, .opendir => {
                // The flags are read only to reject a short body: this
                // filesystem's permission model is the mode bits each synthetic
                // file reports from GETATTR, which the kernel enforces itself,
                // so the access mode has nothing left to say here.
                if (fs.arg(fuse_open_in, body) == null) return null;
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = .open,
                    .node = h.nodeid,
                });
            },
            .read, .readdir => {
                const in = fs.arg(fuse_read_in, body) orelse return null;
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = if (op == .readdir) .readdir else .read,
                    .node = h.nodeid,
                    .handle = @truncate(in.fh),
                    .off = in.offset,
                    .size = in.size,
                });
            },
            .write => {
                const in = fs.arg(fuse_write_in, body) orelse return null;
                const payload = body[@sizeOf(fuse_write_in)..];
                // Trust the header's length over the struct's: a `size` larger
                // than what arrived would read past the request.
                const len = @min(@as(usize, in.size), payload.len);
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = .write,
                    .node = h.nodeid,
                    .handle = @truncate(in.fh),
                    .off = in.offset,
                    .size = @intCast(len),
                    .data = payload[0..len],
                });
            },
            .release, .releasedir => {
                const in = fs.arg(fuse_release_in, body) orelse return null;
                return fs.take(op, .{
                    .tag = h.unique,
                    .op = .release,
                    .node = h.nodeid,
                    .handle = @truncate(in.fh),
                });
            },
            .statfs => return fs.take(op, .{
                .tag = h.unique,
                .op = .statfs,
                .node = h.nodeid,
            }),
            // Everything else. -ENOSYS is not a shrug: for most of these the
            // kernel caches the answer and stops asking (`no_access`,
            // `no_getxattr`, `no_statx`, `no_poll`, `no_lseek`, `no_create`),
            // so one refusal switches the whole feature off for the connection.
            // The mutations (mkdir, unlink, rename, link, symlink) are refused
            // because this tree is generated: its shape follows the pane list
            // and there is nothing for a user to create or remove in it.
            // READDIRPLUS is not in this list by accident — it is unreachable,
            // because INIT never sets FUSE_DO_READDIRPLUS, and it has to stay
            // that way: its -ENOSYS has NO fallback in the kernel and would
            // fail every getdents through the mount.
            else => {
                fs.answerErr(h.unique, .NOSYS);
                return null;
            },
        }
    }

    /// Point a request struct at the read buffer. Null (and an EINVAL reply)
    /// when the kernel sent less than the struct, which cannot happen but would
    /// otherwise be a read past the buffer.
    fn arg(fs: *Fs, comptime T: type, body: []align(8) const u8) ?*const T {
        if (body.len < @sizeOf(T)) {
            const h: *const fuse_in_header = @ptrCast(fs.buf.ptr);
            fs.answerErr(h.unique, .INVAL);
            return null;
        }
        return @ptrCast(body.ptr);
    }

    /// Move a parsed request into a slot and hand it to the caller. Small
    /// payloads are copied in here so that a later park has stable bytes; a
    /// large one stays borrowed (see `park_data_max`).
    ///
    /// Null (and an EAGAIN reply) when the table is full. That is the whole
    /// reason `next` reads unconditionally instead of gating on a free slot:
    /// gating looks like polite backpressure and is a deadlock. With 32 readers
    /// blocked on `event`, refusing to read the descriptor means the INTERRUPT
    /// that would free a slot is never read either, so a SIGKILLed reader stays
    /// in uninterruptible sleep forever and every unrelated `ls` of the mount
    /// hangs behind it. Reading and answering EAGAIN keeps FORGET, INTERRUPT,
    /// DESTROY and the ENOSYS family flowing — none of which need a slot — and
    /// turns "too many blocked readers" into one failed syscall the caller can
    /// see and retry.
    fn take(fs: *Fs, op: Opcode, req: acmefs.Req) ?acmefs.Req {
        const i = fs.freeSlot() orelse {
            fs.answerErr(req.tag, .AGAIN);
            return null;
        };
        const s = &fs.slots[i];
        s.* = .{
            .used = true,
            .seq = fs.seq,
            .op = op,
            .req = req,
        };
        fs.seq += 1;
        if (req.data.len != 0 and req.data.len <= park_data_max) {
            @memcpy(s.data[0..req.data.len], req.data);
            s.copied = true;
            s.req.data = s.data[0..req.data.len];
        }
        return s.req;
    }

    fn freeSlot(fs: *Fs) ?usize {
        for (&fs.slots, 0..) |*s, i| if (!s.used) return i;
        return null;
    }

    fn findSlot(fs: *Fs, tag: u64) ?usize {
        for (&fs.slots, 0..) |*s, i| if (s.used and s.req.tag == tag) return i;
        return null;
    }

    fn release(fs: *Fs, i: usize) void {
        fs.slots[i] = .{};
    }

    /// `Reply.Attr` -> `fuse_attr`. `node` is the fallback inode for replies
    /// that do not name one (a getattr answers about a node the request already
    /// identified); a zero `st_ino` is a value no filesystem is allowed to
    /// report and some tools treat it as a deleted entry.
    fn attr(fs: *const Fs, a: acmefs.Reply.Attr, node: u64) fuse_attr {
        const ino = if (a.node != 0) a.node else node;
        return .{
            .ino = ino,
            .size = a.size,
            // 512-byte units, as `stat` wants them. Rounded up so a nonempty
            // file never reports zero blocks, which `du` reads as a hole.
            .blocks = (a.size + 511) / 512,
            .atime = 0,
            .mtime = 0,
            .ctime = 0,
            .atimensec = 0,
            .mtimensec = 0,
            .ctimensec = 0,
            .mode = (if (a.dir) S_IFDIR else S_IFREG) | @as(u32, a.mode),
            // 2 for a directory (itself and `.`) is what every tool expects;
            // `find` in particular uses it to decide whether to recurse.
            .nlink = if (a.dir) 2 else 1,
            // The mounting user owns everything: without `allow_other` nobody
            // else can reach the mount at all, and reporting some other owner
            // would only make `ls -l` lie.
            .uid = fs.uid,
            .gid = fs.gid,
            .rdev = 0,
            .blksize = 4096,
            .flags = 0,
        };
    }

    // -- reply framing ------------------------------------------------------

    /// One `writev` per reply: header, then the op's fixed out struct, then the
    /// payload. Split into iovecs rather than assembled in a buffer so that a
    /// megabyte read out of a pane's text is written straight from the core's
    /// bytes — the whole point of `Reply.Payload.region`.
    fn answer(fs: *Fs, unique: u64, fixed: []const u8, payload: []const u8) void {
        var header: fuse_out_header = .{
            .len = @intCast(@sizeOf(fuse_out_header) + fixed.len + payload.len),
            .@"error" = 0,
            .unique = unique,
        };
        var iov: [3]std.posix.iovec_const = undefined;
        var n: usize = 1;
        iov[0] = .{ .base = std.mem.asBytes(&header).ptr, .len = @sizeOf(fuse_out_header) };
        if (fixed.len != 0) {
            iov[n] = .{ .base = fixed.ptr, .len = fixed.len };
            n += 1;
        }
        if (payload.len != 0) {
            iov[n] = .{ .base = payload.ptr, .len = payload.len };
            n += 1;
        }
        fs.writeReply(iov[0..n], header.len);
    }

    /// An error reply is header-only: the kernel checks `nbytes ==
    /// sizeof(oh)` when `error != 0` and answers -EINVAL otherwise, which
    /// leaves the original request pending forever.
    fn answerErr(fs: *Fs, unique: u64, e: libc.E) void {
        var header: fuse_out_header = .{
            .len = @sizeOf(fuse_out_header),
            .@"error" = -@as(i32, @intFromEnum(e)),
            .unique = unique,
        };
        const iov = [1]std.posix.iovec_const{
            .{ .base = std.mem.asBytes(&header).ptr, .len = @sizeOf(fuse_out_header) },
        };
        fs.writeReply(&iov, header.len);
    }

    fn writeReply(fs: *Fs, iov: []const std.posix.iovec_const, expect: u32) void {
        if (fs.fd < 0) return;
        while (true) {
            const n = libc.writev(fs.fd, iov.ptr, @intCast(iov.len));
            if (n < 0) switch (libc.errno(n)) {
                .INTR => continue,
                // /dev/fuse writes never block, so this is not the usual
                // EAGAIN; retrying is the only thing that can make progress and
                // it cannot loop forever because the kernel is not waiting on
                // us.
                .AGAIN => continue,
                // The request is no longer pending: it was interrupted or the
                // connection was aborted between the read and this write.
                // Dropping it is correct — there is nothing left to answer.
                .NOENT => return,
                else => {
                    fs.dead = true;
                    return;
                },
            };
            // A short write to /dev/fuse is not a thing (the kernel takes the
            // whole reply or none of it), so this can only mean the reply was
            // malformed and the request is still pending. Nothing useful is
            // left to do about it here, and pretending otherwise would hide it.
            std.debug.assert(@as(u32, @intCast(n)) == expect);
            return;
        }
    }

    // -- poll thread --------------------------------------------------------

    /// Start the one background thread: it waits for POLLIN and calls `wake`.
    /// It never touches the descriptor's data, never sees a request and never
    /// calls the core; the host's `wake` is expected to do nothing but post an
    /// event on the loop, exactly like the inotify thread's.
    ///
    /// Optional by design. A host with no threads simply does not call this and
    /// drains from its frame poll instead; it loses wake latency and nothing
    /// else, which is what makes the no-parallelism backend work unchanged.
    pub fn wakeThread(fs: *Fs, ctx: ?*anyopaque, wake: *const fn (?*anyopaque) void) !void {
        if (comptime !supported) return;
        if (fs.thread != null) return;
        // Blocking on purpose: `consume` is a blocking read on this descriptor.
        // The write side cannot block anyway — an eventfd write only waits for
        // a counter one short of `maxInt(u64)` to be drained, which is not
        // reachable at one increment per drain.
        const efd = libc.eventfd(0, linux.EFD.CLOEXEC);
        if (efd < 0) return error.EventFdFailed;
        fs.ctl = efd;
        fs.wake_ctx = ctx;
        fs.wake_fn = wake;
        fs.thread = std.Thread.spawn(.{}, pollLoop, .{fs}) catch |err| {
            _ = libc.close(efd);
            fs.ctl = -1;
            return err;
        };
    }

    fn stopThread(fs: *Fs) void {
        if (comptime !supported) return;
        // `ctl` and `thread` are set and cleared together, so there is no
        // descriptor to close on the path where no poller was ever started.
        const t = fs.thread orelse return;
        // The flag before the wake, never after: a poller that reads the
        // increment must not then find `stopping` false and go back to sleep on
        // a counter nobody will raise again. With this order every state the
        // poller can be in ends in an exit — the loop condition, the `poll()`
        // (the eventfd becomes readable) and the blocking wait for a drain
        // acknowledgement (the read returns) all re-read the flag.
        fs.stopping.store(true, .release);
        fs.post();
        t.join();
        fs.thread = null;
        _ = libc.close(fs.ctl);
        fs.ctl = -1;
    }

    /// Raise the counter by one: "the descriptor has been drained, you may poll
    /// again", or during teardown "look at `stopping`". Without the drain half
    /// of that handshake the poller re-polls a level-triggered descriptor that
    /// is still readable and spins a core until the main thread catches up;
    /// with it, one wake serves one batch.
    fn post(fs: *Fs) void {
        if (fs.ctl < 0) return;
        const one: u64 = 1;
        _ = libc.write(fs.ctl, std.mem.asBytes(&one), @sizeOf(u64));
    }

    fn pollLoop(fs: *Fs) void {
        if (comptime !supported) return;
        while (!fs.stopping.load(.acquire)) {
            var fds = [2]libc.pollfd{
                .{ .fd = fs.fd, .events = libc.POLL.IN, .revents = 0 },
                .{ .fd = fs.ctl, .events = libc.POLL.IN, .revents = 0 },
            };
            const rc = libc.poll(&fds, 2, -1);
            if (rc < 0) {
                if (libc.errno(rc) == .INTR) continue;
                return;
            }
            // Shutdown, or an acknowledgement for a drain that happened without
            // us. Take the whole counter and re-poll either way: a leftover
            // count would make the wait below return instantly and turn the
            // next wake into a spin.
            if (fds[1].revents != 0 and fs.consume()) return;
            if (fds[0].revents & (libc.POLL.ERR | libc.POLL.HUP | libc.POLL.NVAL) != 0) return;
            if (fds[0].revents & libc.POLL.IN == 0) continue;

            (fs.wake_fn.?)(fs.wake_ctx);
            // Wait for the main thread to finish the batch. This is the whole
            // anti-spin mechanism; see `post`.
            if (fs.consume()) return;
        }
    }

    /// Block until the counter is nonzero, then take all of it. True when the
    /// poller must exit, which is `stopping` and nothing else: the count itself
    /// carries no meaning beyond "look again".
    ///
    /// The read blocks, including on the branch that reached here from a
    /// `poll()` that only *said* the descriptor was readable. That is safe
    /// because `stopThread` closes `ctl` after `join()` and never before: an
    /// eventfd raises neither POLLERR nor POLLHUP, so the one revents value
    /// that would be readable-but-not-readable is POLLNVAL, and a closed
    /// descriptor is the only thing that produces it.
    fn consume(fs: *Fs) bool {
        var v: u64 = undefined;
        while (true) {
            const n = libc.read(fs.ctl, std.mem.asBytes(&v), @sizeOf(u64));
            // A short read and an EOF do not exist on an eventfd: the read
            // returns 8 or -1. So anything but EINTR means this descriptor is
            // not the one we opened, and exiting beats spinning on it.
            if (n < 0) {
                if (libc.errno(n) == .INTR) continue;
                return true;
            }
            return fs.stopping.load(.acquire);
        }
    }
};

// ---------------------------------------------------------------------------
// descriptor flags
// ---------------------------------------------------------------------------

fn setCloexec(fd: c_int) void {
    const FD_CLOEXEC: c_int = 1;
    _ = libc.fcntl(fd, libc.F.SETFD, FD_CLOEXEC);
}

/// The child of the mount fork must KEEP this descriptor across execve — it is
/// the whole channel the setuid helper answers on.
fn clearCloexec(fd: c_int) void {
    _ = libc.fcntl(fd, libc.F.SETFD, @as(c_int, 0));
}

fn setNonblock(fd: c_int) void {
    const flags = libc.fcntl(fd, libc.F.GETFL, @as(c_int, 0));
    if (flags < 0) return;
    var o: libc.O = @bitCast(@as(u32, @bitCast(flags)));
    o.NONBLOCK = true;
    _ = libc.fcntl(fd, libc.F.SETFL, @as(c_int, @bitCast(@as(u32, @bitCast(o)))));
}

/// One blocking read, EINTR-safe. Used only for the INIT handshake, where the
/// descriptor is still blocking; every later read goes through `next()`.
fn readFull(fd: c_int, buf: []u8) usize {
    while (true) {
        const n = libc.read(fd, buf.ptr, buf.len);
        if (n < 0) {
            if (libc.errno(n) == .INTR) continue;
            return 0;
        }
        return @intCast(n);
    }
}

// ---------------------------------------------------------------------------
// tests
// ---------------------------------------------------------------------------
//
// No test here mounts anything: a real mount needs the setuid helper, a
// writable runtime directory and a kernel that will let go of it again, which
// is a snapshot test's job and not a unit test's. What is testable without a
// mount is everything that has ever actually been wrong in a FUSE server —
// struct sizes, dirent alignment, cookies, the INIT reply, the park table, and
// the argv handed to a setuid program. Those are what follows, driven through a
// socketpair standing in for /dev/fuse.

const testing = std.testing;

/// Build an `Fs` with no mount, wired to `fd`. The socketpair replaces
/// /dev/fuse for the codec tests: the kernel's side of the conversation is
/// written by hand and the reply is read back and compared byte for byte.
fn testFs(gpa: std.mem.Allocator, fd: c_int) !*Fs {
    const fs = try gpa.create(Fs);
    fs.* = .{
        .gpa = gpa,
        .fd = fd,
        .path = try gpa.dupeZ(u8, "/nonexistent"),
        .buf = try gpa.alignedAlloc(u8, .@"8", min_read_buffer),
        .uid = 1000,
        .gid = 1000,
        .max_write = 4096,
    };
    return fs;
}

fn testFsFree(fs: *Fs) void {
    const gpa = fs.gpa;
    gpa.free(fs.path);
    gpa.free(fs.buf);
    gpa.destroy(fs);
}

/// Frame a request the way the kernel does and push it at the server.
fn pushRequest(fd: c_int, unique: u64, op: Opcode, nodeid: u64, body: []const u8) !void {
    var buf: [4096]u8 align(8) = undefined;
    const h: fuse_in_header = .{
        .len = @intCast(@sizeOf(fuse_in_header) + body.len),
        .opcode = @intFromEnum(op),
        .unique = unique,
        .nodeid = nodeid,
        .uid = 1000,
        .gid = 1000,
        .pid = 1,
        .total_extlen = 0,
        .padding = 0,
    };
    @memcpy(buf[0..@sizeOf(fuse_in_header)], std.mem.asBytes(&h));
    @memcpy(buf[@sizeOf(fuse_in_header)..][0..body.len], body);
    const total = @sizeOf(fuse_in_header) + body.len;
    try testing.expectEqual(@as(isize, @intCast(total)), libc.write(fd, &buf, total));
}

/// Read one reply back off the socketpair.
fn readReply(fd: c_int, buf: []u8) ![]u8 {
    const n = libc.read(fd, buf.ptr, buf.len);
    try testing.expect(n >= @sizeOf(fuse_out_header));
    return buf[0..@intCast(n)];
}

fn outHeader(bytes: []const u8) fuse_out_header {
    var h: fuse_out_header = undefined;
    @memcpy(std.mem.asBytes(&h), bytes[0..@sizeOf(fuse_out_header)]);
    return h;
}

/// A socketpair standing in for /dev/fuse. SEQPACKET, not STREAM, and that is
/// the whole point: the kernel's character device hands over exactly one
/// request per read(2) and takes exactly one reply per write(2), and a stream
/// socket would coalesce three requests into one read and let a codec that
/// ignores `fuse_in_header.len` pass anyway.
///
/// Both ends non-blocking. The server's end so `next()` meets EAGAIN where it
/// would on the real descriptor; the kernel's end so a test can assert that
/// NOTHING was written — which is what "a held request has no reply" and "a
/// FORGET is never answered" mean, and a blocking read would simply hang there
/// instead of failing.
fn testPair() ![2]c_int {
    var sv: [2]c_int = undefined;
    if (libc.socketpair(libc.AF.UNIX, libc.SOCK.SEQPACKET, 0, &sv) != 0) return error.SocketPairFailed;
    setNonblock(sv[0]);
    setNonblock(sv[1]);
    return sv;
}

test "lookup round trip: parse borrows the name, reply frames an entry" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    try pushRequest(sv[1], 100, .lookup, 1, "index\x00");
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.lookup, req.op);
    try testing.expectEqual(@as(u64, 100), req.tag);
    try testing.expectEqual(@as(u64, 1), req.node);
    try testing.expectEqualStrings("index", req.data);
    // Drained, and nothing else was invented.
    try testing.expect(fs.next() == null);

    fs.reply(&.{
        .tag = 100,
        .attr = .{ .node = 7, .size = 42, .mode = 0o444 },
    }, &.{});

    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    const h = outHeader(got);
    try testing.expectEqual(@as(u32, @sizeOf(fuse_out_header) + @sizeOf(fuse_entry_out)), h.len);
    try testing.expectEqual(@as(u32, @intCast(got.len)), h.len);
    try testing.expectEqual(@as(i32, 0), h.@"error");
    try testing.expectEqual(@as(u64, 100), h.unique);

    var entry: fuse_entry_out = undefined;
    @memcpy(std.mem.asBytes(&entry), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_entry_out)]);
    try testing.expectEqual(@as(u64, 7), entry.nodeid);
    // Caching off in both directions, or `new/<name>` creates a pane once and
    // then serves the cached negative lookup forever.
    try testing.expectEqual(@as(u64, 0), entry.entry_valid);
    try testing.expectEqual(@as(u64, 0), entry.attr_valid);
    try testing.expectEqual(@as(u64, 7), entry.attr.ino);
    try testing.expectEqual(@as(u64, 42), entry.attr.size);
    try testing.expectEqual(S_IFREG | @as(u32, 0o444), entry.attr.mode);
    try testing.expectEqual(@as(u32, 1), entry.attr.nlink);
    try testing.expectEqual(@as(u32, 1000), entry.attr.uid);
    // The slot went back.
    try testing.expect(fs.freeSlot() != null);
    try testing.expectEqual(@as(?usize, null), fs.findSlot(100));
}

test "read reply is capped at the requested size and written as one frame" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const in: fuse_read_in = .{
        .fh = 3,
        .offset = 8,
        .size = 4,
        .read_flags = 0,
        .lock_owner = 0,
        .flags = 0,
        .padding = 0,
    };
    try pushRequest(sv[1], 200, .read, 5, std.mem.asBytes(&in));
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.read, req.op);
    try testing.expectEqual(@as(u32, 3), req.handle);
    try testing.expectEqual(@as(u64, 8), req.off);
    try testing.expectEqual(@as(u32, 4), req.size);

    // The core offers more than was asked for; a reply longer than `size` is
    // answered EIO by the kernel, so it has to be clamped here.
    fs.reply(&.{ .tag = 200, .payload = .{ .staged = 9 } }, "abcdefghi");
    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header) + 4), got.len);
    try testing.expectEqual(@as(u32, @intCast(got.len)), outHeader(got).len);
    try testing.expectEqualStrings("abcd", got[@sizeOf(fuse_out_header)..]);
}

test "an error reply is header only" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    try pushRequest(sv[1], 300, .lookup, 1, "nope\x00");
    _ = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = 300, .status = .err, .errno = @intFromEnum(libc.E.NOENT) }, &.{});

    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    // len MUST be exactly the header when error is set; anything else makes the
    // kernel answer -EINVAL and leaves the request pending forever.
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header)), got.len);
    const h = outHeader(got);
    try testing.expectEqual(@as(u32, @sizeOf(fuse_out_header)), h.len);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.NOENT)), h.@"error");
}

test "opcodes the core never sees are answered here" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);
    var buf: [512]u8 = undefined;

    // FORGET and BATCH_FORGET get NO reply, ever: the kernel keeps no pending
    // entry for them, so a reply would carry a unique it does not recognise.
    const forget: fuse_forget_in = .{ .nlookup = 1 };
    try pushRequest(sv[1], 400, .forget, 7, std.mem.asBytes(&forget));
    const batch: fuse_batch_forget_in = .{ .count = 0, .dummy = 0 };
    try pushRequest(sv[1], 402, .batch_forget, 0, std.mem.asBytes(&batch));
    // ...and a mutation is refused, which is the first thing that produces a
    // reply, proving nothing was written for the two above.
    try pushRequest(sv[1], 404, .mkdir, 1, "x\x00");
    try testing.expect(fs.next() == null);

    const got = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header)), got.len);
    const h = outHeader(got);
    try testing.expectEqual(@as(u64, 404), h.unique);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.NOSYS)), h.@"error");

    // DESTROY must be answered or umount hangs.
    try pushRequest(sv[1], 406, .destroy, 0, &.{});
    try testing.expect(fs.next() == null);
    const destroyed = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header)), destroyed.len);
    try testing.expectEqual(@as(i32, 0), outHeader(destroyed).@"error");
    try testing.expectEqual(@as(u64, 406), outHeader(destroyed).unique);

    // FLUSH is refused so the kernel stops sending one per close(2).
    fs.dead = false;
    const flush: fuse_flush_in = .{ .fh = 1, .unused = 0, .padding = 0, .lock_owner = 0 };
    try pushRequest(sv[1], 408, .flush, 1, std.mem.asBytes(&flush));
    try testing.expect(fs.next() == null);
    const flushed = try readReply(sv[1], &buf);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.NOSYS)), outHeader(flushed).@"error");
}

test "setattr size=0 is the truncate the kernel sends instead of O_TRUNC" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    var in: fuse_setattr_in = std.mem.zeroes(fuse_setattr_in);
    in.valid = FATTR_SIZE;
    in.size = 0;
    try pushRequest(sv[1], 500, .setattr, 9, std.mem.asBytes(&in));
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.setattr, req.op);
    try testing.expect(req.truncate);

    // A nonzero size is not a truncate this ABI can express, and must not be
    // reported as one: the core would clear a pane on `ftruncate(fd, 10)`.
    in.size = 10;
    try pushRequest(sv[1], 502, .setattr, 9, std.mem.asBytes(&in));
    fs.reply(&.{ .tag = 500, .attr = .{ .node = 9 } }, &.{});
    const req2 = fs.next() orelse return error.NoRequest;
    try testing.expect(!req2.truncate);

    fs.reply(&.{ .tag = 502, .attr = .{ .node = 9, .size = 3, .dir = true } }, &.{});
    var buf: [512]u8 = undefined;
    _ = try readReply(sv[1], &buf); // the first reply
    const got = try readReply(sv[1], &buf);
    var out: fuse_attr_out = undefined;
    @memcpy(std.mem.asBytes(&out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_attr_out)]);
    try testing.expectEqual(S_IFDIR | @as(u32, 0o600), out.attr.mode);
    try testing.expectEqual(@as(u32, 2), out.attr.nlink);
    try testing.expectEqual(@as(u64, 0), out.attr_valid);
}

test "open reports direct io for files and nothing for directories" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);
    var buf: [512]u8 = undefined;

    // O_WRONLY
    const wr: fuse_open_in = .{ .flags = 1, .open_flags = 0 };
    try pushRequest(sv[1], 600, .open, 4, std.mem.asBytes(&wr));
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.open, req.op);
    fs.reply(&.{ .tag = 600, .handle = 11 }, &.{});
    var got = try readReply(sv[1], &buf);
    var open_out: fuse_open_out = undefined;
    @memcpy(std.mem.asBytes(&open_out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_open_out)]);
    try testing.expectEqual(@as(u64, 11), open_out.fh);
    try testing.expectEqual(FOPEN_DIRECT_IO, open_out.open_flags);

    // O_RDONLY on a directory
    const rd: fuse_open_in = .{ .flags = 0, .open_flags = 0 };
    try pushRequest(sv[1], 602, .opendir, 1, std.mem.asBytes(&rd));
    const dir_req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.open, dir_req.op);
    fs.reply(&.{ .tag = 602, .handle = 12 }, &.{});
    got = try readReply(sv[1], &buf);
    @memcpy(std.mem.asBytes(&open_out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_open_out)]);
    // No FOPEN_CACHE_DIR either: the pane list changes between two `ls`.
    try testing.expectEqual(@as(u32, 0), open_out.open_flags);
}

test "write borrows the payload and reports the core's own count" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    var body: [@sizeOf(fuse_write_in) + 5]u8 = undefined;
    const in: fuse_write_in = .{
        .fh = 2,
        .offset = 0,
        .size = 5,
        .write_flags = 0,
        .lock_owner = 0,
        .flags = 0,
        .padding = 0,
    };
    @memcpy(body[0..@sizeOf(fuse_write_in)], std.mem.asBytes(&in));
    @memcpy(body[@sizeOf(fuse_write_in)..], "hello");
    try pushRequest(sv[1], 700, .write, 6, &body);
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.write, req.op);
    try testing.expectEqualStrings("hello", req.data);

    fs.reply(&.{ .tag = 700, .written = 5 }, &.{});
    var buf: [512]u8 = undefined;
    var got = try readReply(sv[1], &buf);
    var out: fuse_write_out = undefined;
    @memcpy(std.mem.asBytes(&out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_write_out)]);
    try testing.expectEqual(@as(u32, 5), out.size);

    // A short count is a real answer — `data` refusing a partial grapheme —
    // and must reach write(2) as a short write rather than as a full one.
    try pushRequest(sv[1], 704, .write, 6, &body);
    _ = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = 704, .written = 3 }, &.{});
    got = try readReply(sv[1], &buf);
    @memcpy(std.mem.asBytes(&out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_write_out)]);
    try testing.expectEqual(@as(u32, 3), out.size);

    // A count larger than what was offered would advance the file offset past
    // bytes that never existed.
    try pushRequest(sv[1], 706, .write, 6, &body);
    _ = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = 706, .written = 99 }, &.{});
    got = try readReply(sv[1], &buf);
    @memcpy(std.mem.asBytes(&out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_write_out)]);
    try testing.expectEqual(@as(u32, 5), out.size);
}

test "a write whose size lies about the payload is clamped to what arrived" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    var body: [@sizeOf(fuse_write_in) + 2]u8 = undefined;
    var in: fuse_write_in = std.mem.zeroes(fuse_write_in);
    in.size = 4096; // more than the two bytes that follow
    @memcpy(body[0..@sizeOf(fuse_write_in)], std.mem.asBytes(&in));
    @memcpy(body[@sizeOf(fuse_write_in)..], "hi");
    try pushRequest(sv[1], 702, .write, 6, &body);
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqualStrings("hi", req.data);
    try testing.expectEqual(@as(u32, 2), req.size);
}

test "dirent encoding: 8-byte records, cookies from the request offset" {
    var staged: [64]u8 = undefined;
    var w: usize = 0;
    // node=2 kind=file name="addr"
    std.mem.writeInt(u64, staged[w..][0..8], 2, .little);
    staged[w + 8] = 0;
    staged[w + 9] = 4;
    @memcpy(staged[w + 10 ..][0..4], "addr");
    w += 14;
    // node=3 kind=dir name="new"
    std.mem.writeInt(u64, staged[w..][0..8], 3, .little);
    staged[w + 8] = 1;
    staged[w + 9] = 3;
    @memcpy(staged[w + 10 ..][0..3], "new");
    w += 13;

    var out: [128]u8 = undefined;
    const n = encodeDirents(&out, staged[0..w], 5);
    // 24 + 4 -> 32; 24 + 3 -> 32. A record that is not a multiple of 8
    // desynchronises the kernel's parse of everything after it.
    try testing.expectEqual(@as(usize, 64), n);
    try testing.expectEqual(@as(u64, 0), n % rec_align);

    try testing.expectEqual(@as(u64, 2), std.mem.readInt(u64, out[0..8], .little));
    // Cookies continue from the request's offset: the kernel sends the last
    // `off` it saw as the next request's offset, so restarting at 1 would loop
    // the directory forever.
    try testing.expectEqual(@as(u64, 6), std.mem.readInt(u64, out[8..16], .little));
    try testing.expectEqual(@as(u32, 4), std.mem.readInt(u32, out[16..20], .little));
    try testing.expectEqual(DT_REG, std.mem.readInt(u32, out[20..24], .little));
    try testing.expectEqualStrings("addr", out[24..28]);
    // Padding zeroed, so the wire is deterministic.
    try testing.expectEqualSlices(u8, &.{ 0, 0, 0, 0 }, out[28..32]);

    try testing.expectEqual(@as(u64, 3), std.mem.readInt(u64, out[32..40], .little));
    try testing.expectEqual(@as(u64, 7), std.mem.readInt(u64, out[40..48], .little));
    try testing.expectEqual(DT_DIR, std.mem.readInt(u32, out[52..56], .little));
    try testing.expectEqualStrings("new", out[56..59]);
}

test "dirent encoding stops cleanly when the reply buffer or the staging runs out" {
    var staged: [64]u8 = undefined;
    std.mem.writeInt(u64, staged[0..8], 9, .little);
    staged[8] = 0;
    staged[9] = 4;
    @memcpy(staged[10..14], "body");
    std.mem.writeInt(u64, staged[14..22], 10, .little);
    staged[22] = 0;
    staged[23] = 4;
    @memcpy(staged[24..28], "ctl!");

    // Room for one record only: the second comes back at the higher cookie.
    var out: [40]u8 = undefined;
    try testing.expectEqual(@as(usize, 32), encodeDirents(&out, staged[0..28], 0));

    // A truncated staging record is dropped rather than guessed at.
    try testing.expectEqual(@as(usize, 32), encodeDirents(&out, staged[0..26], 0));
    // Zero staged bytes is EOF, not an error.
    try testing.expectEqual(@as(usize, 0), encodeDirents(&out, &.{}, 4));
    // A zero name length would make the kernel parse the padding as an entry.
    var bad: [10]u8 = @splat(0);
    try testing.expectEqual(@as(usize, 0), encodeDirents(&out, &bad, 0));
}

test "readdir reply carries encoded dirents built from the staged names" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const in: fuse_read_in = .{
        .fh = 1,
        .offset = 0,
        .size = 4096,
        .read_flags = 0,
        .lock_owner = 0,
        .flags = 0,
        .padding = 0,
    };
    try pushRequest(sv[1], 800, .readdir, 1, std.mem.asBytes(&in));
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.readdir, req.op);

    var staged: [16]u8 = undefined;
    std.mem.writeInt(u64, staged[0..8], 4, .little);
    staged[8] = 1;
    staged[9] = 5;
    @memcpy(staged[10..15], "panes");
    fs.reply(&.{ .tag = 800, .payload = .{ .staged = 15 } }, staged[0..15]);

    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header) + 32), got.len);
    const rec = got[@sizeOf(fuse_out_header)..];
    try testing.expectEqual(@as(u64, 4), std.mem.readInt(u64, rec[0..8], .little));
    try testing.expectEqual(@as(u64, 1), std.mem.readInt(u64, rec[8..16], .little));
    try testing.expectEqual(DT_DIR, std.mem.readInt(u32, rec[20..24], .little));
    try testing.expectEqualStrings("panes", rec[24..29]);
}

test "INIT reply negotiates nothing and caps the minor at ours" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);
    fs.max_write = 64 * 1024;

    const in: fuse_init_in = .{
        .major = 7,
        .minor = 45,
        .max_readahead = 131072,
        // Everything the kernel is willing to do. The point of the test is that
        // none of it comes back.
        .flags = 0xffff_ffff,
        .flags2 = 0xffff_ffff,
        .unused = @splat(0),
    };
    try pushRequest(sv[1], 1, .init, 0, std.mem.asBytes(&in));
    try fs.handshake();
    try testing.expectEqual(@as(u32, 45), fs.minor);

    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header) + @sizeOf(fuse_init_out)), got.len);
    try testing.expectEqual(@as(u64, 1), outHeader(got).unique);
    var out: fuse_init_out = undefined;
    @memcpy(std.mem.asBytes(&out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_init_out)]);
    try testing.expectEqual(@as(u32, 7), out.major);
    try testing.expectEqual(@as(u32, 45), out.minor);
    // The one assertion this test exists for. Every bit here is a kernel
    // behaviour we would owe forever: readdirplus whose ENOSYS has no fallback,
    // atomic O_TRUNC that would bypass the SETATTR the core handles, locks.
    try testing.expectEqual(@as(u32, 0), out.flags);
    try testing.expectEqual(@as(u32, 0), out.flags2);
    try testing.expectEqual(@as(u32, 0), out.max_readahead);
    try testing.expectEqual(@as(u32, 64 * 1024), out.max_write);
    // A time granularity of zero is not a legal value.
    try testing.expectEqual(@as(u32, 1), out.time_gran);
    try testing.expectEqual(@as(u16, 0), out.request_timeout);

    // A newer kernel's minor is CAPPED, not echoed. `fc->minor` is our own
    // declared level and it is what sizes the replies the kernel reads back
    // from us, so claiming 7.99 on these structs promises fields they do not
    // have. This assertion is the one the old `@min(in.minor, in.minor)` could
    // not make.
    const newer: fuse_init_in = .{
        .major = 7,
        .minor = kernel_minor + 54,
        .max_readahead = 0,
        .flags = 0,
        .flags2 = 0,
        .unused = @splat(0),
    };
    try pushRequest(sv[1], 2, .init, 0, std.mem.asBytes(&newer));
    try fs.handshake();
    const capped = try readReply(sv[1], &buf);
    @memcpy(std.mem.asBytes(&out), capped[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_init_out)]);
    try testing.expectEqual(kernel_minor, out.minor);

    // A foreign major is fatal, and answering it anyway only moves the failure
    // to the first syscall through the mount.
    const bad: fuse_init_in = .{
        .major = 8,
        .minor = 0,
        .max_readahead = 0,
        .flags = 0,
        .flags2 = 0,
        .unused = @splat(0),
    };
    try pushRequest(sv[1], 3, .init, 0, std.mem.asBytes(&bad));
    try testing.expectError(error.InitVersion, fs.handshake());
}

test "park table: again holds the request, retry offers it back once per round" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const in: fuse_read_in = .{
        .fh = 1,
        .offset = 0,
        .size = 64,
        .read_flags = 0,
        .lock_owner = 0,
        .flags = 0,
        .padding = 0,
    };
    // Two blocked readers of `event`, in arrival order.
    try pushRequest(sv[1], 900, .read, 20, std.mem.asBytes(&in));
    try pushRequest(sv[1], 902, .read, 21, std.mem.asBytes(&in));
    const a = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = a.tag, .status = .again }, &.{});
    const b = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = b.tag, .status = .again }, &.{});
    try testing.expect(fs.next() == null);
    // Nothing was written: a held request has no reply, which is the only way
    // FUSE expresses blocking.
    var buf: [512]u8 = undefined;
    try testing.expect(libc.read(sv[1], &buf, buf.len) < 0);

    // One round offers each parked request exactly once, oldest first, and then
    // ends. Without the per-round flag the oldest would be offered forever and
    // the second reader would never be looked at again.
    const r1 = fs.retry() orelse return error.NoRetry;
    try testing.expectEqual(@as(u64, 900), r1.tag);
    fs.reply(&.{ .tag = r1.tag, .status = .again }, &.{});
    const r2 = fs.retry() orelse return error.NoRetry;
    try testing.expectEqual(@as(u64, 902), r2.tag);
    fs.reply(&.{ .tag = r2.tag, .status = .again }, &.{});
    try testing.expect(fs.retry() == null);

    // ...and the next round starts over.
    const r3 = fs.retry() orelse return error.NoRetry;
    try testing.expectEqual(@as(u64, 900), r3.tag);
    fs.reply(&.{ .tag = r3.tag, .payload = .{ .staged = 3 } }, "ev\n");
    const got = try readReply(sv[1], &buf);
    try testing.expectEqualStrings("ev\n", got[@sizeOf(fuse_out_header)..]);
    // The answered one is gone; the other is still held.
    try testing.expectEqual(@as(?usize, null), fs.findSlot(900));
    try testing.expect(fs.findSlot(902) != null);
}

test "park table: interrupt answers the original with EINTR and drops it" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const in: fuse_read_in = .{
        .fh = 1,
        .offset = 0,
        .size = 64,
        .read_flags = 0,
        .lock_owner = 0,
        .flags = 0,
        .padding = 0,
    };
    try pushRequest(sv[1], 1000, .read, 20, std.mem.asBytes(&in));
    try pushRequest(sv[1], 1002, .read, 21, std.mem.asBytes(&in));
    const a = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = a.tag, .status = .again }, &.{});
    const b = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = b.tag, .status = .again }, &.{});

    // The kernel's interrupt names the ORIGINAL unique in its body; its own
    // unique is `original | 1`, which is why it must not be echoed.
    const intr: fuse_interrupt_in = .{ .unique = 1002 };
    try pushRequest(sv[1], 1002 | 1, .interrupt, 0, std.mem.asBytes(&intr));
    try testing.expect(fs.next() == null);

    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    // Exactly one reply, to the interrupted request, not to the interrupt.
    // Getting this wrong leaves a SIGKILLed reader in uninterruptible sleep.
    try testing.expectEqual(@as(usize, @sizeOf(fuse_out_header)), got.len);
    const h = outHeader(got);
    try testing.expectEqual(@as(u64, 1002), h.unique);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.INTR)), h.@"error");
    try testing.expectEqual(@as(?usize, null), fs.findSlot(1002));
    try testing.expect(fs.findSlot(1000) != null);

    // An interrupt for something we do not hold is ignored, not answered.
    const stale: fuse_interrupt_in = .{ .unique = 4242 };
    try pushRequest(sv[1], 4243, .interrupt, 0, std.mem.asBytes(&stale));
    try testing.expect(fs.next() == null);
    try testing.expect(libc.read(sv[1], &buf, buf.len) < 0);
}

test "park table: a full table answers EAGAIN and keeps the descriptor flowing" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const in: fuse_read_in = .{
        .fh = 1,
        .offset = 0,
        .size = 8,
        .read_flags = 0,
        .lock_owner = 0,
        .flags = 0,
        .padding = 0,
    };
    for (0..max_slots) |i| {
        try pushRequest(sv[1], 2000 + i * 2, .read, 30, std.mem.asBytes(&in));
        const req = fs.next() orelse return error.NoRequest;
        fs.reply(&.{ .tag = req.tag, .status = .again }, &.{});
    }
    var buf: [512]u8 = undefined;

    // One more than the table holds. It is READ and refused, not left queued.
    // Gating the read on a free slot is a deadlock dressed as backpressure:
    // the INTERRUPT that frees a slot would never be read either, so a
    // SIGKILLed reader would stay in uninterruptible sleep and every unrelated
    // `ls` of the mount would hang behind the 32 blocked ones. Measured: that
    // wedges a real mount.
    try pushRequest(sv[1], 9998, .read, 30, std.mem.asBytes(&in));
    try testing.expect(fs.next() == null);
    const refused = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(u64, 9998), outHeader(refused).unique);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.AGAIN)), outHeader(refused).@"error");

    // And the requests that need no slot keep being answered with the table
    // still full — DESTROY above all, since a missing reply to it hangs umount.
    try pushRequest(sv[1], 9990, .access, 1, &.{});
    try testing.expect(fs.next() == null);
    const nosys = try readReply(sv[1], &buf);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.NOSYS)), outHeader(nosys).@"error");

    // An interrupt still lands, which is what lets a full table recover at all.
    const intr: fuse_interrupt_in = .{ .unique = 2000 };
    try pushRequest(sv[1], 2001, .interrupt, 0, std.mem.asBytes(&intr));
    try testing.expect(fs.next() == null);
    const killed = try readReply(sv[1], &buf);
    try testing.expectEqual(@as(u64, 2000), outHeader(killed).unique);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.INTR)), outHeader(killed).@"error");

    // ...and the freed slot takes the next request.
    try pushRequest(sv[1], 9996, .read, 30, std.mem.asBytes(&in));
    const late = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(@as(u64, 9996), late.tag);
}

test "park table: a payload too large to copy is refused rather than dangled" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const payload_len = park_data_max + 1;
    var body: [@sizeOf(fuse_write_in) + payload_len]u8 = undefined;
    var in: fuse_write_in = std.mem.zeroes(fuse_write_in);
    in.size = payload_len;
    @memcpy(body[0..@sizeOf(fuse_write_in)], std.mem.asBytes(&in));
    @memset(body[@sizeOf(fuse_write_in)..], 'z');
    try pushRequest(sv[1], 3000, .write, 6, &body);
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(@as(usize, payload_len), req.data.len);

    // Parking this would park a slice of the read buffer, which the next
    // `next()` overwrites. EAGAIN is the honest answer.
    fs.reply(&.{ .tag = 3000, .status = .again }, &.{});
    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    try testing.expectEqual(-@as(i32, @intFromEnum(libc.E.AGAIN)), outHeader(got).@"error");
    try testing.expectEqual(@as(?usize, null), fs.findSlot(3000));

    // A payload that fits IS copied, so parking it is safe even after the read
    // buffer has been reused.
    var small: [@sizeOf(fuse_write_in) + 4]u8 = undefined;
    in.size = 4;
    @memcpy(small[0..@sizeOf(fuse_write_in)], std.mem.asBytes(&in));
    @memcpy(small[@sizeOf(fuse_write_in)..], "keep");
    try pushRequest(sv[1], 3002, .write, 6, &small);
    const kept = fs.next() orelse return error.NoRequest;
    fs.reply(&.{ .tag = kept.tag, .status = .again }, &.{});
    // Something else lands in the read buffer...
    try pushRequest(sv[1], 3004, .statfs, 1, &.{});
    _ = fs.next() orelse return error.NoRequest;
    // ...and the parked bytes survived it.
    const again = fs.retry() orelse return error.NoRetry;
    try testing.expectEqualStrings("keep", again.data);
}

test "a reply for a tag we no longer hold is dropped, not written" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    // The interrupt path already answered and freed this one; a second reply
    // would carry a unique the kernel does not recognise, and could in
    // principle be matched against a live request that reused the number.
    fs.reply(&.{ .tag = 12345 }, &.{});
    var buf: [512]u8 = undefined;
    try testing.expect(libc.read(sv[1], &buf, buf.len) < 0);
}

test "statfs reports a usable namelen" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    try pushRequest(sv[1], 1100, .statfs, 1, &.{});
    const req = fs.next() orelse return error.NoRequest;
    try testing.expectEqual(acmefs.Op.statfs, req.op);
    fs.reply(&.{ .tag = 1100 }, &.{});

    var buf: [512]u8 = undefined;
    const got = try readReply(sv[1], &buf);
    var out: fuse_statfs_out = undefined;
    @memcpy(std.mem.asBytes(&out), got[@sizeOf(fuse_out_header)..][0..@sizeOf(fuse_statfs_out)]);
    // Zero here makes pathconf(_PC_NAME_MAX) return 0 and some tools then
    // refuse to create any name at all.
    try testing.expectEqual(@as(u32, 255), out.st.namelen);
    try testing.expectEqual(@as(u32, 4096), out.st.bsize);
}

test "the fusermount command line and environment" {
    var opts_buf: [128:0]u8 = undefined;
    const opts = mountOpts(&opts_buf);
    try testing.expectEqualStrings("fsname=pardes,subtype=pardes,nosuid,nodev", opts);
    // allow_other needs user_allow_other in /etc/fuse.conf, which is commented
    // out on a stock install, and asking for it FAILS the whole mount rather
    // than being ignored. default_permissions would move access control out of
    // the core and into a mode nibble.
    try testing.expect(std.mem.indexOf(u8, opts, "allow_other") == null);
    try testing.expect(std.mem.indexOf(u8, opts, "default_permissions") == null);

    var env_buf: [32:0]u8 = undefined;
    try testing.expectEqualStrings("_FUSE_COMMFD=7", commfdEnv(&env_buf, 7));

    var argv: [6:null]?[*:0]const u8 = undefined;
    mountArgv(&argv, "/usr/bin/fusermount3", opts.ptr, "/run/user/1000/pardes/42");
    try testing.expectEqualStrings("/usr/bin/fusermount3", std.mem.span(argv[0].?));
    try testing.expectEqualStrings("-o", std.mem.span(argv[1].?));
    try testing.expectEqualStrings("fsname=pardes,subtype=pardes,nosuid,nodev", std.mem.span(argv[2].?));
    // Without the `--` a mountpoint beginning with a dash is parsed as a flag
    // by a setuid program.
    try testing.expectEqualStrings("--", std.mem.span(argv[3].?));
    try testing.expectEqualStrings("/run/user/1000/pardes/42", std.mem.span(argv[4].?));
    try testing.expectEqual(@as(?[*:0]const u8, null), argv[5]);

    var uargv: [7:null]?[*:0]const u8 = undefined;
    unmountArgv(&uargv, "/usr/bin/fusermount3", "/run/user/1000/pardes/42");
    try testing.expectEqualStrings("-u", std.mem.span(uargv[1].?));
    try testing.expectEqualStrings("-q", std.mem.span(uargv[2].?));
    // Lazy, or a pane shell with a cwd inside the mount makes the unmount fail
    // with EBUSY and the mount outlives the editor.
    try testing.expectEqualStrings("-z", std.mem.span(uargv[3].?));
    try testing.expectEqualStrings("--", std.mem.span(uargv[4].?));
    try testing.expectEqual(@as(?[*:0]const u8, null), uargv[6]);
}

test "the child environment drops an inherited comm descriptor" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    var buf: [32:0]u8 = undefined;
    const commfd = commfdEnv(&buf, 5);
    const env = try buildEnv(gpa, commfd);
    defer gpa.free(env);

    // Exactly one _FUSE_COMMFD, and it is ours: getenv returns the FIRST match,
    // so an inherited stale entry would win and fusermount3 would send the
    // descriptor to a closed socket.
    var seen: usize = 0;
    var i: usize = 0;
    while (env[i]) |entry| : (i += 1) {
        if (std.mem.startsWith(u8, std.mem.span(entry), commfd_env ++ "=")) {
            seen += 1;
            try testing.expectEqualStrings("_FUSE_COMMFD=5", std.mem.span(entry));
        }
    }
    try testing.expectEqual(@as(usize, 1), seen);
    try testing.expectEqual(@as(?[*:0]const u8, null), env[env.len - 1]);
}

test "poll thread: one wake per drained batch, and stop joins from either state" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    var wakes: std.atomic.Value(u32) = .init(0);
    const Sink = struct {
        fn wake(ctx: ?*anyopaque) void {
            const c: *std.atomic.Value(u32) = @ptrCast(@alignCast(ctx.?));
            _ = c.fetchAdd(1, .release);
        }
    };
    try fs.wakeThread(&wakes, Sink.wake);

    // One pending request, one wake. A FORGET is answered inside `next()` and
    // never surfaces, so draining to null is the whole batch — and it is that
    // null which raises the eventfd and lets the poller poll again.
    const forget: fuse_forget_in = .{ .nlookup = 1 };
    try pushRequest(sv[1], 7000, .forget, 2, std.mem.asBytes(&forget));
    while (wakes.load(.acquire) == 0) std.Thread.yield() catch {};
    try testing.expectEqual(@as(?acmefs.Req, null), fs.next());

    // The poller is now in one of the two states a stop has to break: still in
    // the blocking wait, or back in `poll()` because the drain above beat the
    // stop there. Which one is a race, deliberately unresolved — the assertion
    // is that either joins, and a hang here is this test's only failure mode.
    fs.stopThread();
    try testing.expect(fs.thread == null);
    try testing.expectEqual(@as(c_int, -1), fs.ctl);
}

test "poll thread: stop breaks a poller that never saw a request" {
    if (comptime !supported) return;
    const gpa = testing.allocator;
    const sv = try testPair();
    defer {
        _ = libc.close(sv[0]);
        _ = libc.close(sv[1]);
    }
    const fs = try testFs(gpa, sv[0]);
    defer testFsFree(fs);

    const Sink = struct {
        fn wake(_: ?*anyopaque) void {
            unreachable; // nothing is ever pending on this descriptor
        }
    };
    try fs.wakeThread(null, Sink.wake);
    // Covers the two states with no acknowledgement in them at all: blocked in
    // `poll()` with an idle descriptor, and not yet past the loop condition.
    fs.stopThread();
    try testing.expect(fs.thread == null);
}

test "mount refuses a relative point" {
    if (comptime !supported) return;
    try testing.expectError(
        error.MountPathNotAbsolute,
        Fs.mount(testing.allocator, .{ .mount = "relative/dir" }),
    );
}