path: root/9ns/src/fuse.zig

//! Kernel FUSE protocol subset (no libfuse, no libc, no policy).
//!
//! Extern structs mirror `/usr/include/linux/fuse.h` (kernel header 7.45);
//! every layout is checked against the header's size at comptime. Only the
//! opcodes and structs 9ns needs are here. The I/O helpers are blocking
//! and allocation-free: the caller owns a single request buffer.
//!
//! Wire rules worth remembering:
//! * The kernel delivers exactly one request per `read(2)` on `/dev/fuse`,
//!   and a reply must be exactly one `write(2)`/`writev(2)`.
//! * Request bodies start right after the 40-byte `InHeader`; since every
//!   in-struct is 8-byte aligned in the header, `body()` requires the caller's
//!   buffer to be 8-byte aligned (`std.heap` page allocations and
//!   `align(8)` arrays both qualify).
//! * A write that fails with `ENOENT` means the request was interrupted and
//!   the kernel already forgot it: the reply is silently dropped.
//! * `ENODEV` on read means the filesystem was unmounted.

const std = @import("std");
const linux = std.os.linux;

pub const kernel_version: u32 = 7;
/// The minor we answer; the kernel adapts to the lower of the two.
pub const kernel_minor: u32 = 31;
pub const root_id: u64 = 1;

pub const FOPEN_DIRECT_IO: u32 = 1 << 0;
pub const FOPEN_KEEP_CACHE: u32 = 1 << 1;
pub const FOPEN_NONSEEKABLE: u32 = 1 << 2;

pub const FUSE_ASYNC_READ: u32 = 1 << 0;
/// The kernel passes O_TRUNC in OPEN instead of a separate SETATTR(size=0); 9P has OTRUNC for exactly this.
pub const FUSE_ATOMIC_O_TRUNC: u32 = 1 << 3;
/// Without this the kernel's cached-write path (`--no-direct-io`) sends one 4 KiB WRITE per page.
pub const FUSE_BIG_WRITES: u32 = 1 << 5;
/// Re-fetch a cached inode's size/mtime and drop stale pages when they change.
/// Required for `--no-direct-io` correctness: 9P sizes change under us, and
/// without this the kernel trusts a stale cached size and truncates reads.
pub const FUSE_AUTO_INVAL_DATA: u32 = 1 << 12;
pub const FUSE_MAX_PAGES: u32 = 1 << 22;

pub const FATTR_MODE: u32 = 1 << 0;
pub const FATTR_UID: u32 = 1 << 1;
pub const FATTR_GID: u32 = 1 << 2;
pub const FATTR_SIZE: u32 = 1 << 3;
pub const FATTR_ATIME: u32 = 1 << 4;
pub const FATTR_MTIME: u32 = 1 << 5;
pub const FATTR_FH: u32 = 1 << 6;
pub const FATTR_ATIME_NOW: u32 = 1 << 7;
pub const FATTR_MTIME_NOW: u32 = 1 << 8;
pub const FATTR_LOCKOWNER: u32 = 1 << 9;
pub const FATTR_CTIME: u32 = 1 << 10;

/// File type bits for `Attr.mode` and `Dirent.type` (used by the bridge).
pub const S_IFDIR: u32 = linux.S.IFDIR;
pub const S_IFREG: u32 = linux.S.IFREG;
pub const DT_DIR: u32 = linux.DT.DIR;
pub const DT_REG: u32 = linux.DT.REG;

pub 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,
    _,
};

// ---------------------------------------------------------------------------
// Structs (field order and widths follow linux/fuse.h exactly)
// ---------------------------------------------------------------------------

pub const InHeader = extern struct {
    len: u32,
    opcode: u32,
    unique: u64,
    nodeid: u64,
    uid: u32,
    gid: u32,
    pid: u32,
    total_extlen: u16,
    padding: u16,

    pub fn op(h: InHeader) Opcode {
        return @enumFromInt(h.opcode);
    }
};

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

pub const Attr = extern struct {
    ino: u64 = 0,
    size: u64 = 0,
    blocks: u64 = 0,
    atime: u64 = 0,
    mtime: u64 = 0,
    ctime: u64 = 0,
    atimensec: u32 = 0,
    mtimensec: u32 = 0,
    ctimensec: u32 = 0,
    mode: u32 = 0,
    nlink: u32 = 0,
    uid: u32 = 0,
    gid: u32 = 0,
    rdev: u32 = 0,
    blksize: u32 = 0,
    flags: u32 = 0,
};

pub const EntryOut = extern struct {
    nodeid: u64 = 0,
    generation: u64 = 0,
    entry_valid: u64 = 0,
    attr_valid: u64 = 0,
    entry_valid_nsec: u32 = 0,
    attr_valid_nsec: u32 = 0,
    attr: Attr = .{},
};

pub const AttrOut = extern struct {
    attr_valid: u64 = 0,
    attr_valid_nsec: u32 = 0,
    dummy: u32 = 0,
    attr: Attr = .{},
};

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

pub const SetattrIn = 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,
};

pub const OpenIn = extern struct { flags: u32, open_flags: u32 };
pub const OpenOut = extern struct { fh: u64 = 0, open_flags: u32 = 0, backing_id: i32 = 0 };
pub const ReleaseIn = extern struct { fh: u64, flags: u32, release_flags: u32, lock_owner: u64 };
pub const FlushIn = extern struct { fh: u64, unused: u32, padding: u32, lock_owner: u64 };

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

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

pub const WriteOut = extern struct { size: u32, padding: u32 = 0 };
pub const CreateIn = extern struct { flags: u32, mode: u32, umask: u32, open_flags: u32 };
pub const MkdirIn = extern struct { mode: u32, umask: u32 };
pub const RenameIn = extern struct { newdir: u64 };
pub const Rename2In = extern struct { newdir: u64, flags: u32, padding: u32 };
pub const ForgetIn = extern struct { nlookup: u64 };
pub const BatchForgetIn = extern struct { count: u32, dummy: u32 };
pub const ForgetOne = extern struct { nodeid: u64, nlookup: u64 };
pub const FsyncIn = extern struct { fh: u64, fsync_flags: u32, padding: u32 };
pub const AccessIn = extern struct { mask: u32, padding: u32 };
pub const InterruptIn = extern struct { unique: u64 };
pub const LseekIn = extern struct { fh: u64, offset: u64, whence: u32, padding: u32 };

pub const Kstatfs = extern struct {
    blocks: u64 = 0,
    bfree: u64 = 0,
    bavail: u64 = 0,
    files: u64 = 0,
    ffree: u64 = 0,
    bsize: u32 = 0,
    namelen: u32 = 0,
    frsize: u32 = 0,
    padding: u32 = 0,
    spare: [6]u32 = [_]u32{0} ** 6,
};

pub const StatfsOut = extern struct { st: Kstatfs = .{} };

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

/// 64 bytes; the kernel accepts this size whenever the answered minor >= 23.
pub const InitOut = extern struct {
    major: u32 = kernel_version,
    minor: u32 = kernel_minor,
    max_readahead: u32 = 0,
    flags: u32 = 0,
    max_background: u16 = 0,
    congestion_threshold: u16 = 0,
    max_write: u32 = 0,
    time_gran: u32 = 0,
    max_pages: u16 = 0,
    map_alignment: u16 = 0,
    flags2: u32 = 0,
    max_stack_depth: u32 = 0,
    request_timeout: u16 = 0,
    unused: [11]u16 = [_]u16{0} ** 11,
};

/// Fixed 24-byte head of `fuse_dirent`; the name follows, padded to 8 bytes.
pub const Dirent = extern struct { ino: u64, off: u64, namelen: u32, type: u32 };

comptime {
    std.debug.assert(@sizeOf(InHeader) == 40);
    std.debug.assert(@sizeOf(OutHeader) == 16);
    std.debug.assert(@sizeOf(Attr) == 88);
    std.debug.assert(@sizeOf(EntryOut) == 128);
    std.debug.assert(@sizeOf(AttrOut) == 104);
    std.debug.assert(@sizeOf(GetattrIn) == 16);
    std.debug.assert(@sizeOf(SetattrIn) == 88);
    std.debug.assert(@sizeOf(OpenIn) == 8);
    std.debug.assert(@sizeOf(OpenOut) == 16);
    std.debug.assert(@sizeOf(ReleaseIn) == 24);
    std.debug.assert(@sizeOf(FlushIn) == 24);
    std.debug.assert(@sizeOf(ReadIn) == 40);
    std.debug.assert(@sizeOf(WriteIn) == 40);
    std.debug.assert(@sizeOf(WriteOut) == 8);
    std.debug.assert(@sizeOf(CreateIn) == 16);
    std.debug.assert(@sizeOf(MkdirIn) == 8);
    std.debug.assert(@sizeOf(RenameIn) == 8);
    std.debug.assert(@sizeOf(Rename2In) == 16);
    std.debug.assert(@sizeOf(ForgetIn) == 8);
    std.debug.assert(@sizeOf(BatchForgetIn) == 8);
    std.debug.assert(@sizeOf(ForgetOne) == 16);
    std.debug.assert(@sizeOf(FsyncIn) == 16);
    std.debug.assert(@sizeOf(AccessIn) == 8);
    std.debug.assert(@sizeOf(InterruptIn) == 8);
    std.debug.assert(@sizeOf(Kstatfs) == 80);
    std.debug.assert(@sizeOf(StatfsOut) == 80);
    std.debug.assert(@sizeOf(InitIn) == 64);
    std.debug.assert(@sizeOf(InitOut) == 64);
    std.debug.assert(@sizeOf(Dirent) == 24);
    std.debug.assert(@sizeOf(LseekIn) == 24);
}

// ---------------------------------------------------------------------------
// Request / reply helpers
// ---------------------------------------------------------------------------

pub const Error = error{ Protocol, Io, TooManyPayloads, Retry };

pub const Request = struct {
    header: InHeader,
    /// Bytes after the header; a slice into the caller's buffer.
    body: []const u8,
};

/// Reads one kernel request with a single `read(2)`. Returns null on ENODEV
/// (unmounted). Retries EINTR/EAGAIN/ENOENT. `buf` should be at least
/// `max_write + 4096` bytes and 8-byte aligned so `body()` can view it.
pub fn readRequest(fd: i32, buf: []u8) Error!?Request {
    while (true) {
        return readRequestOnce(fd, buf) catch |e| switch (e) {
            error.Retry => continue,
            else => return e,
        };
    }
}

/// One `read(2)` attempt: like `readRequest` but EINTR/EAGAIN/ENOENT surface as
/// `error.Retry` instead of being retried, so a caller that only reads after
/// `poll` (or on a non-blocking fd) never blocks in here.
pub fn readRequestOnce(fd: i32, buf: []u8) Error!?Request {
    const rc = linux.read(fd, buf.ptr, buf.len);
    switch (linux.errno(rc)) {
        .SUCCESS => {
            const n: usize = rc;
            if (n < @sizeOf(InHeader)) return error.Protocol;
            const header = std.mem.bytesToValue(InHeader, buf[0..@sizeOf(InHeader)]);
            if (header.len != n) return error.Protocol;
            return .{ .header = header, .body = buf[@sizeOf(InHeader)..n] };
        },
        .INTR, .AGAIN, .NOENT => return error.Retry,
        .NODEV => return null,
        else => return error.Io,
    }
}

/// Sets O_NONBLOCK on `fd` so that a read after `poll` cannot block when the
/// kernel withdrew the request in between (a killed waiter, for instance).
pub fn setNonblocking(fd: i32) Error!void {
    const cur = linux.fcntl(fd, linux.F.GETFL, 0);
    if (linux.errno(cur) != .SUCCESS) return error.Io;
    const nonblock: u32 = @bitCast(linux.O{ .NONBLOCK = true });
    const rc = linux.fcntl(fd, linux.F.SETFL, @as(usize, cur) | nonblock);
    if (linux.errno(rc) != .SUCCESS) return error.Io;
}

/// Maximum number of payload slices a single `reply` can carry.
pub const max_payloads = 7;

/// Success reply: `OutHeader` followed by the concatenated `payloads`, sent in
/// one `writev(2)`. An ENOENT from the kernel means the request was
/// interrupted; the reply is dropped and this returns normally.
pub fn reply(fd: i32, unique: u64, payloads: []const []const u8) Error!void {
    if (payloads.len > max_payloads) return error.TooManyPayloads;
    var total: usize = @sizeOf(OutHeader);
    for (payloads) |p| total += p.len;
    if (total > std.math.maxInt(u32)) return error.Protocol;
    const header = OutHeader{ .len = @intCast(total), .@"error" = 0, .unique = unique };
    var iov: [max_payloads + 1]std.posix.iovec_const = undefined;
    iov[0] = .{ .base = @ptrCast(&header), .len = @sizeOf(OutHeader) };
    for (payloads, 1..) |p, i| iov[i] = .{ .base = p.ptr, .len = p.len };
    return writeAll(fd, &iov, payloads.len + 1, total);
}

/// Error reply: an `OutHeader` carrying `-errno` and no payload.
pub fn replyError(fd: i32, unique: u64, err: linux.E) Error!void {
    const code: i32 = @intCast(@intFromEnum(err));
    const header = OutHeader{ .len = @sizeOf(OutHeader), .@"error" = -code, .unique = unique };
    var iov = [_]std.posix.iovec_const{.{ .base = @ptrCast(&header), .len = @sizeOf(OutHeader) }};
    return writeAll(fd, &iov, 1, @sizeOf(OutHeader));
}

fn writeAll(fd: i32, iov: [*]const std.posix.iovec_const, count: usize, total: usize) Error!void {
    while (true) {
        const rc = linux.writev(fd, iov, count);
        switch (linux.errno(rc)) {
            .SUCCESS => return if (rc == total) {} else error.Protocol,
            .INTR => continue,
            .NOENT => return, // request was interrupted; reply dropped
            else => return error.Io,
        }
    }
}

/// Appends a `fuse_dirent` (head + name, padded to a multiple of 8) at
/// `buf[used.*..]`. Returns false and leaves `buf`/`used` unchanged if the
/// record does not fit.
pub fn addDirent(buf: []u8, used: *usize, ino: u64, off: u64, dtype: u32, name: []const u8) bool {
    const raw = @sizeOf(Dirent) + name.len;
    const rec = (raw + 7) & ~@as(usize, 7);
    if (used.* > buf.len or buf.len - used.* < rec) return false;
    const dst = buf[used.*..][0..rec];
    const head = Dirent{ .ino = ino, .off = off, .namelen = @intCast(name.len), .type = dtype };
    @memcpy(dst[0..@sizeOf(Dirent)], std.mem.asBytes(&head));
    @memcpy(dst[@sizeOf(Dirent)..raw], name);
    @memset(dst[raw..rec], 0);
    used.* += rec;
    return true;
}

/// Views the first `@sizeOf(T)` bytes of `req.body` as `T` (copy-free).
/// Fails with `error.Protocol` if the body is too short or misaligned.
pub fn body(comptime T: type, req: Request) Error!*const T {
    if (req.body.len < @sizeOf(T)) return error.Protocol;
    if (@intFromPtr(req.body.ptr) % @alignOf(T) != 0) return error.Protocol;
    return @ptrCast(@alignCast(req.body.ptr));
}

/// The NUL-terminated string at `req.body[offset..]`, without the NUL.
pub fn nameAt(req: Request, offset: usize) Error![]const u8 {
    if (offset > req.body.len) return error.Protocol;
    const rest = req.body[offset..];
    const end = std.mem.indexOfScalar(u8, rest, 0) orelse return error.Protocol;
    return rest[0..end];
}

/// The NUL-terminated string following a `T` body (or at offset 0 when
/// `T == void`), e.g. LOOKUP's name (`void`) or MKDIR's name (`MkdirIn`).
pub fn nameAfter(comptime T: type, req: Request) Error![]const u8 {
    const offset = if (T == void) 0 else @sizeOf(T);
    return nameAt(req, offset);
}

/// The string that follows `first` (obtained via `nameAt(req, offset)`),
/// for "old\0new\0" pairs such as RENAME's.
pub fn secondName(req: Request, first: []const u8, offset: usize) Error![]const u8 {
    return nameAt(req, offset + first.len + 1);
}

/// Builds the INIT reply per docs/DESIGN.md.
pub fn initReply(in: *const InitIn, max_write: u32) InitOut {
    var out = InitOut{
        .major = kernel_version,
        .minor = @min(kernel_minor, in.minor),
        .max_readahead = in.max_readahead,
        .flags = FUSE_ASYNC_READ | FUSE_ATOMIC_O_TRUNC | FUSE_BIG_WRITES | FUSE_AUTO_INVAL_DATA,
        .max_background = 16,
        .congestion_threshold = 12,
        .max_write = max_write,
        .time_gran = 1,
    };
    if (in.flags & FUSE_MAX_PAGES != 0) {
        out.flags |= FUSE_MAX_PAGES;
        out.max_pages = 256;
    }
    return out;
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

const testing = std.testing;

test "struct sizes match linux/fuse.h" {
    // The comptime block above is the real check; this makes it run under
    // `zig test` even if the module is otherwise unreferenced.
    try testing.expectEqual(@as(usize, 40), @sizeOf(InHeader));
    try testing.expectEqual(@as(usize, 64), @sizeOf(InitOut));
    try testing.expectEqual(@as(usize, 24), @sizeOf(Dirent));
    try testing.expectEqual(@as(u32, 26), @intFromEnum(Opcode.init));
    try testing.expectEqual(Opcode.statx, @as(Opcode, @enumFromInt(52)));
}

test "addDirent pads records to 8 bytes and refuses when full" {
    var buf: [1024]u8 = undefined;
    var used: usize = 0;
    const name = "abcdefghijklmnopq"; // 17 chars
    var expect_total: usize = 0;
    var n: usize = 1;
    while (n <= 17) : (n += 1) {
        const before = used;
        try testing.expect(addDirent(&buf, &used, n, n, DT_REG, name[0..n]));
        const rec = used - before;
        try testing.expectEqual(@as(usize, 0), rec % 8);
        try testing.expectEqual((24 + n + 7) & ~@as(usize, 7), rec);
        // check head fields and NUL padding
        const head = std.mem.bytesToValue(Dirent, buf[before..][0..24]);
        try testing.expectEqual(n, head.ino);
        try testing.expectEqual(@as(u32, @intCast(n)), head.namelen);
        try testing.expectEqualStrings(name[0..n], buf[before + 24 ..][0..n]);
        for (buf[before + 24 + n .. used]) |b| try testing.expectEqual(@as(u8, 0), b);
        expect_total += rec;
    }
    try testing.expectEqual(expect_total, used);

    // A record that does not fit leaves everything untouched.
    var small: [40]u8 = undefined;
    var used2: usize = 0;
    try testing.expect(addDirent(&small, &used2, 1, 1, DT_DIR, "0123456789abcdef")); // 24+16 = 40
    try testing.expectEqual(@as(usize, 40), used2);
    try testing.expect(!addDirent(&small, &used2, 2, 2, DT_DIR, "x"));
    try testing.expectEqual(@as(usize, 40), used2);
    var tight: [31]u8 = undefined;
    var used3: usize = 0;
    try testing.expect(!addDirent(&tight, &used3, 1, 1, DT_REG, "a")); // needs 32
    try testing.expectEqual(@as(usize, 0), used3);
}

test "body/nameAfter/secondName on hand-built requests" {
    var buf: [128]u8 align(8) = undefined;
    // LOOKUP(parent=1, "hello")
    const name = "hello";
    const hdr = InHeader{
        .len = @intCast(@sizeOf(InHeader) + name.len + 1),
        .opcode = @intFromEnum(Opcode.lookup),
        .unique = 7,
        .nodeid = root_id,
        .uid = 1000,
        .gid = 1000,
        .pid = 42,
        .total_extlen = 0,
        .padding = 0,
    };
    @memcpy(buf[0..40], std.mem.asBytes(&hdr));
    @memcpy(buf[40..45], name);
    buf[45] = 0;
    const req = Request{ .header = hdr, .body = buf[40..hdr.len] };
    try testing.expectEqual(Opcode.lookup, req.header.op());
    try testing.expectEqualStrings("hello", try nameAfter(void, req));
    try testing.expectError(error.Protocol, body(MkdirIn, Request{ .header = hdr, .body = buf[40..44] }));

    // MKDIR(mode=0o755) + "dir"
    const mk = MkdirIn{ .mode = 0o755, .umask = 0o22 };
    @memcpy(buf[40..48], std.mem.asBytes(&mk));
    @memcpy(buf[48..51], "dir");
    buf[51] = 0;
    const mreq = Request{ .header = hdr, .body = buf[40..52] };
    const got = try body(MkdirIn, mreq);
    try testing.expectEqual(@as(u32, 0o755), got.mode);
    try testing.expectEqualStrings("dir", try nameAfter(MkdirIn, mreq));

    // RENAME(newdir) + "old\0new\0"
    const rn = RenameIn{ .newdir = 9 };
    @memcpy(buf[40..48], std.mem.asBytes(&rn));
    @memcpy(buf[48..56], "old\x00new\x00");
    const rreq = Request{ .header = hdr, .body = buf[40..56] };
    try testing.expectEqual(@as(u64, 9), (try body(RenameIn, rreq)).newdir);
    const old = try nameAfter(RenameIn, rreq);
    try testing.expectEqualStrings("old", old);
    try testing.expectEqualStrings("new", try secondName(rreq, old, @sizeOf(RenameIn)));
    try testing.expectError(error.Protocol, secondName(rreq, "new", @sizeOf(RenameIn) + 4));

    // Missing NUL and misalignment are protocol errors.
    try testing.expectError(error.Protocol, nameAt(Request{ .header = hdr, .body = buf[48..51] }, 0));
    try testing.expectError(error.Protocol, body(MkdirIn, Request{ .header = hdr, .body = buf[41..57] }));
}

test "initReply fields" {
    var in = InitIn{ .major = 7, .minor = 45, .max_readahead = 131072, .flags = 0, .flags2 = 0, .unused = [_]u32{0} ** 11 };
    const a = initReply(&in, 1 << 20);
    try testing.expectEqual(@as(u32, 7), a.major);
    try testing.expectEqual(@as(u32, 31), a.minor);
    try testing.expectEqual(@as(u32, 131072), a.max_readahead);
    try testing.expectEqual(FUSE_ASYNC_READ | FUSE_ATOMIC_O_TRUNC | FUSE_BIG_WRITES | FUSE_AUTO_INVAL_DATA, a.flags);
    try testing.expectEqual(@as(u16, 0), a.max_pages);
    try testing.expectEqual(@as(u16, 16), a.max_background);
    try testing.expectEqual(@as(u16, 12), a.congestion_threshold);
    try testing.expectEqual(@as(u32, 1 << 20), a.max_write);
    try testing.expectEqual(@as(u32, 1), a.time_gran);

    in.flags = FUSE_MAX_PAGES | FUSE_ASYNC_READ;
    in.minor = 27;
    const b = initReply(&in, 4096);
    try testing.expectEqual(@as(u32, 27), b.minor);
    try testing.expectEqual(FUSE_ASYNC_READ | FUSE_ATOMIC_O_TRUNC | FUSE_BIG_WRITES | FUSE_AUTO_INVAL_DATA | FUSE_MAX_PAGES, b.flags);
    try testing.expectEqual(@as(u16, 256), b.max_pages);
    try testing.expectEqual(@as(u32, 4096), b.max_write);
}

fn makePipe() ![2]i32 {
    var fds: [2]i32 = undefined;
    if (linux.errno(linux.pipe2(&fds, .{ .CLOEXEC = true })) != .SUCCESS) return error.Io;
    return fds;
}

fn readExact(fd: i32, out: []u8) !void {
    var got: usize = 0;
    while (got < out.len) {
        const rc = linux.read(fd, out[got..].ptr, out.len - got);
        if (linux.errno(rc) != .SUCCESS or rc == 0) return error.Io;
        got += rc;
    }
}

test "reply writes header + payloads through a pipe" {
    const fds = try makePipe();
    defer _ = linux.close(fds[0]);
    defer _ = linux.close(fds[1]);

    const oo = OpenOut{ .fh = 0x1234, .open_flags = FOPEN_DIRECT_IO };
    try reply(fds[1], 99, &.{ std.mem.asBytes(&oo), "tail" });

    var out: [16 + 16 + 4]u8 = undefined;
    try readExact(fds[0], &out);
    const h = std.mem.bytesToValue(OutHeader, out[0..16]);
    try testing.expectEqual(@as(u32, 36), h.len);
    try testing.expectEqual(@as(i32, 0), h.@"error");
    try testing.expectEqual(@as(u64, 99), h.unique);
    try testing.expectEqualSlices(u8, std.mem.asBytes(&oo), out[16..32]);
    try testing.expectEqualStrings("tail", out[32..36]);

    // Empty payload list: header only.
    try reply(fds[1], 5, &.{});
    var only: [16]u8 = undefined;
    try readExact(fds[0], &only);
    try testing.expectEqual(@as(u32, 16), std.mem.bytesToValue(OutHeader, &only).len);

    var too_many: [max_payloads + 1][]const u8 = undefined;
    for (&too_many) |*p| p.* = "x";
    try testing.expectError(error.TooManyPayloads, reply(fds[1], 1, &too_many));
}

test "replyError writes a negative errno" {
    const fds = try makePipe();
    defer _ = linux.close(fds[0]);
    defer _ = linux.close(fds[1]);

    try replyError(fds[1], 0xdead_beef, .NOENT);
    var out: [16]u8 = undefined;
    try readExact(fds[0], &out);
    const h = std.mem.bytesToValue(OutHeader, &out);
    try testing.expectEqual(@as(u32, 16), h.len);
    try testing.expectEqual(@as(i32, -2), h.@"error");
    try testing.expectEqual(@as(u64, 0xdead_beef), h.unique);

    try replyError(fds[1], 1, .NOSYS);
    try readExact(fds[0], &out);
    try testing.expectEqual(-@as(i32, @intCast(@intFromEnum(linux.E.NOSYS))), std.mem.bytesToValue(OutHeader, &out).@"error");
}

test "readRequest parses one request from a pipe and rejects bad lengths" {
    const fds = try makePipe();
    defer _ = linux.close(fds[0]);
    defer _ = linux.close(fds[1]);

    var wire: [48]u8 align(8) = undefined;
    const hdr = InHeader{ .len = 48, .opcode = @intFromEnum(Opcode.forget), .unique = 3, .nodeid = 2, .uid = 0, .gid = 0, .pid = 0, .total_extlen = 0, .padding = 0 };
    @memcpy(wire[0..40], std.mem.asBytes(&hdr));
    @memcpy(wire[40..48], std.mem.asBytes(&ForgetIn{ .nlookup = 11 }));
    try testing.expectEqual(@as(usize, 48), linux.write(fds[1], &wire, wire.len));

    var buf: [4096]u8 align(8) = undefined;
    const req = (try readRequest(fds[0], &buf)) orelse return error.Io;
    try testing.expectEqual(Opcode.forget, req.header.op());
    try testing.expectEqual(@as(u64, 2), req.header.nodeid);
    try testing.expectEqual(@as(u64, 11), (try body(ForgetIn, req)).nlookup);

    // Header length disagreeing with what was read is a protocol error.
    var bad = wire;
    std.mem.bytesAsValue(InHeader, bad[0..40]).len = 40;
    try testing.expectEqual(@as(usize, 48), linux.write(fds[1], &bad, bad.len));
    try testing.expectError(error.Protocol, readRequest(fds[0], &buf));

    // Non-blocking and empty: readRequestOnce reports Retry instead of waiting.
    try setNonblocking(fds[0]);
    try testing.expectError(error.Retry, readRequestOnce(fds[0], &buf));
    try testing.expectEqual(@as(usize, 48), linux.write(fds[1], &wire, wire.len));
    const again = (try readRequestOnce(fds[0], &buf)) orelse return error.Io;
    try testing.expectEqual(@as(u64, 3), again.header.unique);
    try testing.expectError(error.Retry, readRequestOnce(fds[0], &buf));
}