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

//! Synchronous 9P2000 session over a blocking file descriptor.
//!
//! A thin RPC layer over `cloud9.Client` (push/take, allocation-free). One request
//! is outstanding at a time: the FUSE loop that drives this is single-threaded, so
//! every call here blocks until its reply (or the connection's death) arrives.
//! Fids are handed out from a free list; fid 0 is reserved for the root.
const std = @import("std");
const cloud9 = @import("cloud9");
const linux = std.os.linux;

pub const Address = union(enum) {
    unix: []const u8,
    tcp: struct { host: []const u8, port: u16 },
    fd: i32,
};

/// A Stat whose every field means "leave unchanged" in a Twstat.
pub const dontcare = cloud9.Stat{
    .type = 0xFFFF,
    .dev = 0xFFFF_FFFF,
    .qid = .{ .type = 0xFF, .version = 0xFFFF_FFFF, .path = 0xFFFF_FFFF_FFFF_FFFF },
    .mode = 0xFFFF_FFFF,
    .atime = 0xFFFF_FFFF,
    .mtime = 0xFFFF_FFFF,
    .length = 0xFFFF_FFFF_FFFF_FFFF,
    .name = "",
    .uid = "",
    .gid = "",
    .muid = "",
};

/// How the owner of a session (the FUSE bridge) gets a say while an rpc waits
/// for its reply. `watch` names a descriptor to poll alongside the socket, or
/// -1 to poll nothing extra right now; when it becomes readable `onReadable`
/// consumes whatever is there and returns true if the request in flight should
/// be cancelled with a Tflush. `armed` reports whether such a cancellation was
/// requested earlier for the operation in progress; the chunked read/write
/// loops stop between chunks when it is set (a reply that raced the flush still
/// leaves the caller wanting out).
pub const Interrupt = struct {
    ctx: *anyopaque,
    watch: *const fn (ctx: *anyopaque) i32,
    onReadable: *const fn (ctx: *anyopaque) Session.Error!bool,
    armed: *const fn (ctx: *anyopaque) bool,
};

pub const Session = struct {
    pub const Error = error{ Nine, Protocol, Io, Closed, Stopped, Interrupted, TooLarge, OutOfMemory };

    pub const Walk = struct { nwqid: u16, wqid: [cloud9.max_welem]cloud9.Qid };
    pub const Open = struct { qid: cloud9.Qid, iounit: u32 };

    gpa: std.mem.Allocator,
    fd: i32,
    client: cloud9.Client,
    in_buf: []u8,
    out_buf: []u8,
    /// After `error.Nine`, the server's Rerror text (copied, bounded).
    ename: [256]u8 = undefined,
    ename_len: usize = 0,
    /// Negotiated maximum message size.
    msize: u32,
    next_fid: u32 = 1,
    free_fids: std.ArrayList(u32) = .empty,
    /// Per-fid iounit learned from open/create (0 = none); used to chunk read/write.
    iounits: std.AutoHashMapUnmanaged(u32, u32) = .empty,
    /// Optional descriptor watched while waiting for a reply: when it becomes
    /// readable (the bridge's "child exited" pipe) the pending rpc fails with
    /// `error.Stopped` instead of blocking on a server that never answers.
    stop_fd: i32 = -1,
    /// Optional interrupt source (the bridge's FUSE descriptor) consulted while
    /// a reply is outstanding; see `Interrupt`.
    interrupt: ?Interrupt = null,

    /// Connect to `address`, then negotiate the protocol version.
    /// `msize` is the maximum message size to ask for (0 = the buffers' size).
    pub fn connect(gpa: std.mem.Allocator, address: Address, msize: u32) !Session {
        const want: u32 = if (msize == 0) 8192 else @max(msize, 24);
        const fd = try openTransport(address);
        errdefer if (address != .fd) {
            _ = linux.close(fd);
        };

        const in_buf = try gpa.alloc(u8, want);
        errdefer gpa.free(in_buf);
        const out_buf = try gpa.alloc(u8, want);
        errdefer gpa.free(out_buf);

        var s: Session = .{
            .gpa = gpa,
            .fd = fd,
            .client = .init(.{ .in = in_buf, .out = out_buf }),
            .in_buf = in_buf,
            .out_buf = out_buf,
            .msize = want,
        };
        const r = try s.rpc(.{ .version = .{ .msize = want } });
        if (!std.mem.eql(u8, r.version.version, "9P2000")) return error.Protocol;
        s.msize = r.version.msize;
        return s;
    }

    /// Closes the descriptor and frees the buffers. Fids are not clunked.
    pub fn deinit(s: *Session) void {
        _ = linux.close(s.fd);
        s.free_fids.deinit(s.gpa);
        s.iounits.deinit(s.gpa);
        s.gpa.free(s.in_buf);
        s.gpa.free(s.out_buf);
        s.* = undefined;
    }

    pub fn attach(s: *Session, fid: u32, uname: []const u8, aname: []const u8) Error!cloud9.Qid {
        const r = try s.rpc(.{ .attach = .{ .fid = fid, .uname = uname, .aname = aname } });
        return r.attach;
    }

    /// Fid 0 is never handed out: it belongs to the root attach.
    pub fn allocFid(s: *Session) u32 {
        if (s.free_fids.pop()) |fid| return fid;
        const fid = s.next_fid;
        s.next_fid += 1;
        return fid;
    }

    /// Fids currently bound (excluding fid 0); a debugging aid for leak hunting.
    pub fn fidsInUse(s: *const Session) usize {
        return (s.next_fid - 1) - s.free_fids.items.len;
    }

    pub fn freeFid(s: *Session, fid: u32) void {
        _ = s.iounits.remove(fid);
        // If the free list cannot grow the fid is simply leaked; the counter keeps going.
        s.free_fids.append(s.gpa, fid) catch {};
    }

    /// Generic RPC. Result slices borrow the input buffer until the next call.
    ///
    /// While the reply is outstanding the socket is polled together with
    /// `stop_fd` (→ `error.Stopped`) and the interrupt source's descriptor. When
    /// the latter asks for a cancellation a Tflush for the request's tag goes out
    /// and the wait continues until either the original reply arrives (the flush
    /// lost the race; the result is returned as if nothing happened and the
    /// Rflush is swallowed by a later call) or the Rflush does (→
    /// `error.Interrupted`; the server has dropped the request).
    pub fn rpc(s: *Session, req: cloud9.Client.Request) Error!cloud9.Client.Result {
        s.ename_len = 0;
        const tag = s.client.submit(req) catch |e| switch (e) {
            error.NoTags, error.Handshake, error.Dead => return error.Protocol,
            error.NoSpace, error.TooLarge => return error.TooLarge,
            error.BadRequest => {
                s.setEname("bad request");
                return error.Nine;
            },
        };
        try s.flush();
        var flush_tag: ?u16 = null;
        var tmp: [64 * 1024]u8 = undefined;
        while (true) {
            while (s.client.take()) |done| {
                if (done.tag == tag) {
                    switch (done.result) {
                        .fail => |ename| {
                            s.setEname(ename);
                            return error.Nine;
                        },
                        else => return done.result,
                    }
                }
                if (flush_tag != null and done.tag == flush_tag.?) return error.Interrupted;
                // An Rflush for a flush whose original reply won the race in an
                // earlier call: the client has released both tags; nothing to do.
                if (done.op == .flush) continue;
                return error.Protocol;
            }
            if (s.client.dead) return error.Protocol;
            // After take() returned null the previous frame is gone, so the free
            // space is at least what the pending frame still needs.
            const room = s.client.in.len - s.client.in_len;
            if (room == 0) return error.Protocol;
            switch (try s.wait()) {
                .socket => {
                    const n = try readSocket(s.fd, tmp[0..@min(room, tmp.len)]);
                    if (n == 0) return error.Closed;
                    const pushed = s.client.push(tmp[0..n]);
                    if (pushed != n) return error.Protocol;
                },
                .cancel => if (flush_tag == null) {
                    flush_tag = s.client.submit(.{ .flush = .{ .oldtag = tag } }) catch return error.Protocol;
                    try s.flush();
                },
            }
        }
    }

    /// True when the interrupt source has asked for the operation in progress to
    /// stop; consulted between the chunks of a read or write.
    pub fn interruptArmed(s: *const Session) bool {
        const i = s.interrupt orelse return false;
        return i.armed(i.ctx);
    }

    /// Walk `names` from `fid` to `newfid`. A partial walk leaves `newfid` unbound
    /// (9P semantics) and reports `error.Nine` with ename "file does not exist".
    pub fn walk(s: *Session, fid: u32, newfid: u32, names: []const []const u8) Error!Walk {
        const r = try s.rpc(.{ .walk = .{ .fid = fid, .newfid = newfid, .names = names } });
        if (r.walk.nwqid < names.len) {
            s.setEname("file does not exist");
            return error.Nine;
        }
        return .{ .nwqid = r.walk.nwqid, .wqid = r.walk.wqid };
    }

    /// allocFid + zero-element walk. The fid is released again on failure.
    pub fn clone(s: *Session, fid: u32) Error!u32 {
        const newfid = s.allocFid();
        errdefer s.freeFid(newfid);
        _ = try s.walk(fid, newfid, &.{});
        return newfid;
    }

    pub fn open(s: *Session, fid: u32, mode: u8) Error!Open {
        const r = try s.rpc(.{ .open = .{ .fid = fid, .mode = mode } });
        s.noteIounit(fid, r.open.iounit);
        return .{ .qid = r.open.qid, .iounit = r.open.iounit };
    }

    pub fn create(s: *Session, fid: u32, name: []const u8, perm: u32, mode: u8) Error!Open {
        const r = try s.rpc(.{ .create = .{ .fid = fid, .name = name, .perm = perm, .mode = mode } });
        s.noteIounit(fid, r.create.iounit);
        return .{ .qid = r.create.qid, .iounit = r.create.iounit };
    }

    /// Reads into `buf`, chunking by min(maxRead, iounit) and stopping at the first
    /// short read. Returns the number of bytes read (0 at end of file).
    pub fn read(s: *Session, fid: u32, offset: u64, buf: []u8) Error!usize {
        return readWith(s, rpc, fid, offset, buf, s.chunk(fid));
    }

    /// Writes `data`, chunking like `read` and stopping at the first short write.
    pub fn write(s: *Session, fid: u32, offset: u64, data: []const u8) Error!usize {
        return writeWith(s, rpc, fid, offset, data, s.chunkWrite(fid));
    }

    /// The returned Stat's strings (name/uid/gid/muid) borrow the session's input
    /// buffer: they are valid only until the next rpc. Copy what must outlive it.
    pub fn stat(s: *Session, fid: u32) Error!cloud9.Stat {
        const r = try s.rpc(.{ .stat = .{ .fid = fid } });
        return r.stat;
    }

    pub fn wstat(s: *Session, fid: u32, st: cloud9.Stat) Error!void {
        _ = try s.rpc(.{ .wstat = .{ .fid = fid, .stat = st } });
    }

    /// Frees the fid locally even when the server reports an error.
    pub fn clunk(s: *Session, fid: u32) Error!void {
        defer s.freeFid(fid);
        _ = try s.rpc(.{ .clunk = .{ .fid = fid } });
    }

    /// Frees the fid locally even when the server reports an error.
    pub fn remove(s: *Session, fid: u32) Error!void {
        defer s.freeFid(fid);
        _ = try s.rpc(.{ .remove = .{ .fid = fid } });
    }

    /// Maps the last Rerror text to an errno (case-insensitive substring match).
    pub fn errno(s: *const Session) linux.E {
        return enameToErrno(s.ename[0..s.ename_len]);
    }

    // -- internals --------------------------------------------------------------

    fn setEname(s: *Session, text: []const u8) void {
        const n = @min(text.len, 255);
        @memcpy(s.ename[0..n], text[0..n]);
        s.ename_len = n;
    }

    fn noteIounit(s: *Session, fid: u32, iounit: u32) void {
        if (iounit == 0) {
            _ = s.iounits.remove(fid);
        } else {
            s.iounits.put(s.gpa, fid, iounit) catch {};
        }
    }

    fn chunk(s: *Session, fid: u32) u32 {
        return chunkSize(s.client.maxRead(), s.iounits.get(fid) orelse 0);
    }

    fn chunkWrite(s: *Session, fid: u32) u32 {
        return chunkSize(s.client.maxWrite(), s.iounits.get(fid) orelse 0);
    }

    const Ready = enum { socket, cancel };

    /// Blocks until the socket is readable (`.socket`), the interrupt source
    /// wants the request in flight cancelled (`.cancel`), or `stop_fd` fires
    /// (`error.Stopped`). Anything the interrupt source consumes without asking
    /// for a cancellation simply resumes the wait.
    fn wait(s: *Session) Error!Ready {
        while (true) {
            var pfds: [3]linux.pollfd = undefined;
            var n: usize = 0;
            pfds[n] = .{ .fd = s.fd, .events = linux.POLL.IN, .revents = 0 };
            n += 1;
            const stop_at: ?usize = if (s.stop_fd >= 0) n else null;
            if (stop_at != null) {
                pfds[n] = .{ .fd = s.stop_fd, .events = linux.POLL.IN, .revents = 0 };
                n += 1;
            }
            const ifd: i32 = if (s.interrupt) |i| i.watch(i.ctx) else -1;
            const int_at: ?usize = if (ifd >= 0) n else null;
            if (int_at != null) {
                pfds[n] = .{ .fd = ifd, .events = linux.POLL.IN, .revents = 0 };
                n += 1;
            }
            if (n == 1) return .socket;
            const prc = linux.poll(&pfds, @intCast(n), -1);
            switch (linux.errno(prc)) {
                .SUCCESS => {},
                .INTR, .AGAIN => continue,
                else => return error.Io,
            }
            // A reply that is already there wins over everything else.
            if (pfds[0].revents != 0) return .socket;
            if (stop_at) |i| {
                if (pfds[i].revents != 0) return error.Stopped;
            }
            if (int_at) |i| {
                if (pfds[i].revents != 0) {
                    const src = s.interrupt.?;
                    if (try src.onReadable(src.ctx)) return .cancel;
                }
            }
        }
    }

    /// Writes everything in the client's output buffer to the socket.
    fn flush(s: *Session) Error!void {
        while (s.client.output().len != 0) {
            const out = s.client.output();
            const rc = linux.write(s.fd, out.ptr, out.len);
            switch (linux.errno(rc)) {
                .SUCCESS => {
                    if (rc == 0) return error.Closed;
                    s.client.wrote(rc);
                },
                .INTR, .AGAIN => continue,
                .PIPE, .CONNRESET => return error.Closed,
                else => return error.Io,
            }
        }
    }
};

fn chunkSize(max: u32, iounit: u32) u32 {
    if (iounit != 0 and iounit < max) return iounit;
    return max;
}

/// Chunked read over any rpc-shaped function (injected so the loop is testable).
fn readWith(
    s: anytype,
    comptime rpcFn: anytype,
    fid: u32,
    offset: u64,
    buf: []u8,
    max_chunk: u32,
) Session.Error!usize {
    if (max_chunk == 0) return error.Protocol;
    var done: usize = 0;
    while (done < buf.len) {
        // Like read(2): an interruption after some data arrived is a short read.
        if (done != 0 and s.interruptArmed()) break;
        const want: u32 = @intCast(@min(buf.len - done, max_chunk));
        const r = rpcFn(s, .{ .read = .{ .fid = fid, .offset = offset + done, .count = want } }) catch |e| {
            if (e == error.Interrupted and done != 0) break;
            return e;
        };
        const data = r.read;
        @memcpy(buf[done..][0..data.len], data);
        done += data.len;
        if (data.len < want) break;
    }
    return done;
}

/// Chunked write over any rpc-shaped function.
fn writeWith(
    s: anytype,
    comptime rpcFn: anytype,
    fid: u32,
    offset: u64,
    data: []const u8,
    max_chunk: u32,
) Session.Error!usize {
    if (max_chunk == 0) return error.Protocol;
    var done: usize = 0;
    while (done < data.len) {
        if (done != 0 and s.interruptArmed()) break;
        const want: usize = @min(data.len - done, max_chunk);
        const r = rpcFn(s, .{ .write = .{ .fid = fid, .offset = offset + done, .data = data[done..][0..want] } }) catch |e| {
            if (e == error.Interrupted and done != 0) break;
            return e;
        };
        done += r.write;
        if (r.write < want) break;
    }
    return done;
}

fn readSocket(fd: i32, buf: []u8) Session.Error!usize {
    while (true) {
        const rc = linux.read(fd, buf.ptr, buf.len);
        switch (linux.errno(rc)) {
            .SUCCESS => return rc,
            .INTR, .AGAIN => continue,
            .CONNRESET => return error.Closed,
            else => return error.Io,
        }
    }
}

/// Rerror text → errno, per docs/DESIGN.md (first match wins).
pub fn enameToErrno(ename: []const u8) linux.E {
    const Rule = struct { needle: []const u8, err: linux.E };
    const rules = [_]Rule{
        // A server that answers a flushed request with an error (Pardes says
        // "Interrupted system call") should look like a flush to the caller.
        .{ .needle = "interrupt", .err = .INTR },
        .{ .needle = "not exist", .err = .NOENT },
        .{ .needle = "not found", .err = .NOENT },
        .{ .needle = "no such", .err = .NOENT },
        .{ .needle = "exists", .err = .EXIST },
        .{ .needle = "not empty", .err = .NOTEMPTY },
        .{ .needle = "not a dir", .err = .NOTDIR },
        .{ .needle = "is a dir", .err = .ISDIR },
        .{ .needle = "permission", .err = .ACCES },
        .{ .needle = "denied", .err = .ACCES },
        .{ .needle = "read-only", .err = .ROFS },
        .{ .needle = "read only", .err = .ROFS },
        .{ .needle = "readonly", .err = .ROFS },
        .{ .needle = "no space", .err = .NOSPC },
        .{ .needle = "not allowed", .err = .PERM },
        .{ .needle = "not permitted", .err = .PERM },
        .{ .needle = "cannot", .err = .PERM },
        .{ .needle = "fid", .err = .BADF },
        .{ .needle = "bad offset", .err = .INVAL },
        .{ .needle = "invalid", .err = .INVAL },
        .{ .needle = "bad ", .err = .INVAL },
        .{ .needle = "busy", .err = .BUSY },
        .{ .needle = "in use", .err = .BUSY },
        .{ .needle = "too long", .err = .NAMETOOLONG },
        .{ .needle = "not supported", .err = .OPNOTSUPP },
        .{ .needle = "unsupported", .err = .OPNOTSUPP },
    };
    for (rules) |rule| {
        if (std.ascii.findIgnoreCase(ename, rule.needle) != null) return rule.err;
    }
    return .IO;
}

// -- transport ------------------------------------------------------------------

fn openTransport(address: Address) !i32 {
    switch (address) {
        .fd => |fd| return fd,
        .unix => |path| {
            if (path.len == 0 or path.len >= 108) return error.NameTooLong;
            var sa: linux.sockaddr.un = .{ .path = @splat(0) };
            @memcpy(sa.path[0..path.len], path);
            const fd = try newSocket(linux.AF.UNIX, 0);
            errdefer _ = linux.close(fd);
            try doConnect(fd, @ptrCast(&sa), @sizeOf(linux.sockaddr.un));
            return fd;
        },
        .tcp => |t| {
            const ip = std.Io.net.IpAddress.parse(t.host, t.port) catch return error.InvalidAddress;
            switch (ip) {
                .ip4 => |a| {
                    const sa: linux.sockaddr.in = .{
                        .port = std.mem.nativeToBig(u16, t.port),
                        .addr = @bitCast(a.bytes),
                    };
                    const fd = try newSocket(linux.AF.INET, linux.IPPROTO.TCP);
                    errdefer _ = linux.close(fd);
                    setNodelay(fd);
                    try doConnect(fd, @ptrCast(&sa), @sizeOf(linux.sockaddr.in));
                    return fd;
                },
                .ip6 => |a| {
                    const sa: linux.sockaddr.in6 = .{
                        .port = std.mem.nativeToBig(u16, t.port),
                        .flowinfo = 0,
                        .addr = a.bytes,
                        .scope_id = 0,
                    };
                    const fd = try newSocket(linux.AF.INET6, linux.IPPROTO.TCP);
                    errdefer _ = linux.close(fd);
                    setNodelay(fd);
                    try doConnect(fd, @ptrCast(&sa), @sizeOf(linux.sockaddr.in6));
                    return fd;
                },
            }
        },
    }
}

fn newSocket(domain: u32, protocol: u32) !i32 {
    const rc = linux.socket(domain, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, protocol);
    switch (linux.errno(rc)) {
        .SUCCESS => return @intCast(rc),
        .MFILE, .NFILE => return error.ProcessFdQuotaExceeded,
        .AFNOSUPPORT, .PROTONOSUPPORT => return error.AddressFamilyNotSupported,
        .ACCES => return error.AccessDenied,
        .NOMEM, .NOBUFS => return error.SystemResources,
        else => return error.Unexpected,
    }
}

fn setNodelay(fd: i32) void {
    const one: u32 = 1;
    _ = linux.setsockopt(fd, linux.IPPROTO.TCP, linux.TCP.NODELAY, @ptrCast(&one), @sizeOf(u32));
}

fn doConnect(fd: i32, addr: *const linux.sockaddr, len: linux.socklen_t) !void {
    while (true) {
        const rc = linux.connect(fd, addr, len);
        switch (linux.errno(rc)) {
            .SUCCESS => return,
            .INTR => continue,
            .CONNREFUSED => return error.ConnectionRefused,
            .NOENT, .NOTDIR => return error.FileNotFound,
            .ACCES, .PERM => return error.AccessDenied,
            .TIMEDOUT => return error.ConnectionTimedOut,
            .NETUNREACH, .HOSTUNREACH => return error.NetworkUnreachable,
            .ADDRNOTAVAIL => return error.AddressNotAvailable,
            .AGAIN, .INPROGRESS => return error.WouldBlock,
            else => return error.Unexpected,
        }
    }
}

// -- tests ----------------------------------------------------------------------

const testing = std.testing;

test {
    testing.refAllDecls(@This());
}

test "ename → errno mapping" {
    try testing.expectEqual(linux.E.INTR, enameToErrno("Interrupted system call"));
    try testing.expectEqual(linux.E.INTR, enameToErrno("read interrupted"));
    try testing.expectEqual(linux.E.NOENT, enameToErrno("file does not exist"));
    try testing.expectEqual(linux.E.NOENT, enameToErrno("No Such File"));
    try testing.expectEqual(linux.E.NOENT, enameToErrno("directory entry not found"));
    try testing.expectEqual(linux.E.EXIST, enameToErrno("file already exists"));
    try testing.expectEqual(linux.E.NOTEMPTY, enameToErrno("directory not empty"));
    try testing.expectEqual(linux.E.NOTDIR, enameToErrno("not a directory"));
    try testing.expectEqual(linux.E.ISDIR, enameToErrno("is a directory"));
    try testing.expectEqual(linux.E.ACCES, enameToErrno("permission denied"));
    try testing.expectEqual(linux.E.ACCES, enameToErrno("access denied"));
    try testing.expectEqual(linux.E.ROFS, enameToErrno("read-only file system"));
    try testing.expectEqual(linux.E.NOSPC, enameToErrno("no space left"));
    try testing.expectEqual(linux.E.PERM, enameToErrno("operation not permitted"));
    try testing.expectEqual(linux.E.PERM, enameToErrno("cannot remove root"));
    try testing.expectEqual(linux.E.BADF, enameToErrno("unknown fid"));
    try testing.expectEqual(linux.E.BADF, enameToErrno("fid in use")); // "fid" precedes "in use"
    try testing.expectEqual(linux.E.INVAL, enameToErrno("bad offset"));
    try testing.expectEqual(linux.E.INVAL, enameToErrno("invalid argument"));
    try testing.expectEqual(linux.E.INVAL, enameToErrno("bad request"));
    try testing.expectEqual(linux.E.BUSY, enameToErrno("device busy"));
    try testing.expectEqual(linux.E.NAMETOOLONG, enameToErrno("name too long"));
    try testing.expectEqual(linux.E.OPNOTSUPP, enameToErrno("operation not supported"));
    try testing.expectEqual(linux.E.IO, enameToErrno("something odd happened"));
    try testing.expectEqual(linux.E.IO, enameToErrno(""));
}

test "fid allocator recycles and never hands out 0" {
    var s: Session = undefined;
    s.gpa = testing.allocator;
    s.next_fid = 1;
    s.free_fids = .empty;
    s.iounits = .empty;
    defer s.free_fids.deinit(s.gpa);
    defer s.iounits.deinit(s.gpa);

    const a = s.allocFid();
    const b = s.allocFid();
    const c = s.allocFid();
    try testing.expectEqual(@as(u32, 1), a);
    try testing.expectEqual(@as(u32, 2), b);
    try testing.expectEqual(@as(u32, 3), c);
    s.freeFid(b);
    try testing.expectEqual(b, s.allocFid());
    s.freeFid(a);
    s.freeFid(c);
    const x = s.allocFid();
    const y = s.allocFid();
    try testing.expect((x == a and y == c) or (x == c and y == a));
    try testing.expectEqual(@as(u32, 4), s.allocFid());
    try testing.expect(a != 0 and b != 0 and c != 0);
}

test "chunkSize honours iounit only when smaller" {
    try testing.expectEqual(@as(u32, 100), chunkSize(100, 0));
    try testing.expectEqual(@as(u32, 40), chunkSize(100, 40));
    try testing.expectEqual(@as(u32, 100), chunkSize(100, 400));
}

/// Fake rpc for the chunked read/write loops: a file of `len` bytes where byte i == i & 0xff.
const FakeFile = struct {
    len: usize,
    calls: usize = 0,
    max_count: u32 = 0,
    short_write_at: ?usize = null,
    /// Fail the call with `error.Interrupted` once this many calls were made.
    interrupt_at: ?usize = null,
    /// Report an armed interrupt once this many calls were made.
    armed_at: ?usize = null,
    scratch: [4096]u8 = undefined,

    fn interruptArmed(f: *const FakeFile) bool {
        return if (f.armed_at) |at| f.calls >= at else false;
    }

    fn rpc(f: *FakeFile, req: cloud9.Client.Request) Session.Error!cloud9.Client.Result {
        f.calls += 1;
        if (f.interrupt_at) |at| if (f.calls > at) return error.Interrupted;
        switch (req) {
            .read => |r| {
                f.max_count = @max(f.max_count, r.count);
                if (r.offset >= f.len) return .{ .read = "" };
                const n: usize = @min(@as(usize, r.count), f.len - @as(usize, @intCast(r.offset)));
                for (f.scratch[0..n], 0..) |*b, i| b.* = @truncate(r.offset + i);
                return .{ .read = f.scratch[0..n] };
            },
            .write => |w| {
                f.max_count = @max(f.max_count, @as(u32, @intCast(w.data.len)));
                if (f.short_write_at) |at| {
                    if (w.offset + w.data.len > at) {
                        const n: usize = if (w.offset >= at) 0 else @intCast(at - w.offset);
                        return .{ .write = @intCast(n) };
                    }
                }
                return .{ .write = @intCast(w.data.len) };
            },
            else => unreachable,
        }
    }
};

test "read chunks by max_chunk and stops at a short read" {
    var f: FakeFile = .{ .len = 2500 };
    var buf: [4000]u8 = undefined;
    const n = try readWith(&f, FakeFile.rpc, 7, 0, &buf, 1000);
    try testing.expectEqual(@as(usize, 2500), n);
    try testing.expectEqual(@as(usize, 3), f.calls); // 1000, 1000, 500 (short → stop)
    try testing.expectEqual(@as(u32, 1000), f.max_count);
    for (buf[0..n], 0..) |b, i| try testing.expectEqual(@as(u8, @truncate(i)), b);

    // Reading exactly up to a chunk boundary uses one call per chunk and no more.
    f = .{ .len = 2000 };
    try testing.expectEqual(@as(usize, 2000), try readWith(&f, FakeFile.rpc, 7, 0, buf[0..2000], 1000));
    try testing.expectEqual(@as(usize, 2), f.calls);

    // Offset past EOF → 0.
    f = .{ .len = 10 };
    try testing.expectEqual(@as(usize, 0), try readWith(&f, FakeFile.rpc, 7, 50, &buf, 1000));
}

test "write chunks and stops at a short write" {
    var f: FakeFile = .{ .len = 0 };
    var data: [2500]u8 = undefined;
    for (&data, 0..) |*b, i| b.* = @truncate(i);
    try testing.expectEqual(@as(usize, 2500), try writeWith(&f, FakeFile.rpc, 7, 0, &data, 1000));
    try testing.expectEqual(@as(usize, 3), f.calls);
    try testing.expectEqual(@as(u32, 1000), f.max_count);

    f = .{ .len = 0, .short_write_at = 1500 };
    try testing.expectEqual(@as(usize, 1500), try writeWith(&f, FakeFile.rpc, 7, 0, &data, 1000));
    try testing.expectEqual(@as(usize, 2), f.calls);
}

test "interrupted chunk loops: partial count if data moved, Interrupted otherwise" {
    var buf: [4000]u8 = undefined;
    // The second chunk's rpc is interrupted: the first chunk is returned.
    var f: FakeFile = .{ .len = 2500, .interrupt_at = 1 };
    try testing.expectEqual(@as(usize, 1000), try readWith(&f, FakeFile.rpc, 7, 0, &buf, 1000));
    // The first chunk's rpc is interrupted: nothing was transferred.
    f = .{ .len = 2500, .interrupt_at = 0 };
    try testing.expectError(error.Interrupted, readWith(&f, FakeFile.rpc, 7, 0, &buf, 1000));
    // A reply that raced the flush arms the interrupt: stop before the next chunk.
    f = .{ .len = 2500, .armed_at = 2 };
    try testing.expectEqual(@as(usize, 2000), try readWith(&f, FakeFile.rpc, 7, 0, &buf, 1000));
    try testing.expectEqual(@as(usize, 2), f.calls);
    // Same for writes.
    var data: [2500]u8 = undefined;
    for (&data, 0..) |*b, i| b.* = @truncate(i);
    f = .{ .len = 0, .interrupt_at = 2 };
    try testing.expectEqual(@as(usize, 2000), try writeWith(&f, FakeFile.rpc, 7, 0, &data, 1000));
    f = .{ .len = 0, .interrupt_at = 0 };
    try testing.expectError(error.Interrupted, writeWith(&f, FakeFile.rpc, 7, 0, &data, 1000));
    f = .{ .len = 0, .armed_at = 1 };
    try testing.expectEqual(@as(usize, 1000), try writeWith(&f, FakeFile.rpc, 7, 0, &data, 1000));
}

// -- in-process server test ---------------------------------------------------------

/// A tiny 9P2000 backend on a cloud9.Server: answers version/attach/walk/stat/open/
/// read/clunk/remove with canned data. Runs in its own thread over a socketpair.
/// Test support only (bridge.zig's tests use it too).
pub const FakeServer = struct {
    fd: i32,
    msize: u32,
    max_read_count: u32 = 0,
    file_len: usize,
    /// A Tread at this offset is never answered (a blocked stream read); the
    /// server keeps serving whatever else arrives, notably a Tflush.
    hang_offset: ?u64 = null,
    /// What a Tflush gets: the connection dropped (`.hangup`), an Rflush, or
    /// first the Rread the flush was aimed at and then the Rflush (the race).
    on_flush: enum { hangup, rflush, reply_then_rflush } = .hangup,
    /// Observed by the test thread: number of Tflush seen and the last oldtag.
    flushes: std.atomic.Value(u32) = .init(0),
    flush_oldtag: std.atomic.Value(u32) = .init(0xFFFF),
    /// The tag of the hung Tread, for the test to compare with `flush_oldtag`.
    hung_tag: std.atomic.Value(u32) = .init(0xFFFF),
    /// When set, the server injects that source's INTERRUPT the moment a read
    /// hangs, so the cancellation provably arrives while the wait is on.
    on_hang_inject: ?*PipeInterrupt = null,
    /// Delay before every Rread, so a test can be sure the client is waiting.
    read_delay_ns: u64 = 0,

    pub const file_qid: cloud9.Qid = .{ .type = 0, .version = 3, .path = 0x1234 };
    pub const dir_qid: cloud9.Qid = .{ .type = cloud9.qtdir, .version = 1, .path = 0x1 };

    pub fn run(fs: *FakeServer) void {
        fs.loop() catch |e| std.debug.print("fake server: {s}\n", .{@errorName(e)});
        _ = linux.close(fs.fd);
    }

    fn loop(fs: *FakeServer) !void {
        const gpa = testing.allocator;
        const in = try gpa.alloc(u8, fs.msize);
        defer gpa.free(in);
        const out = try gpa.alloc(u8, fs.msize * 2);
        defer gpa.free(out);
        var srv: cloud9.Server = .init(.{ .in = in, .out = out });
        var tmp: [4096]u8 = undefined;
        var data: [8192]u8 = undefined;
        var hung: ?struct { tag: u16, offset: u64, count: u32 } = null;
        while (true) {
            while (try srv.receive()) |req| {
                const tag = req.tag;
                switch (req.msg) {
                    .tversion => |m| try srv.negotiate(m.msize, m.version),
                    .tattach => try srv.reply(tag, .{ .rattach = .{ .qid = dir_qid } }),
                    .twalk => |m| {
                        var wq: [cloud9.max_welem]cloud9.Qid = @splat(dir_qid);
                        var n: u16 = 0;
                        for (m.wname[0..m.nwname]) |name| {
                            if (std.mem.eql(u8, name, "file")) {
                                wq[n] = file_qid;
                            } else if (std.mem.eql(u8, name, "dir")) {
                                wq[n] = dir_qid;
                            } else break;
                            n += 1;
                        }
                        if (n == 0 and m.nwname != 0) {
                            try srv.reply(tag, .{ .rerror = .{ .ename = "file does not exist" } });
                        } else {
                            try srv.reply(tag, .{ .rwalk = .{ .nwqid = n, .wqid = wq } });
                        }
                    },
                    .tstat => try srv.reply(tag, .{ .rstat = .{ .stat = .{
                        .type = 0,
                        .dev = 0,
                        .qid = file_qid,
                        .mode = 0o644,
                        .atime = 1,
                        .mtime = 2,
                        .length = fs.file_len,
                        .name = "file",
                        .uid = "u",
                        .gid = "g",
                        .muid = "u",
                    } } }),
                    .topen => |m| try srv.reply(tag, .{ .ropen = .{ .qid = file_qid, .iounit = if (m.mode == cloud9.owrite) 700 else 0 } }),
                    .tread => |m| {
                        fs.max_read_count = @max(fs.max_read_count, m.count);
                        if (fs.hang_offset != null and fs.hang_offset.? == m.offset) {
                            hung = .{ .tag = tag, .offset = m.offset, .count = m.count };
                            fs.hung_tag.store(tag, .seq_cst);
                            if (fs.on_hang_inject) |p| try p.inject(p.unique);
                        } else {
                            if (fs.read_delay_ns != 0) {
                                const ts: linux.timespec = .{ .sec = @intCast(fs.read_delay_ns / std.time.ns_per_s), .nsec = @intCast(fs.read_delay_ns % std.time.ns_per_s) };
                                _ = linux.nanosleep(&ts, null);
                            }
                            try srv.reply(tag, .{ .rread = .{ .data = fs.fill(&data, m.offset, m.count) } });
                        }
                    },
                    .twrite => |m| try srv.reply(tag, .{ .rwrite = .{ .count = @intCast(m.data.len) } }),
                    .tclunk => try srv.reply(tag, .rclunk),
                    .tremove => try srv.reply(tag, .{ .rerror = .{ .ename = "permission denied" } }),
                    .twstat => try srv.reply(tag, .rwstat),
                    .tflush => |m| {
                        fs.flush_oldtag.store(m.oldtag, .seq_cst);
                        _ = fs.flushes.fetchAdd(1, .seq_cst);
                        switch (fs.on_flush) {
                            // The test's "hang up now" signal.
                            .hangup => return,
                            .rflush => {
                                if (hung != null and hung.?.tag == m.oldtag) hung = null;
                                try srv.reply(tag, .rflush);
                            },
                            .reply_then_rflush => {
                                if (hung) |h| if (h.tag == m.oldtag) {
                                    try srv.reply(h.tag, .{ .rread = .{ .data = fs.fill(&data, h.offset, h.count) } });
                                    hung = null;
                                };
                                try srv.reply(tag, .rflush);
                            },
                        }
                    },
                    else => try srv.reply(tag, .{ .rerror = .{ .ename = "not supported" } }),
                }
                srv.release();
            }
            while (srv.output().len != 0) {
                const o = srv.output();
                const rc = linux.write(fs.fd, o.ptr, o.len);
                if (linux.errno(rc) != .SUCCESS) return error.Write;
                srv.wrote(rc);
            }
            const rc = linux.read(fs.fd, &tmp, tmp.len);
            if (linux.errno(rc) != .SUCCESS) return error.Read;
            if (rc == 0) return;
            if (srv.push(tmp[0..rc]) != rc) return error.Overflow;
        }
    }

    /// File contents: byte i == i & 0xff, `file_len` bytes long.
    fn fill(fs: *const FakeServer, data: []u8, offset: u64, count: u32) []const u8 {
        var n: usize = 0;
        if (offset < fs.file_len) n = @min(@as(usize, count), fs.file_len - @as(usize, @intCast(offset)));
        n = @min(n, data.len);
        for (data[0..n], 0..) |*b, i| b.* = @truncate(offset + i);
        return data[0..n];
    }

    /// A connected session (fid 0 attached, fid 1 walked to "file" and opened
    /// for reading) plus the server thread; `close` when done.
    pub const Pair = struct {
        server: *FakeServer,
        session: Session,
        thread: std.Thread,

        pub fn close(p: *Pair) void {
            p.session.deinit();
            p.thread.join();
        }
    };

    pub fn start(fs: *FakeServer) !Pair {
        var fds: [2]i32 = undefined;
        if (linux.errno(linux.socketpair(linux.AF.UNIX, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, 0, &fds)) != .SUCCESS) return error.Io;
        fs.fd = fds[1];
        const th = try std.Thread.spawn(.{}, FakeServer.run, .{fs});
        var s = try Session.connect(testing.allocator, .{ .fd = fds[0] }, fs.msize);
        errdefer s.deinit();
        _ = try s.attach(0, "me", "");
        const fid = s.allocFid();
        _ = try s.walk(0, fid, &.{"file"});
        _ = try s.open(fid, cloud9.oread);
        return .{ .server = fs, .session = s, .thread = th };
    }
};

/// A fake interrupt source for the rpc wait loop: a SOCK_SEQPACKET pair stands
/// in for the FUSE descriptor (one datagram per request, like /dev/fuse
/// delivers one request per read). Mirrors the bridge's rules: an INTERRUPT for
/// `unique` arms and cancels, anything else is consumed and ignored.
pub const PipeInterrupt = struct {
    read_end: i32,
    write_end: i32,
    unique: u64,
    armed_flag: bool = false,
    /// Requests consumed that were not the matching INTERRUPT.
    ignored: usize = 0,

    const fuse_interrupt_opcode: u32 = 36;

    pub fn init(unique: u64) !PipeInterrupt {
        var fds: [2]i32 = undefined;
        const flags = linux.SOCK.SEQPACKET | linux.SOCK.CLOEXEC | linux.SOCK.NONBLOCK;
        if (linux.errno(linux.socketpair(linux.AF.UNIX, flags, 0, &fds)) != .SUCCESS) return error.Io;
        return .{ .read_end = fds[0], .write_end = fds[1], .unique = unique };
    }

    pub fn deinit(p: *PipeInterrupt) void {
        _ = linux.close(p.read_end);
        _ = linux.close(p.write_end);
    }

    pub fn interface(p: *PipeInterrupt) Interrupt {
        return .{ .ctx = p, .watch = watch, .onReadable = onReadable, .armed = armed };
    }

    /// Writes a FUSE_INTERRUPT request (InHeader + InterruptIn) naming `target`.
    pub fn inject(p: *PipeInterrupt, target: u64) !void {
        var wire: [48]u8 = undefined;
        std.mem.writeInt(u32, wire[0..4], 48, .little); // len
        std.mem.writeInt(u32, wire[4..8], fuse_interrupt_opcode, .little); // opcode
        std.mem.writeInt(u64, wire[8..16], 0x8000_0000_0000_0001, .little); // the interrupt's own unique
        @memset(wire[16..40], 0); // nodeid, uid, gid, pid, extlen, padding
        std.mem.writeInt(u64, wire[40..48], target, .little); // InterruptIn.unique
        if (linux.write(p.write_end, &wire, wire.len) != wire.len) return error.Io;
    }

    fn watch(ctx: *anyopaque) i32 {
        const p: *PipeInterrupt = @ptrCast(@alignCast(ctx));
        return p.read_end;
    }

    fn onReadable(ctx: *anyopaque) Session.Error!bool {
        const p: *PipeInterrupt = @ptrCast(@alignCast(ctx));
        var buf: [4096]u8 = undefined;
        const rc = linux.read(p.read_end, &buf, buf.len);
        if (linux.errno(rc) != .SUCCESS or rc != 48) return error.Io;
        const opcode = std.mem.readInt(u32, buf[4..8], .little);
        const target = std.mem.readInt(u64, buf[40..48], .little);
        if (opcode == fuse_interrupt_opcode and target == p.unique) {
            p.armed_flag = true;
            return true;
        }
        p.ignored += 1;
        return false;
    }

    fn armed(ctx: *anyopaque) bool {
        const p: *PipeInterrupt = @ptrCast(@alignCast(ctx));
        return p.armed_flag;
    }
};

test "rpc wait loop: INTERRUPT → Tflush → Rflush → error.Interrupted; session still usable" {
    var fs: FakeServer = .{ .fd = -1, .msize = 8192, .file_len = 50, .hang_offset = 0, .on_flush = .rflush };
    var pair = try fs.start();
    defer pair.close();
    const s = &pair.session;
    var pi = try PipeInterrupt.init(77);
    defer pi.deinit();
    s.interrupt = pi.interface();
    defer s.interrupt = null;

    // An INTERRUPT for some other request is consumed and ignored: the wait
    // goes on, and the reply (a read past the hang offset) arrives normally.
    var buf: [100]u8 = undefined;
    try pi.inject(78);
    try testing.expectEqual(@as(usize, 40), try s.read(1, 10, &buf));
    try testing.expect(!pi.armed_flag);

    // The read at offset 0 hangs; the INTERRUPT for our request cancels it
    // (the stray one above is consumed along the way if the reply beat it).
    try pi.inject(77);
    try testing.expectError(error.Interrupted, s.read(1, 0, &buf));
    try testing.expect(pi.armed_flag);
    try testing.expectEqual(@as(usize, 1), pi.ignored);
    try testing.expectEqual(@as(u32, 1), fs.flushes.load(.seq_cst));
    try testing.expectEqual(fs.hung_tag.load(.seq_cst), fs.flush_oldtag.load(.seq_cst));

    // Both tags are free again: further rpcs work.
    const st = try s.stat(1);
    try testing.expectEqual(@as(u64, 50), st.length);
    try testing.expectEqual(@as(usize, 40), try s.read(1, 10, &buf));
    try testing.expectEqual(@as(usize, 0), s.client.pending());
}

test "rpc wait loop: the reply beats the Rflush → data returned, stray Rflush swallowed" {
    var fs: FakeServer = .{ .fd = -1, .msize = 8192, .file_len = 50, .hang_offset = 0, .on_flush = .reply_then_rflush };
    var pair = try fs.start();
    defer pair.close();
    const s = &pair.session;
    var pi = try PipeInterrupt.init(5);
    defer pi.deinit();
    s.interrupt = pi.interface();
    defer s.interrupt = null;

    var buf: [40]u8 = undefined;
    try pi.inject(5);
    try testing.expectEqual(@as(usize, 30), try s.read(1, 0, buf[0..30]));
    for (buf[0..30], 0..) |b, i| try testing.expectEqual(@as(u8, @truncate(i)), b);
    try testing.expectEqual(@as(u32, 1), fs.flushes.load(.seq_cst));
    try testing.expectEqual(fs.hung_tag.load(.seq_cst), fs.flush_oldtag.load(.seq_cst));

    // The Rflush is still in flight (or already buffered): the next rpcs must
    // step over it, and afterwards nothing is pending in the client.
    pi.armed_flag = false;
    const st = try s.stat(1);
    try testing.expectEqual(@as(u64, 50), st.length);
    try testing.expectEqual(@as(usize, 40), try s.read(1, 10, &buf));
    try testing.expectEqual(@as(usize, 0), s.client.pending());
}

test "rpc wait loop: a read interrupted after some data is a short read" {
    // Chunks of maxRead = 1013 (msize 1024); the third chunk (offset 2026) hangs
    // and the server fires the INTERRUPT at that moment.
    var pi = try PipeInterrupt.init(9);
    defer pi.deinit();
    var fs: FakeServer = .{ .fd = -1, .msize = 1024, .file_len = 5000, .hang_offset = 2026, .on_flush = .rflush, .on_hang_inject = &pi };
    var pair = try fs.start();
    defer pair.close();
    const s = &pair.session;
    s.interrupt = pi.interface();
    defer s.interrupt = null;

    var buf: [4000]u8 = undefined;
    try testing.expectEqual(@as(usize, 2026), try s.read(1, 0, &buf));
    for (buf[0..2026], 0..) |b, i| try testing.expectEqual(@as(u8, @truncate(i)), b);
    try testing.expect(pi.armed_flag);
    try testing.expectEqual(@as(u32, 1), fs.flushes.load(.seq_cst));
    try testing.expectEqual(@as(usize, 0), s.client.pending());

    // An INTERRUPT already waiting when the read starts: the first chunk's
    // reply races the flush and wins, the armed flag then stops the loop.
    pi.armed_flag = false;
    try pi.inject(9);
    try testing.expectEqual(@as(usize, 1013), try s.read(1, 0, &buf));
    // The stray Rflush is consumed by the next call.
    _ = try s.stat(1);
    try testing.expectEqual(@as(usize, 0), s.client.pending());
}

test "session against an in-process cloud9.Server" {
    var fds: [2]i32 = undefined;
    try testing.expectEqual(linux.E.SUCCESS, linux.errno(linux.socketpair(linux.AF.UNIX, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, 0, &fds)));

    var fs: FakeServer = .{ .fd = fds[1], .msize = 8192, .file_len = 20_000 };
    const th = try std.Thread.spawn(.{}, FakeServer.run, .{&fs});

    var s = try Session.connect(testing.allocator, .{ .fd = fds[0] }, 8192);
    defer {
        s.deinit();
        th.join();
    }
    try testing.expectEqual(@as(u32, 8192), s.msize);

    const root = try s.attach(0, "me", "");
    try testing.expectEqual(FakeServer.dir_qid.path, root.path);

    // Plain rpc + stat borrowing the input buffer.
    const fid = s.allocFid();
    const w = try s.walk(0, fid, &.{"file"});
    try testing.expectEqual(@as(u16, 1), w.nwqid);
    try testing.expectEqual(FakeServer.file_qid.path, w.wqid[0].path);
    const st = try s.stat(fid);
    try testing.expectEqualStrings("file", st.name);
    try testing.expectEqual(@as(u64, 20_000), st.length);

    // Chunked read: 20000 bytes at maxRead = msize - 11 = 8181 per chunk.
    _ = try s.open(fid, cloud9.oread);
    const buf = try testing.allocator.alloc(u8, 30_000);
    defer testing.allocator.free(buf);
    const n = try s.read(fid, 0, buf);
    try testing.expectEqual(@as(usize, 20_000), n);
    for (buf[0..n], 0..) |b, i| try testing.expectEqual(@as(u8, @truncate(i)), b);
    try testing.expectEqual(@as(u32, 8181), fs.max_read_count);
    try testing.expectEqual(@as(usize, 0), try s.read(fid, 20_000, buf));

    // iounit from open bounds the chunk.
    const wfid = try s.clone(fid);
    _ = try s.open(wfid, cloud9.owrite);
    fs.max_read_count = 0;
    _ = try s.read(wfid, 0, buf[0..3000]);
    try testing.expectEqual(@as(u32, 700), fs.max_read_count);
    try testing.expectEqual(@as(usize, 3000), try s.write(wfid, 0, buf[0..3000]));

    // Partial walk → error.Nine with a "not exist" ename → ENOENT.
    const pfid = s.allocFid();
    try testing.expectError(error.Nine, s.walk(0, pfid, &.{ "dir", "nope" }));
    try testing.expectEqual(linux.E.NOENT, s.errno());
    try testing.expectEqualStrings("file does not exist", s.ename[0..s.ename_len]);
    s.freeFid(pfid);

    // Server Rerror → error.Nine, ename copied, fid freed by remove even on error.
    try testing.expectError(error.Nine, s.remove(wfid));
    try testing.expectEqual(linux.E.ACCES, s.errno());
    try testing.expectEqual(wfid, s.allocFid()); // recycled
    s.freeFid(wfid);

    // Unsupported op → "not supported" → ENOTSUP; a plain wstat succeeds.
    try testing.expectError(error.Nine, s.rpc(.{ .auth = .{ .afid = 5, .uname = "me" } }));
    try testing.expectEqual(linux.E.OPNOTSUPP, s.errno());
    try s.wstat(fid, dontcare);
    try s.clunk(fid);
    try testing.expectEqual(fid, s.allocFid());
    s.freeFid(fid);

    // A clone bound to a fid that then fails to walk must release the fid.
    const before = s.next_fid;
    const cfid = s.allocFid();
    s.freeFid(cfid);
    try testing.expectError(error.Nine, s.walk(0, cfid, &.{"nope"}));
    try testing.expectEqual(before, s.next_fid);

    // The server hanging up makes the pending rpc fail with error.Closed.
    try testing.expectError(error.Closed, s.rpc(.{ .flush = .{ .oldtag = 0 } }));
}