const std = @import("std");
const libc = std.c;
const builtin = @import("builtin");
const ninep = @import("9p.zig");
const cloud9 = @import("cloud9");
const transport = cloud9.transport;
const pardes = @import("pardes.zig");
const limits = @import("memory.zig").limits;
pub const quic_enabled = @import("9p_options").quic;
const quic = if (quic_enabled) @import("9p_quic.zig") else struct {};

const log = std.log.scoped(.ninep);

pub const darwin = switch (builtin.os.tag) {
    .macos, .ios, .tvos, .watchos, .visionos => true,
    else => false,
};
pub const supported = builtin.os.tag == .linux or darwin;
pub const sun_path_len = @typeInfo(@FieldType(libc.sockaddr.un, "path")).array.len;

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

pub fn socketDir(buf: *[sun_path_len:0]u8) ?[:0]const u8 {
    if (libc.getenv("XDG_RUNTIME_DIR")) |path|
        return std.fmt.bufPrintSentinel(buf, "{s}", .{std.mem.span(path)}, 0) catch null;
    const home = libc.getenv("HOME") orelse return null;
    return std.fmt.bufPrintSentinel(buf, "{s}/.local/state/pardes", .{std.mem.span(home)}, 0) catch null;
}

pub const FileFacts = struct { mode: u32, uid: libc.uid_t };

pub fn statNoFollow(path: [:0]const u8) ?FileFacts {
    if (comptime darwin) {
        var stat: libc.Stat = undefined;
        if (libc.fstatat(libc.AT.FDCWD, path, &stat, libc.AT.SYMLINK_NOFOLLOW) != 0) return null;
        return .{ .mode = stat.mode, .uid = stat.uid };
    } else {
        const linux = std.os.linux;
        var stat: linux.Statx = undefined;
        const fields: linux.STATX = .{ .TYPE = true, .MODE = true, .UID = true };
        if (libc.statx(linux.AT.FDCWD, path, linux.AT.SYMLINK_NOFOLLOW, fields, &stat) != 0) return null;
        return .{ .mode = stat.mode, .uid = stat.uid };
    }
}

pub fn ensureSocketDir(dir: [:0]const u8) bool {
    if (dir.len == 0) return false;
    var partial: [sun_path_len:0]u8 = undefined;
    @memcpy(partial[0 .. dir.len + 1], dir[0 .. dir.len + 1]);
    for (1..dir.len) |i| {
        if (dir[i] != '/') continue;
        partial[i] = 0;
        _ = libc.mkdir(partial[0..i :0], 0o700);
        partial[i] = '/';
    }
    _ = libc.mkdir(dir, 0o700);
    const stat = statNoFollow(dir) orelse return false;
    return stat.mode & 0o170000 == 0o040000 and stat.uid == libc.getuid() and stat.mode & 0o077 == 0;
}

const prefix = "pardes-9p-";

pub const msize = ninep.msize;

/// Scripts, a mount and an agent or two at once; each slot holds its
/// buffers (a few msize) whether used or not.
pub const max_conns = 16;

extern "c" fn inet_pton(family: c_int, src: [*:0]const u8, dst: *anyopaque) c_int;

pub fn networkAddress(dial: []const u8, allow_zero_port: bool) error{BadDial}!std.Io.net.IpAddress {
    if (comptime !supported) return error.BadDial;
    const host_start: usize = if (std.mem.startsWith(u8, dial, "tcp!")) 4 else if (std.mem.startsWith(u8, dial, "quic!")) 5 else return error.BadDial;
    const split = std.mem.lastIndexOfScalar(u8, dial, '!') orelse return error.BadDial;
    if (split <= host_start or split + 1 == dial.len) return error.BadDial;
    const port_text = dial[split + 1 ..];
    for (port_text) |c| if (c < '0' or c > '9') return error.BadDial;
    const port = std.fmt.parseInt(u16, port_text, 10) catch return error.BadDial;
    if (port == 0 and !allow_zero_port) return error.BadDial;
    const host = dial[host_start..split];
    if (std.mem.indexOfScalar(u8, host, 0) != null) return error.BadDial;
    var host_buf: [46]u8 = undefined;
    const host_z = std.fmt.bufPrintSentinel(&host_buf, "{s}", .{host}, 0) catch return error.BadDial;
    var ip4: std.Io.net.Ip4Address = .{ .port = port, .bytes = undefined };
    if (inet_pton(libc.AF.INET, host_z, &ip4.bytes) == 1) return .{ .ip4 = ip4 };
    var ip6: std.Io.net.Ip6Address = .{ .port = port, .bytes = undefined };
    if (inet_pton(libc.AF.INET6, host_z, &ip6.bytes) == 1) return canonicalIp(.{ .ip6 = ip6 });
    return error.BadDial;
}

fn canonicalIp(address: std.Io.net.IpAddress) std.Io.net.IpAddress {
    if (address == .ip6) {
        if (std.Io.net.Ip4Address.fromIp6(address.ip6)) |ip4| return .{ .ip4 = ip4 };
    }
    return address;
}

fn sockaddrIp(address: *const libc.sockaddr) ?std.Io.net.IpAddress {
    return switch (address.family) {
        libc.AF.INET => blk: {
            const addr: *const libc.sockaddr.in = @ptrCast(@alignCast(address));
            break :blk .{ .ip4 = .{ .port = std.mem.bigToNative(u16, addr.port), .bytes = @bitCast(addr.addr) } };
        },
        libc.AF.INET6 => blk: {
            const addr: *const libc.sockaddr.in6 = @ptrCast(@alignCast(address));
            break :blk canonicalIp(.{ .ip6 = .{ .port = std.mem.bigToNative(u16, addr.port), .bytes = addr.addr } });
        },
        else => null,
    };
}

const IfAddr = extern struct {
    next: ?*IfAddr,
    name: ?[*:0]u8,
    flags: c_uint,
    address: ?*libc.sockaddr,
    netmask: ?*libc.sockaddr,
    destination: ?*libc.sockaddr,
    data: ?*anyopaque,
};

extern "c" fn getifaddrs(out: *?*IfAddr) c_int;
extern "c" fn freeifaddrs(first: *IfAddr) void;

fn localIp(address: std.Io.net.IpAddress) bool {
    switch (address) {
        .ip4 => |ip| if (ip.bytes[0] == 127 or std.mem.allEqual(u8, &ip.bytes, 0)) return true,
        .ip6 => |ip| if (ip.isLoopBack() or std.mem.allEqual(u8, &ip.bytes, 0)) return true,
    }
    var first: ?*IfAddr = null;
    if (getifaddrs(&first) != 0) return false;
    defer if (first) |head| freeifaddrs(head);
    var next = first;
    while (next) |entry| : (next = entry.next) {
        var local = sockaddrIp(entry.address orelse continue) orelse continue;
        local.setPort(address.getPort());
        if (address.eql(&local)) return true;
    }
    return false;
}

const Srv = ninep.Server(pardes.ctlfs, ninep.editor);

/// The Unix and TCP listeners run on cloud9's `std.Io` runner: it accepts,
/// reads frames, steps each connection's engine and writes replies on its
/// own tasks, and every backend request is answered right there, on the
/// connection's task, by taking the editor's turn (`pardes.turn`): while the
/// editor waits for input, or while a step of it is out in a syscall --
/// which is what lets the editor read its own tree through a mount. QUIC
/// keeps the poll loop below, on the editor's thread.
const Runner = if (supported) cloud9.serve.Runner(pardes.ctlfs, ninep.editor, .{
    .msize = msize,
    .connections = max_conns,
    .listeners = 2,
}) else void;

const quic_slots = if (quic_enabled) max_conns else 0;

/// A QUIC connection on the poll path.
const Conn = struct {
    quic: if (quic_enabled) ?quic.Connection else void = if (quic_enabled) null else {},
    draining: bool = false,
    accepted_ms: i64 = 0,
    srv: Srv = undefined,
    in: [msize]u8 = undefined,
    out: [2 * msize]u8 = undefined,
};

const accept_pause_ms: i64 = 100;

pub const Listener = struct {
    io: std.Io,
    /// The core that answers; `reset` puts a replacement in.
    core: *pardes.Pardes,
    runner: Runner = undefined,
    stopping: std.atomic.Value(bool) = .init(false),
    /// A connection wrote the Restore and has yet to answer it.
    restore_writer: std.atomic.Value(bool) = .init(false),
    /// Clients turned away with every slot taken, since the log last said so.
    refused: std.atomic.Value(u32) = .init(0),
    /// Connections accepted so far, and the count each slot's connection
    /// was accepted at: a Restore cuts those accepted before it (`reset`),
    /// and a new client in a freed slot has a later stamp. Written on the
    /// connection's task as it opens, read with the turn.
    accepts: std.atomic.Value(u64) = .init(0),
    accepted: [max_conns]std.atomic.Value(u64) = @splat(.init(0)),
    /// Connections accepted before this count are being cut by a Restore.
    cut_before: u64 = 0,
    tcp_address: ?std.Io.net.IpAddress = null,
    quic: if (quic_enabled) ?quic.Listener else void = if (quic_enabled) null else {},
    quic_address: ?std.Io.net.IpAddress = null,
    paused_ms: i64 = 0,
    path_buf: [sun_path_len]u8 = undefined,
    path_len: usize = 0,
    /// The registry entry this editor posted
    /// (`$XDG_RUNTIME_DIR/9p/pardes/<name>`), zero when it did not.
    posted_buf: [sun_path_len]u8 = undefined,
    posted_len: usize = 0,
    conns: [quic_slots]Conn = @splat(.{}),
    control: [2]c_int = .{ -1, -1 },
    watcher: ?std.Thread = null,
    watch_stop: std.atomic.Value(bool) = .init(false),
    watch_lock: std.atomic.Mutex = .unlocked,
    watch_fds: [2]libc.pollfd = undefined,
    watch_len: usize = 0,
    watch_timeout: c_int = -1,
    wake_ctx: ?*anyopaque = null,
    wake: ?*const fn (?*anyopaque) void = null,

    pub fn path(l: *const Listener) []const u8 {
        return l.path_buf[0..l.path_len];
    }

    fn of(ctx: ?*anyopaque) *Listener {
        return @ptrCast(@alignCast(ctx.?));
    }

    /// The runner's handler, on the connection's task: takes the turn and
    /// answers. A request that would change a pane while the editor is out
    /// in a syscall mid-step is parked in the engine instead, and retried
    /// when the turn is next given up between steps (`wakeParked`).
    /// On the accepting task, without the turn: counted, and logged with
    /// the turn (`answerHeld`).
    fn onRefused(ctx: ?*anyopaque) void {
        const l = of(ctx);
        _ = l.refused.fetchAdd(1, .acq_rel);
        l.kick();
    }

    fn onOpened(ctx: ?*anyopaque, conn: *Runner.Conn) void {
        const l = of(ctx);
        l.accepted[conn.index].store(l.accepts.fetchAdd(1, .acq_rel) + 1, .release);
    }

    /// A live connection accepted before the Restore under way.
    fn cutting(l: *Listener, conn: *Runner.Conn) bool {
        return conn.live() and l.accepted[conn.index].load(.acquire) <= l.cut_before and l.cut_before != 0;
    }

    fn onServe(ctx: ?*anyopaque, conn: *Runner.Conn, req: pardes.ctlfs.Req) void {
        const l = of(ctx);
        // Once `stop` has begun the editor is tearing down and may never
        // rest again: answer without waiting on it.
        if (l.stopping.load(.acquire)) {
            const refused = pardes.ctlfs.Reply.fail(req.tag, pardes.ctlfs.E.IO);
            return conn.reply(&refused, "");
        }
        const quiet = pardes.turn.take();
        defer pardes.turn.give();
        // A connection a Restore is cutting (`reset`) is the old editor's
        // client: its fids name the old editor's panes and opens.
        if (l.cutting(conn)) {
            const refused = pardes.ctlfs.Reply.fail(req.tag, pardes.ctlfs.E.IO);
            return conn.reply(&refused, "");
        }
        if (!quiet and pardes.ctlfs.needsQuiet(l.core, req)) {
            pardes.turn.parked = true;
            const later: pardes.ctlfs.Reply = .{ .tag = req.tag, .status = .again };
            return conn.reply(&later, "");
        }
        const core = l.core;
        core.fs.write_room = if (conn.engine.protocol.msize > 24) conn.engine.protocol.msize - 24 else 0;
        const epoch = pardes.turn.epoch;
        const restores = pardes.turn.restores;
        // Asked first: a release that runs a held line has none after.
        const changes = pardes.ctlfs.changesPane(core, req);
        const reply = core.serveFs(req);
        if (req.op == .release and quiet) l.collectOs();
        // A read with nothing yet stays parked in the engine, and the core
        // keeps its ticket to answer it when what it waits on has something.
        if (reply.status == .again and req.op == .read) {
            // Only an open's record can hold a read; one that waits with
            // none would sit parked until some unrelated write parks.
            const o = pardes.ctlfs.openOf(core, req) orelse {
                log.err("a read of node {x} waits with no open record to hold it", .{req.node});
                std.debug.assert(false);
                return conn.reply(&reply, "");
            };
            // One read waits on an open at a time, as acme's window keeps
            // one `eventx`; a second is refused rather than left parked
            // where nothing would ever answer it. The first asked again by
            // a retry is the same request, and holds its place.
            if (o.held) |held| if (held.req.tag != req.tag) {
                const other: *Runner.Conn = @ptrCast(@alignCast(held.asker));
                if (other.waiting(held.ticket))
                    return conn.reply(&pardes.ctlfs.failText(req.tag, pardes.ctlfs.E.BUSY, pardes.ctlfs.e_in_use), "");
            };
            o.held = null;
            const ticket = conn.hold(&reply) orelse return;
            o.held = .{ .asker = conn, .req = req, .ticket = ticket };
            return;
        }
        if (!changes) return conn.reply(&reply, core.fsPayload(reply));
        // The editor draws the change and performs what it asked for; when
        // it asked for something -- a save, a shell, a watch -- the answer
        // waits until that is done, so `echo Save > exec` returns with the
        // file written. A change carries no payload, so the reply is still
        // whole after the wait.
        l.kick();
        // A refusal answers at once: its text may be in the core's one
        // buffer for it, which a request run while this one waited would
        // write over.
        if (reply.status == .err) return conn.reply(&reply, "");
        // A write that quits the editor (Kill) is answered now: the editor
        // is on its way out and will settle nothing this could wait for,
        // and `deinit` lets the answer out before it cuts the connections.
        if (core.quit) return conn.reply(&reply, "");
        const restoring = core.restore_req != null;
        // Set with the turn still held, so `reset` sees it and waits.
        if (restoring) l.restore_writer.store(true, .release);
        defer if (restoring) l.restore_writer.store(false, .release);
        core.fs.late_failure_len = 0;
        // What fails while this waits is this write's err, never a msg too.
        core.fs.write_waits = true;
        if (core.effects_len != 0) pardes.turn.awaitSettled(epoch);
        if (core.fs.lsp_answer_at) |n| {
            core.fs.lsp_answer_at = null;
            pardes.turn.awaitLsp(n);
        }
        core.fs.write_waits = false;
        // ponytail: one slot, so a failure of another client's effects that
        // settle in the same wait is told to this write too.
        if (l.core == core and core.fs.late_failure_len != 0) {
            const late = core.fs.late_failure[0..core.fs.late_failure_len];
            // What is not there (`no such directory`) is ENOENT, as a
            // builtin's failure saying so is (ctl.failureErrno).
            const errno = if (std.mem.indexOf(u8, late, "no such") != null or std.mem.indexOf(u8, late, "not found") != null)
                pardes.ctlfs.E.NOENT
            else if (std.mem.indexOf(u8, late, "no space") != null) pardes.ctlfs.E.NOSPC else pardes.ctlfs.E.IO;
            const failed = pardes.ctlfs.failText(req.tag, errno, late);
            // Its err record says it (the path in it), the one record: it
            // was said with no msg while this waited (fs.write_waits).
            pardes.ctlfs.events.noteError(core, req, failed);
            return conn.reply(&failed, "");
        }
        // The Restore's own write is answered once it is done, and before
        // `reset` hangs every connection up, this one too, so the writer
        // hears that it happened rather than a cut; a failed one leaves the
        // connection and says why on the message row and in the log.
        if (restoring) {
            if (l.core == core) pardes.turn.awaitRestored(restores);
            return conn.reply(&reply, "");
        }
        // Once a wait returns `core` may be gone: a Restore meanwhile put a
        // replacement in and is hanging this connection up.
        if (l.core != core) return;
        conn.reply(&reply, "");
    }

    /// With the turn, as it is given up: answers each read the core holds
    /// whose file now has something, on the connection that asked, with no
    /// retry of anything else parked there. Only while its ticket still
    /// waits, asked in the same hold of the engine as the answer: Tflush
    /// and clunk drop a park without a word to the backend, a `retry` takes
    /// one out to ask again (and that asking answers it), and a record spent
    /// on any of them would be lost to the reader that comes next.
    fn answerHeld(ctx: ?*anyopaque) void {
        if (comptime !supported) return;
        const l = of(ctx);
        if (l.stopping.load(.acquire)) return;
        const core = l.core;
        const refused = l.refused.swap(0, .acq_rel);
        if (refused != 0) {
            var why: [64]u8 = undefined;
            pardes.ctlfs.events.notePath(core, "err -", std.fmt.bufPrint(&why, "9p: too many connections ({d} turned away)", .{refused}) catch "9p: too many connections");
        }
        if (!core.fs.news) return;
        core.fs.news = false;
        for (&core.fs.opens) |*o| {
            const held = o.held orelse continue;
            const conn: *Runner.Conn = @ptrCast(@alignCast(held.asker));
            if (!conn.answerWith(held.ticket, Held{ .core = core, .req = held.req }, Held.make)) o.held = null;
        }
    }

    /// A held read asked again, to make its answer while its park waits.
    const Held = struct {
        core: *pardes.Pardes,
        req: pardes.ctlfs.Req,

        fn make(h: Held) ?Runner.Conn.Answer {
            const reply = h.core.serveFs(h.req);
            if (reply.status == .again) return null;
            return .{ .reply = reply, .bytes = h.core.fsPayload(reply) };
        }
    };

    /// The turn was given up quiet with a request parked: every connection
    /// retries what it parked.
    fn wakeParked(ctx: ?*anyopaque) void {
        const l = of(ctx);
        if (l.stopping.load(.acquire)) return;
        if (comptime supported) l.runner.wakeAll();
    }

    /// A request changed the core: the editor wakes to draw it and perform
    /// what it asked for.
    fn kick(l: *Listener) void {
        if (l.stopping.load(.acquire)) return;
        if (l.wake) |f| f(l.wake_ctx);
    }

    /// Accepts and reads QUIC connections (the runner does its own).
    pub fn accept(l: *Listener) void {
        if (comptime !quic_enabled) return;
        if (l.quic) |*listener| for (0..max_conns + 1) |_| {
            var connection = (listener.accept() catch |err| {
                log.warn("QUIC accept failed: {s}", .{@errorName(err)});
                return;
            }) orelse break;
            const c = for (&l.conns, 0..) |*cand, i| {
                if (!l.live(@intCast(i)) and !cand.draining) break cand;
            } else {
                connection.deinit();
                continue;
            };
            c.quic = connection;
            c.draining = false;
            c.accepted_ms = nowMs();
            c.srv = .init(.{ .in = &c.in, .out = &c.out, .root = pardes.ctlfs.root });
        };
    }

    pub const greet_deadline_ms: i64 = 5000;

    pub fn expire(l: *Listener) void {
        if (comptime !quic_enabled) return;
        const now = nowMs();
        if (now == 0) return;
        for (&l.conns, 0..) |*c, i| {
            if (!l.live(@intCast(i)) or c.srv.protocol.msize != 0) continue;
            if (now - c.accepted_ms < greet_deadline_ms) continue;
            log.debug("QUIC slot {d} never sent Tversion; taking it back", .{i});
            l.drop(@intCast(i));
        }
    }

    pub fn nextDue(l: *const Listener) ?i32 {
        if (comptime !quic_enabled) return null;
        const now = nowMs();
        if (now == 0) return null;
        var due: ?i64 = null;
        if (l.quic) |*listener| if (listener.nextDue()) |ms| {
            const at = now + ms;
            due = if (due) |d| @min(d, at) else at;
        };
        for (&l.conns, 0..) |*c, i| {
            if (!l.live(@intCast(i))) continue;
            if (c.quic) |*connection| if (connection.pending()) return 0;
            if (c.srv.protocol.msize != 0) continue;
            const at = c.accepted_ms + greet_deadline_ms;
            due = if (due) |d| @min(d, at) else at;
        }
        const at = due orelse return null;
        return @intCast(@max(0, at - now));
    }

    const nowMs = transport.nowMs;

    pub fn fill(l: *Listener, i: u8) void {
        if (comptime !quic_enabled) return;
        const c = &l.conns[i];
        if (c.srv.protocol.dead) return l.drop(i);
        const room = c.srv.protocol.in.len - c.srv.protocol.in_len;
        if (room == 0) return;
        var buf: [msize]u8 = undefined;
        const got = (c.quic.?.read(buf[0..@min(room, buf.len)]) catch return l.drop(i)) orelse return;
        if (got == 0) return l.drop(i);
        const n = c.srv.push(buf[0..@intCast(got)]);
        if (c.srv.protocol.dead) return l.drop(i);
        std.debug.assert(n == @as(usize, @intCast(got)));
    }

    pub fn flush(l: *Listener, i: u8) void {
        if (comptime !quic_enabled) return;
        const c = &l.conns[i];
        if (!l.live(i)) return;
        while (true) {
            const bytes = c.srv.output();
            if (bytes.len == 0) return;
            const n = c.quic.?.write(bytes) catch return l.drop(i);
            if (n == 0) return;
            c.srv.wrote(@intCast(n));
        }
    }

    pub fn live(l: *const Listener, i: u8) bool {
        if (comptime !quic_enabled) return false;
        return l.conns[i].quic != null;
    }

    pub fn drop(l: *Listener, i: u8) void {
        if (comptime !quic_enabled) return;
        const c = &l.conns[i];
        if (c.quic) |*connection| {
            connection.deinit();
            c.quic = null;
        }
        if (c.draining or c.accepted_ms == 0) return;
        c.srv.hangup();
        c.draining = true;
    }

    /// What one QUIC poll pass did: `pending` says there is more to do at
    /// once.
    pub const Polled = struct { count: usize = 0, pending: bool = false };

    /// Answers one QUIC request, on the editor's thread.
    fn step(l: *Listener, srv: *Srv, req: pardes.ctlfs.Req) void {
        l.core.fs.write_room = if (srv.protocol.msize > 24) srv.protocol.msize - 24 else 0;
        const reply = l.core.serveFs(req);
        srv.reply(&reply, l.core.fsPayload(reply));
        if (req.op == .release) l.collectOs();
    }

    /// QUIC's events, accepts, reads, requests and replies, on the editor's
    /// thread after each of its steps; `pending` says there is more to do
    /// right away. Unix and TCP need none of this.
    pub fn tick(l: *Listener) Polled {
        var result: Polled = .{};
        // Between the editor's steps, which is when the table may be swept:
        // the editor's own directory listings leave nodes in it too.
        l.collectOs();
        if (comptime quic_enabled) {
            if (l.quic) |*listener| listener.events() catch |err| {
                log.warn("QUIC listener stopped: {s}", .{@errorName(err)});
                for (&l.conns, 0..) |*conn, i| if (conn.quic != null) l.drop(@intCast(i));
                listener.deinit();
                l.quic = null;
                l.quic_address = null;
            };
            l.accept();
            for (0..quic_slots) |i| if (l.live(@intCast(i))) l.fill(@intCast(i));
            l.expire();
            for (&l.conns, 0..) |*conn, i| {
                if (!l.live(@intCast(i)) and !conn.draining) continue;
                var count: usize = 0;
                while (conn.srv.retry()) |req| {
                    l.step(&conn.srv, req);
                    count += 1;
                }
                while (count < 64) {
                    const req = conn.srv.next() orelse break;
                    l.step(&conn.srv, req);
                    count += 1;
                }
                result.count += count;
                result.pending = result.pending or count >= 64;
                if (conn.draining) {
                    if (count == 0) conn.draining = false else result.pending = true;
                } else l.flush(@intCast(i));
                if (conn.quic) |*connection| result.pending = result.pending or connection.pending();
            }
            l.arm();
        }
        return result;
    }

    /// Hangs every connection up, so that `replacement` starts with no
    /// client holding anything. From here requests are answered from the
    /// replacement -- a client that connects meanwhile is a client of the
    /// replacement -- and every task waiting on the old core is let go: the
    /// Restore's own writer answers, the rest find the core changed and
    /// answer nothing, their connections being hung up. The connections pay their releases on their own tasks, so
    /// the editor rests while they do.
    pub fn reset(l: *Listener, replacement: *pardes.Pardes) void {
        for (0..quic_slots) |i| l.drop(@intCast(i));
        if (comptime quic_enabled) while (l.tick().pending) {};
        l.core = replacement;
        replacement.fs.socket_path = l.path();
        replacement.fs.tcp_address = l.tcp_address;
        replacement.fs.quic_address = l.quic_address;
        if (comptime supported) {
            // The Restore's writer answers as it wakes (`onServe`), and its
            // answer gets 200 ms to leave before the cut; every other request
            // from the old editor's clients meanwhile is refused, since the
            // fids it names are the old editor's. A client that connects now
            // is the replacement's, and stays.
            l.cut_before = l.accepts.load(.acquire);
            pardes.turn.settle();
            pardes.turn.restoreSettled();
            pardes.turn.rest();
            const flush_by = nowMs() +| 200;
            while (nowMs() < flush_by) {
                const pending = l.restore_writer.load(.acquire) or for (&l.runner.conns) |*conn| {
                    if (!l.cutting(conn)) continue;
                    conn.lock();
                    const n = conn.engine.output().len;
                    conn.unlock();
                    if (n != 0) break true;
                } else false;
                if (!pending) break;
                Client.nap(1);
            }
            for (&l.runner.conns) |*conn| if (l.cutting(conn)) conn.close();
            const deadline = nowMs() +| 2000;
            while (nowMs() < deadline) {
                const left = for (&l.runner.conns) |*conn| {
                    if (l.cutting(conn)) break true;
                } else false;
                if (!left) break;
                Client.nap(1);
            }
            pardes.turn.wake();
            l.cut_before = 0;
        }
        l.collectOs();
        for (&l.conns) |*conn| conn.accepted_ms = 0;
        l.arm();
    }

    /// Forgets the host paths no connection names any more. With the turn,
    /// and only between the editor's steps: a step out in a syscall may be
    /// reading one of these entries.
    fn collectOs(l: *Listener) void {
        const core = l.core;
        var i: usize = 0;
        while (i < core.fs.os_paths.items.len) {
            const entry = core.fs.os_paths.items[i];
            if (l.references(entry.node)) {
                i += 1;
            } else {
                core.gpa.free(entry.path);
                _ = core.fs.os_paths.swapRemove(i);
            }
        }
    }

    /// Whether any connection names `node`.
    fn references(l: *Listener, node: u64) bool {
        if (comptime supported) {
            for (&l.runner.conns) |*conn| {
                if (!conn.live()) continue;
                conn.lock();
                defer conn.unlock();
                if (conn.engine.references(node)) return true;
            }
        }
        for (&l.conns, 0..) |*conn, j| {
            if (!l.live(@intCast(j)) and !conn.draining) continue;
            if (conn.srv.references(node)) return true;
        }
        return false;
    }

    /// Where the runner's tasks report that a request changed the core,
    /// and for QUIC a thread that watches its socket and calls `wake` when
    /// `tick` has something to read.
    pub fn watch(l: *Listener, ctx: ?*anyopaque, wake: *const fn (?*anyopaque) void) !void {
        l.wake_ctx = ctx;
        l.wake = wake;
        if (comptime !quic_enabled) return;
        if (l.quic == null or l.watcher != null) return;
        if (libc.pipe(&l.control) != 0) return error.PipeFailed;
        errdefer {
            _ = libc.close(l.control[0]);
            _ = libc.close(l.control[1]);
            l.control = .{ -1, -1 };
        }
        for (l.control) |fd| {
            setCloexec(fd);
            _ = setNonblock(fd);
        }
        l.arm();
        l.watcher = try std.Thread.spawn(.{}, watchQuic, .{l});
    }

    fn arm(l: *Listener) void {
        if (comptime !quic_enabled) return;
        if (l.control[1] < 0) return;
        while (!l.watch_lock.tryLock()) std.atomic.spinLoopHint();
        l.watch_fds[0] = .{ .fd = l.control[0], .events = @intCast(libc.POLL.IN), .revents = 0 };
        l.watch_len = 1;
        if (l.quic) |*listener| {
            l.watch_fds[l.watch_len] = listener.poll();
            l.watch_len += 1;
        }
        l.watch_timeout = l.nextDue() orelse -1;
        l.watch_lock.unlock();
        _ = libc.write(l.control[1], "w", 1);
    }

    fn watchQuic(l: *Listener) void {
        var notified = false;
        while (!l.watch_stop.load(.acquire)) {
            var fds: [2]libc.pollfd = undefined;
            while (!l.watch_lock.tryLock()) std.atomic.spinLoopHint();
            const len = if (notified) 1 else l.watch_len;
            @memcpy(fds[0..len], l.watch_fds[0..len]);
            const timeout = if (notified) -1 else l.watch_timeout;
            l.watch_lock.unlock();
            const ready = libc.poll(&fds, @intCast(len), timeout);
            if (ready < 0) continue;
            if (fds[0].revents != 0) {
                var buf: [64]u8 = undefined;
                while (libc.read(l.control[0], &buf, buf.len) > 0) {}
                notified = false;
                continue;
            }
            if (!l.watch_stop.load(.acquire)) l.wake.?(l.wake_ctx);
            notified = true;
        }
    }

    /// Advertises this editor in the posted-9P registry so a
    /// `9ns --mntgen` mount lists it and dials it on a walk:
    /// `$XDG_RUNTIME_DIR/9p/pardes/<name>` is a symlink to the socket
    /// pardes already binds. One directory for the program, one entry
    /// per running editor — the layout zmx uses for its sessions, so
    /// several editors group instead of crowding the registry root.
    ///
    /// Only the runtime-directory socket posts: an instance that fell
    /// back to `~/.local/state/pardes` stays out of the user's
    /// registry, the way a private `ZMX_DIR` does for zmx.
    ///
    /// The socket itself is still bound by `listen` above, not by
    /// `cloud9.post`: `post` claims flat names only, and its claim
    /// protocol derives its lock directory from the registry path, so
    /// a name inside a subdirectory cannot go through it yet. See
    /// docs/cloud9.md.
    fn postToRegistry(l: *Listener, name: []const u8) void {
        if (comptime !supported) return;
        if (name.len == 0 or std.mem.indexOfAny(u8, name, "/\x00") != null) return;
        const xdg_c = libc.getenv("XDG_RUNTIME_DIR") orelse return;
        const xdg = std.mem.span(xdg_c);
        if (xdg.len == 0) return;

        var reg_buf: [sun_path_len:0]u8 = undefined;
        const reg = std.fmt.bufPrintSentinel(&reg_buf, "{s}/9p", .{xdg}, 0) catch return;
        _ = libc.mkdir(reg, 0o750);
        var svc_buf: [sun_path_len:0]u8 = undefined;
        const svc = std.fmt.bufPrintSentinel(&svc_buf, "{s}/9p/pardes", .{xdg}, 0) catch return;
        if (libc.mkdir(svc, 0o750) != 0 and statNoFollow(svc) == null) {
            log.warn("registry post skipped: cannot create {s}", .{svc});
            return;
        }
        sweepRegistry(l.io, svc, xdg);
        var entry_buf: [sun_path_len:0]u8 = undefined;
        const entry = std.fmt.bufPrintSentinel(&entry_buf, "{s}/{s}", .{ svc, name }, 0) catch {
            log.warn("registry post skipped: name too long: {s}", .{name});
            return;
        };
        const target = l.path_buf[0..l.path_len];

        // Replace only what is provably not live: our own entry, or a
        // dead predecessor's symlink. `probe` treats uncertainty as
        // live, so it is only asked about an entry that *is* a symlink;
        // anything else is left strictly alone and the symlink below
        // simply fails.
        var link_buf: [sun_path_len]u8 = undefined;
        const n = libc.readlink(entry, &link_buf, link_buf.len);
        if (n >= 0) {
            const had = link_buf[0..@intCast(n)];
            if (!std.mem.eql(u8, had, target) and probe(entry) == .live) {
                log.warn("registry entry pardes/{s} is live; not re-posted", .{name});
                return;
            }
            _ = libc.unlink(entry);
        }
        var target_z: [sun_path_len:0]u8 = undefined;
        const target_zs = std.fmt.bufPrintSentinel(&target_z, "{s}", .{target}, 0) catch return;
        if (libc.symlink(target_zs, entry) != 0) {
            log.warn("registry post skipped: pardes/{s} is occupied", .{name});
            return;
        }
        @memcpy(l.posted_buf[0..entry.len], entry);
        l.posted_len = entry.len;
        log.info("posted pardes/{s} -> {s}", .{ name, target });
    }

    /// Removes the registry entry, but only while it is still ours: a
    /// name another editor has since claimed is never unlinked.
    fn unpostFromRegistry(l: *Listener) void {
        if (comptime !supported) return;
        if (l.posted_len == 0) return;
        var z: [sun_path_len:0]u8 = undefined;
        @memcpy(z[0..l.posted_len], l.posted_buf[0..l.posted_len]);
        z[l.posted_len] = 0;
        const entry = z[0..l.posted_len :0];
        var link_buf: [sun_path_len]u8 = undefined;
        const n = libc.readlink(entry, &link_buf, link_buf.len);
        if (n >= 0 and std.mem.eql(u8, link_buf[0..@intCast(n)], l.path_buf[0..l.path_len])) {
            _ = libc.unlink(entry);
        }
        l.posted_len = 0;
    }

    pub fn deinit(l: *Listener, gpa: std.mem.Allocator) void {
        l.stopping.store(true, .release);
        l.unpostFromRegistry();
        if (l.watcher) |thread| {
            l.watch_stop.store(true, .release);
            _ = libc.write(l.control[1], "q", 1);
            thread.join();
            for (l.control) |fd| _ = libc.close(fd);
        }
        for (0..quic_slots) |i| l.drop(@intCast(i));
        if (comptime quic_enabled) {
            if (l.quic) |*listener| listener.deinit();
        }
        if (comptime supported) {
            // A connection task may be waiting for the turn; it answers
            // nothing more once stopping, but the wait itself must end.
            pardes.turn.rest();
            // An answer given as the editor quit (a Kill written to /ctl)
            // may still be on its way out: let it go before the cut.
            const deadline = nowMs() +| 200;
            while (nowMs() < deadline) {
                const pending = for (&l.runner.conns) |*conn| {
                    if (!conn.live()) continue;
                    conn.lock();
                    const n = conn.engine.output().len;
                    conn.unlock();
                    if (n != 0) break true;
                } else false;
                if (!pending) break;
                Client.nap(1);
            }
            Client.nap(1); // the last bytes leave the stream's own buffer
            l.runner.stop();
            pardes.turn.wake();
        }
        pardes.turn.wake_parked = null;
        pardes.turn.answer_held = null;
        pardes.turn.stop();
        if (l.path_len != 0) {
            var z: [sun_path_len:0]u8 = undefined;
            @memcpy(z[0..l.path_len], l.path_buf[0..l.path_len]);
            z[l.path_len] = 0;
            _ = libc.unlink(z[0..l.path_len :0]);
        }
        gpa.destroy(l);
    }
};

pub fn socketPath(buf: *[sun_path_len]u8, dir: []const u8, name: []const u8) ?[:0]const u8 {
    if (name.len == 0) return null;
    if (std.mem.indexOfAny(u8, name, "/\x00") != null) return null;
    return std.fmt.bufPrintSentinel(buf, "{s}/" ++ prefix ++ "{s}.sock", .{ dir, name }, 0) catch null;
}

/// Serves `core`. From here on the calling thread has the editor's turn
/// (`pardes.turn`) and must give it up while it waits.
pub fn listen(io: std.Io, gpa: std.mem.Allocator, core: *pardes.Pardes, named: []const u8, fallback: []const u8, tcp_dial: ?[]const u8, quic_dial: ?[]const u8) ?*Listener {
    if (comptime !supported) return null;
    var dir_buf: [sun_path_len:0]u8 = undefined;
    const dir = socketDir(&dir_buf) orelse {
        log.warn("no runtime directory for the socket", .{});
        return null;
    };
    if (!ensureSocketDir(dir)) return null;
    const l = gpa.create(Listener) catch return null;
    l.* = .{ .io = io, .core = core };
    pardes.turn.start(io);
    pardes.turn.wake_parked = Listener.wakeParked;
    pardes.turn.answer_held = Listener.answerHeld;
    pardes.turn.wake_ctx = l;
    l.runner.init(.{
        .io = io,
        .root = pardes.ctlfs.root,
        .handler = .{ .ctx = l, .serve = Listener.onServe, .opened = Listener.onOpened, .refused = Listener.onRefused },
        .greet_timeout_ms = Listener.greet_deadline_ms,
    });
    const entry_name = if (named.len != 0) named else fallback;
    const p = socketPath(&l.path_buf, dir, entry_name) orelse {
        gpa.destroy(l);
        return null;
    };
    _ = l.runner.listen(.{ .unix = p }, max_conns) catch |err| retry: {
        const existing = @import("fs.zig").statPath(io, p, .{ .follow_symlinks = false }) catch null;
        if (err != error.AddressInUse or existing == null or existing.?.kind != .unix_domain_socket or alive(p)) {
            log.warn("something is already listening on {s}", .{p});
            l.deinit(gpa);
            return null;
        }
        if (libc.unlink(p) != 0) {
            l.deinit(gpa);
            return null;
        }
        break :retry l.runner.listen(.{ .unix = p }, max_conns) catch {
            l.deinit(gpa);
            return null;
        };
    };
    l.path_len = p.len;
    if (libc.chmod(p, 0o600) != 0) {
        l.deinit(gpa);
        return null;
    }
    l.postToRegistry(entry_name);
    if (tcp_dial) |dial| {
        if (!std.mem.startsWith(u8, dial, "tcp!")) {
            l.deinit(gpa);
            return null;
        }
        const address = networkAddress(dial, true) catch {
            log.warn("invalid TCP address {s}", .{dial});
            l.deinit(gpa);
            return null;
        };
        const bound = l.runner.listen(.{ .tcp = address }, max_conns) catch |err| {
            log.warn("cannot listen on {s}: {s}", .{ dial, @errorName(err) });
            l.deinit(gpa);
            return null;
        };
        l.tcp_address = canonicalIp(bound);
        log.info("serving 9P2000 over TCP on {f}", .{l.tcp_address.?});
    }
    if (quic_dial) |dial| {
        if (comptime quic_enabled) {
            if (!std.mem.startsWith(u8, dial, "quic!")) {
                l.deinit(gpa);
                return null;
            }
            const address = networkAddress(dial, true) catch {
                log.warn("invalid QUIC address {s}", .{dial});
                l.deinit(gpa);
                return null;
            };
            l.quic = quic.Listener.init(address) catch |err| {
                log.warn("cannot listen on {s}: {s}", .{ dial, @errorName(err) });
                l.deinit(gpa);
                return null;
            };
            l.quic_address = l.quic.?.address;
            log.info("serving 9P2000 over QUIC on {f}", .{l.quic_address.?});
        } else {
            log.warn("QUIC is unavailable in this build", .{});
            l.deinit(gpa);
            return null;
        }
    }
    log.info("serving 9P2000 on {s}", .{p});
    core.fs.socket_path = l.path();
    core.fs.tcp_address = l.tcp_address;
    core.fs.quic_address = l.quic_address;
    return l;
}

const alive = transport.isListening;

/// What is at a socket path, in cloud9's three answers (`post.Probe`).
/// Asked here with libc rather than through `cloud9.post.probe`, whose
/// raw Linux syscalls darwin cannot compile, but the classification is
/// that file's and must not become a second opinion: the one definite
/// refusal is `.stale`, nothing at the path at all is `.none`, and
/// everything else — connected, busy, refused permission, a surprise —
/// is `.live`. Uncertainty belongs to the server that owns the socket,
/// never to a sweeper deciding what to delete.
const Probe = enum { none, stale, live };

/// `SOCK.STREAM`, and the kernel's own non-blocking bit where there is
/// one. Darwin's `SOCK.NONBLOCK` is a Zig shim for `std.posix.socket`
/// to unpack, not an ABI value, so handing it to the raw libc call
/// would ask a kernel that has never heard of it; there it is an
/// `fcntl` instead.
const probe_socket_kind: c_uint = libc.SOCK.STREAM | (if (darwin) 0 else libc.SOCK.NONBLOCK);

fn probe(path: [:0]const u8) Probe {
    if (path.len + 1 > sun_path_len) return .live; // cannot ask; assume occupied
    var addr: libc.sockaddr.un = .{ .path = @splat(0) };
    @memcpy(addr.path[0 .. path.len + 1], path[0 .. path.len + 1]);
    const fd = libc.socket(libc.AF.UNIX, probe_socket_kind, 0);
    if (fd < 0) return .live;
    defer _ = libc.close(fd);

    // Non-blocking is the whole safety of this function, so it is read
    // back rather than assumed: a BLOCKING connect to a live server
    // whose backlog is full parks in the kernel with no timeout to end
    // it — measured, it simply never returns — and a sweep that parks
    // takes the editor's startup with it. An fd that cannot be proven
    // non-blocking is never connected at all, which lands on `.live`,
    // the answer that deletes nothing.
    if (comptime darwin) _ = setNonblock(fd);
    if (!isNonblocking(fd)) return .live;

    if (libc.connect(fd, @ptrCast(&addr), @sizeOf(@TypeOf(addr))) == 0) return .live;
    // cloud9's `post.probe` mapping, answer for answer. The connection
    // is never wanted: EAGAIN (a full Unix backlog) and EINPROGRESS say
    // somebody is listening, which is all that was asked, and the fd
    // closes without ever waiting for it to become writable.
    return switch (libc.errno(-1)) {
        .AGAIN, .INPROGRESS, .PERM, .ACCES => .live,
        .CONNREFUSED => .stale,
        .NOENT, .NOTDIR => .none,
        else => .live,
    };
}

/// Reaps the registry entries whose editor is gone, once per post.
///
/// A clean exit unposts itself (`unpostFromRegistry`), so everything
/// left behind comes from an exit that could not: an aborted test, a
/// kill, a crash. No code in the dead process can ever run, so the cure
/// has to be somebody else's readdir, and the next editor to start is
/// the somebody. It is only ever a symlink that is followed or
/// unlinked, because that is the only thing `postToRegistry` makes and
/// anything else under a name belongs to whoever put it there; and only
/// a definite refusal counts as gone. The socket a stale entry points
/// at goes too, but not before a stat agrees it is a socket of ours: a
/// plain file answers a connect with the same refusal.
/// Is this the kind of path this editor is allowed to delete? A registry
/// entry is a symlink we wrote, but its target is just bytes on disk that
/// anyone could have pointed anywhere, so the sweep only ever follows one
/// into the directory our own sockets live in, and only to a name of the
/// shape we give them. Everything else gets its entry removed and its target
/// left strictly alone.
fn ourSocket(target: []const u8, sockets: []const u8) bool {
    if (sockets.len == 0 or !std.mem.startsWith(u8, target, sockets)) return false;
    if (target.len <= sockets.len or target[sockets.len] != '/') return false;
    const base = target[sockets.len + 1 ..];
    if (std.mem.indexOfScalar(u8, base, '/') != null) return false;
    return std.mem.startsWith(u8, base, "pardes-9p-") and std.mem.endsWith(u8, base, ".sock");
}

fn sweepRegistry(io: std.Io, svc: [:0]const u8, sockets: []const u8) void {
    if (comptime !supported) return;

    // The names are staged before anything is unlinked, so the sweep
    // never asks a directory to keep reading while it is being edited.
    var names: [4096]u8 = undefined;
    var staged: usize = 0;
    {
        const dir = std.Io.Dir.openDirAbsolute(io, svc, .{ .iterate = true }) catch return;
        defer dir.close(io);
        var read_buf: [std.Io.Dir.Iterator.reader_buffer_len]u8 align(@alignOf(usize)) = undefined;
        var reader: std.Io.Dir.Reader = .init(dir, &read_buf);
        while (true) {
            const listed = (reader.next(io) catch break) orelse break;
            if (listed.name.len == 0 or listed.name.len > 255) continue;
            if (staged + 1 + listed.name.len > names.len) break;
            names[staged] = @intCast(listed.name.len);
            @memcpy(names[staged + 1 ..][0..listed.name.len], listed.name);
            staged += 1 + listed.name.len;
        }
    }

    var reaped: usize = 0;
    var i: usize = 0;
    while (i < staged) {
        const name = names[i + 1 ..][0..names[i]];
        i += 1 + name.len;
        var entry_buf: [sun_path_len:0]u8 = undefined;
        const entry = std.fmt.bufPrintSentinel(&entry_buf, "{s}/{s}", .{ svc, name }, 0) catch continue;
        const facts = statNoFollow(entry) orelse continue;
        if (facts.mode & 0o170000 != 0o120000) continue; // not a symlink: not ours to judge
        const state = probe(entry);
        if (state == .live) continue;
        var link_buf: [sun_path_len]u8 = undefined;
        const n = libc.readlink(entry, &link_buf, link_buf.len);
        if (libc.unlink(entry) != 0) continue;
        reaped += 1;
        // A relative target would resolve against this editor's working
        // directory, which says nothing about what the entry named, and a
        // readlink that exactly filled the buffer was truncated, so the path
        // it produced is some other file's.
        if (state != .stale or n <= 0 or link_buf[0] != '/') continue;
        if (@as(usize, @intCast(n)) >= link_buf.len) continue;
        var target_buf: [sun_path_len:0]u8 = undefined;
        const target = std.fmt.bufPrintSentinel(&target_buf, "{s}", .{link_buf[0..@intCast(n)]}, 0) catch continue;
        if (!ourSocket(target, sockets)) continue;
        const t = statNoFollow(target) orelse continue;
        if (t.mode & 0o170000 != 0o140000 or t.uid != libc.getuid()) continue;
        // Ask again, immediately before deleting. The first probe was of the
        // ENTRY and is by now several syscalls old; a socket that is bound but
        // has not reached listen(2) yet answers ECONNREFUSED exactly like a
        // dead one, and that window is every server's startup.
        if (probe(target) != .stale) continue;
        _ = libc.unlink(target);
    }
    if (reaped != 0) log.info("reaped {d} stale registry entries under {s}", .{ reaped, svc });
}

/// Whether it worked, because `probe` is not allowed to find out the
/// hard way: a caller that needs the guarantee has to be able to check.
fn setNonblock(fd: c_int) bool {
    const flags = libc.fcntl(fd, libc.F.GETFL, @as(c_int, 0));
    if (flags < 0) return false;
    var o: libc.O = @bitCast(@as(u32, @bitCast(flags)));
    o.NONBLOCK = true;
    return libc.fcntl(fd, libc.F.SETFL, @as(c_int, @bitCast(@as(u32, @bitCast(o))))) >= 0;
}

fn isNonblocking(fd: c_int) bool {
    const flags = libc.fcntl(fd, libc.F.GETFL, @as(c_int, 0));
    if (flags < 0) return false;
    const o: libc.O = @bitCast(@as(u32, @bitCast(flags)));
    return o.NONBLOCK;
}

const testing = std.testing;

test "the socket name is a third prefix in the shared directory" {
    var buf: [sun_path_len]u8 = undefined;
    const p = socketPath(&buf, "/run/user/1000", "t9srv").?;
    try testing.expectEqualStrings("/run/user/1000/pardes-9p-t9srv.sock", p);
    try testing.expect(!std.mem.startsWith(u8, std.fs.path.basename(p), "pardes-detached-"));
}

test "a name that is not one path component is no address at all" {
    var buf: [sun_path_len]u8 = undefined;
    try testing.expect(socketPath(&buf, "/run", "") == null);
    try testing.expect(socketPath(&buf, "/run", "a/b") == null);
    try testing.expect(socketPath(&buf, "/run", "a\x00b") == null);
}

test "the registry sweep takes the dead entries and leaves everything else" {
    if (comptime !supported) return error.SkipZigTest;
    // Under the real runtime directory, because a sun_path is 108 bytes
    // and the test cache's temporary directories are longer than that.
    var dir_buf: [sun_path_len:0]u8 = undefined;
    const base = socketDir(&dir_buf) orelse return error.SkipZigTest;
    if (!ensureSocketDir(base)) return error.SkipZigTest;

    var svc_buf: [sun_path_len:0]u8 = undefined;
    const svc = std.fmt.bufPrintSentinel(&svc_buf, "{s}/sweep-{d}", .{ base, @as(u32, @intCast(libc.getpid())) }, 0) catch
        return error.SkipZigTest;
    if (libc.mkdir(svc, 0o700) != 0) return error.SkipZigTest;

    // The sockets live where the real ones do -- beside the registry, not in
    // it, and named the way a session names them -- because the sweep only
    // follows an entry to a target of exactly that shape and place.
    var paths: [6][sun_path_len:0]u8 = undefined;
    const pid: u32 = @intCast(libc.getpid());
    const live_sock = try std.fmt.bufPrintSentinel(&paths[0], "{s}/pardes-9p-sweeplive-{d}.sock", .{ base, pid }, 0);
    const dead_sock = try std.fmt.bufPrintSentinel(&paths[1], "{s}/pardes-9p-sweepdead-{d}.sock", .{ base, pid }, 0);
    const live = try std.fmt.bufPrintSentinel(&paths[2], "{s}/live", .{svc}, 0);
    const dead = try std.fmt.bufPrintSentinel(&paths[3], "{s}/dead", .{svc}, 0);
    const stranger = try std.fmt.bufPrintSentinel(&paths[4], "{s}/stranger", .{svc}, 0);
    // A dead entry pointing at something that is NOT one of our sockets: the
    // entry goes, the file it named must not.
    const outsider_sock = try std.fmt.bufPrintSentinel(&paths[5], "{s}/sweep-outsider-{d}.sock", .{ base, pid }, 0);
    var outsider_buf: [sun_path_len:0]u8 = undefined;
    const outsider = try std.fmt.bufPrintSentinel(&outsider_buf, "{s}/outsider", .{svc}, 0);
    defer {
        for ([_][:0]const u8{ live_sock, dead_sock, live, dead, stranger, outsider_sock, outsider }) |p| _ = libc.unlink(p);
        _ = libc.rmdir(svc);
    }

    const listening = bindSocket(live_sock);
    try testing.expect(listening >= 0);
    defer _ = libc.close(listening);
    try testing.expectEqual(@as(c_int, 0), libc.listen(listening, 1));

    // Bound and then dropped: the file stays, and nobody answers it —
    // exactly what an aborted test leaves behind.
    const abandoned = bindSocket(dead_sock);
    try testing.expect(abandoned >= 0);
    _ = libc.close(abandoned);

    try testing.expectEqual(@as(c_int, 0), libc.symlink(live_sock, live));
    try testing.expectEqual(@as(c_int, 0), libc.symlink(dead_sock, dead));
    // Dead too, but its target is not one of our sockets by name, so the
    // entry must go and the file it named must survive untouched.
    const outside = bindSocket(outsider_sock);
    try testing.expect(outside >= 0);
    defer _ = libc.close(outside);
    try testing.expectEqual(@as(c_int, 0), libc.symlink(outsider_sock, outsider));
    // Not a symlink, so not this program's to reason about, even though
    // connecting to it is refused exactly like the dead socket.
    try std.Io.Dir.cwd().writeFile(testing.io, .{ .sub_path = stranger, .data = "" });

    // The guarantee itself, asserted rather than inferred from the fact
    // that the test finished: if a probe socket ever stops being
    // non-blocking the connect below parks instead of failing, and a
    // parked test costs whoever is building far more than a red one.
    const checking = libc.socket(libc.AF.UNIX, probe_socket_kind, 0);
    try testing.expect(checking >= 0);
    if (comptime darwin) try testing.expect(setNonblock(checking));
    try testing.expect(isNonblocking(checking));
    _ = libc.close(checking);

    // The listener's backlog is filled before anything is asked, because
    // a full backlog is the one place a blocking connect parks forever
    // (`unix_wait_for_peer`, no timeout) and a sweep that only answers
    // while nobody is queued is the sweep that hangs a build. Nothing
    // below accepts any of these, so the queue stays full throughout.
    var queued: [8]c_int = @splat(-1);
    defer for (queued) |fd| {
        if (fd >= 0) _ = libc.close(fd);
    };
    for (&queued) |*slot| {
        const fd = libc.socket(libc.AF.UNIX, probe_socket_kind, 0);
        if (fd < 0) break;
        if (comptime darwin) _ = setNonblock(fd);
        var addr: libc.sockaddr.un = .{ .path = @splat(0) };
        @memcpy(addr.path[0 .. live_sock.len + 1], live_sock[0 .. live_sock.len + 1]);
        _ = libc.connect(fd, @ptrCast(&addr), @sizeOf(@TypeOf(addr)));
        slot.* = fd;
    }

    const started = transport.nowMs();
    try testing.expectEqual(Probe.live, probe(live));
    try testing.expectEqual(Probe.stale, probe(dead));

    sweepRegistry(testing.io, svc, base);

    // Promptly, and not "eventually": a blocking probe never comes back
    // at all, so any wall-clock bound at all is the assertion that
    // matters. A second is several thousand times what three connects
    // and a readdir cost.
    try testing.expect(transport.nowMs() - started < 1000);

    try testing.expect(statNoFollow(dead) == null);
    try testing.expect(statNoFollow(dead_sock) == null);
    try testing.expect(statNoFollow(live) != null);
    try testing.expect(statNoFollow(live_sock) != null);
    try testing.expect(statNoFollow(stranger) != null);
    try testing.expect(statNoFollow(outsider) == null);
    try testing.expect(statNoFollow(outsider_sock) != null);
    try testing.expectEqual(Probe.live, probe(live));
}

fn bindSocket(path: [:0]const u8) c_int {
    const fd = libc.socket(libc.AF.UNIX, libc.SOCK.STREAM, 0);
    if (fd < 0) return fd;
    var addr: libc.sockaddr.un = .{ .path = @splat(0) };
    @memcpy(addr.path[0 .. path.len + 1], path[0 .. path.len + 1]);
    if (libc.bind(fd, @ptrCast(&addr), @sizeOf(@TypeOf(addr))) != 0) {
        _ = libc.close(fd);
        return -1;
    }
    return fd;
}

test "one connection's buffers are sized from the one msize constant" {
    try testing.expect(msize >= ninep.min_msize);
    try testing.expectEqual(@as(u32, msize), Runner.msize);
    try testing.expectEqual(@as(usize, msize), @typeInfo(@FieldType(Runner.Conn, "in")).array.len);
    try testing.expectEqual(@as(usize, 2 * msize), @typeInfo(@FieldType(Runner.Conn, "out")).array.len);
    try testing.expectEqual(@as(usize, max_conns), Runner.max_connections);
    if (quic_enabled) {
        const c: Conn = .{};
        try testing.expectEqual(@as(usize, msize), c.in.len);
    }
}

test "TCP addresses are numeric and normalize mapped IPv4" {
    const loopback = try networkAddress("tcp!127.0.0.1!5640", false);
    try testing.expectEqualDeep(loopback, try networkAddress("tcp!::ffff:127.0.0.1!5640", false));
    try testing.expectEqualDeep(loopback, try networkAddress("tcp!::ffff:7f00:1!5640", false));
    try testing.expectEqualDeep(try networkAddress("tcp!::1!5640", false), try networkAddress("tcp!0:0:0:0:0:0:0:1!5640", false));
    try testing.expectEqual(@as(u16, 0), (try networkAddress("tcp!127.0.0.1!0", true)).getPort());
    for ([_][]const u8{ "tcp!localhost!5640", "tcp!127.0.0.1!0", "tcp!127.0.0.1!-1", "tcp!127.0.0.1!65536", "tcp!127.0.0.1!", "tcp!!5640" }) |dial|
        try testing.expectError(error.BadDial, networkAddress(dial, false));
    try Client.validateDial("tcp!127.0.0.1!5640");
    try Client.validateDial("/tmp/pardes-owned.sock");
    try Client.validateDial("unix!/tmp/pardes-owned.sock");
    try testing.expectError(error.BadDial, Client.validateDial("unix!work"));
    try testing.expectError(error.BadDial, Client.validateDial("unix!"));
    try testing.expectError(error.BadDial, Client.validateDial("unix!/tmp/a\x00b"));
    try testing.expectError(error.BadDial, Client.validateDial("tcp!localhost!5640"));
    try testing.expectError(error.BadDial, Client.validateDial("/tmp/a\x00b"));
    if (quic_enabled) {
        try Client.validateDial("quic!127.0.0.1!5640");
    } else try testing.expectError(error.QuicUnavailable, Client.validateDial("quic!127.0.0.1!5640"));
}

extern "c" fn mkdtemp(template: [*:0]u8) ?[*:0]u8;
extern "c" fn rmdir(path: [*:0]const u8) c_int;

test "a listening editor posts itself into the 9P registry and unposts on stop" {
    if (comptime !supported) return error.SkipZigTest;
    const gpa = testing.allocator;
    var directory: [64:0]u8 = undefined;
    _ = try std.fmt.bufPrintSentinel(&directory, "/tmp/pardes-post-XXXXXX", .{}, 0);
    if (mkdtemp(&directory) == null) return error.TempDirectoryFailed;
    const dir = std.mem.span(@as([*:0]const u8, &directory));
    const old_runtime = if (libc.getenv("XDG_RUNTIME_DIR")) |v| try gpa.dupeZ(u8, std.mem.span(v)) else null;
    defer {
        if (old_runtime) |v| {
            _ = setenv("XDG_RUNTIME_DIR", v, 1);
            gpa.free(v);
        } else _ = unsetenv("XDG_RUNTIME_DIR");
    }
    try testing.expectEqual(@as(c_int, 0), setenv("XDG_RUNTIME_DIR", &directory, 1));

    var entry_buf: [sun_path_len:0]u8 = undefined;
    const entry = try std.fmt.bufPrintSentinel(&entry_buf, "{s}/9p/pardes/unit", .{dir}, 0);
    var svc_buf: [sun_path_len:0]u8 = undefined;
    const svc = try std.fmt.bufPrintSentinel(&svc_buf, "{s}/9p/pardes", .{dir}, 0);
    var sock_buf: [sun_path_len:0]u8 = undefined;
    const sock = try std.fmt.bufPrintSentinel(&sock_buf, "{s}/" ++ prefix ++ "unit.sock", .{dir}, 0);
    defer {
        _ = libc.unlink(entry);
        _ = rmdir(svc);
        var reg_buf: [sun_path_len:0]u8 = undefined;
        if (std.fmt.bufPrintSentinel(&reg_buf, "{s}/9p", .{dir}, 0)) |reg| {
            _ = rmdir(reg);
        } else |_| {}
        _ = libc.unlink(sock);
        _ = rmdir(&directory);
    }

    var link: [sun_path_len]u8 = undefined;
    {
        const p = try pardes.Pardes.init(gpa, .{ .tty_only = true, .cols = 40, .rows = 12 });
        defer p.deinit();
        const l = listen(testing.io, gpa, p, "unit", "", null, null) orelse return error.ListenFailed;
        defer l.deinit(gpa);
        // The socket stays exactly where pardes has always bound it:
        // adopting the registry moves nothing, it only advertises.
        try testing.expect(statNoFollow(sock) != null);
        // And the registry holds a symlink to it one directory down, so
        // several editors group under /mnt/9p/pardes/ instead of
        // crowding the registry root — the layout zmx posts its
        // sessions in.
        const n = libc.readlink(entry, &link, link.len);
        try testing.expect(n > 0);
        try testing.expectEqualStrings(sock, link[0..@intCast(n)]);
    }
    // Stopping takes the name back out, so the next editor of that name
    // is not refused by its own corpse.
    try testing.expect(libc.readlink(entry, &link, link.len) < 0);
}

test "Unix TCP and QUIC share one listener through reads writes reconnects and reset" {
    if (comptime !supported) return error.SkipZigTest;
    const gpa = testing.allocator;
    var directory: [64:0]u8 = undefined;
    _ = try std.fmt.bufPrintSentinel(&directory, "/tmp/pardes-tcp-XXXXXX", .{}, 0);
    if (mkdtemp(&directory) == null) return error.TempDirectoryFailed;
    // What the listener posted and left inside goes with it.
    defer std.Io.Dir.cwd().deleteTree(testing.io, std.mem.sliceTo(&directory, 0)) catch |err| std.debug.print("deleteTree: {s}\n", .{@errorName(err)});
    const old_runtime = if (libc.getenv("XDG_RUNTIME_DIR")) |v| try gpa.dupeZ(u8, std.mem.span(v)) else null;
    defer {
        if (old_runtime) |v| {
            _ = setenv("XDG_RUNTIME_DIR", v, 1);
            gpa.free(v);
        } else _ = unsetenv("XDG_RUNTIME_DIR");
    }
    try testing.expectEqual(@as(c_int, 0), setenv("XDG_RUNTIME_DIR", &directory, 1));
    const replacement = try gpa.alloc(u8, 3 * msize + 27);
    defer gpa.free(replacement);
    @memset(replacement, 'x');
    @memcpy(replacement[0.."changed café λ\n".len], "changed café λ\n");
    replacement[replacement.len - 1] = '\n';

    const Worker = struct {
        dial: []const u8,
        body_path: []const u8,
        expected: []const u8,
        replacement: []const u8,
        done: std.atomic.Value(bool) = .init(false),
        failure: ?anyerror = null,

        fn run(w: *@This()) void {
            defer w.done.store(true, .release);
            w.check() catch |err| {
                w.failure = err;
            };
        }

        fn check(w: *@This()) !void {
            const before = try Client.readLimit(testing.allocator, w.dial, w.body_path, w.body_path, w.expected.len);
            defer testing.allocator.free(before);
            try testing.expectEqualStrings(w.expected, before);
            try testing.expectError(error.FileTooLarge, Client.readLimit(testing.allocator, w.dial, w.body_path, w.body_path, w.expected.len - 1));
            const listing = try Client.readLimit(testing.allocator, w.dial, "/pane", "/pane", 128);
            defer testing.allocator.free(listing);
            const exact_listing = try Client.readLimit(testing.allocator, w.dial, "/pane", "/pane", listing.len);
            defer testing.allocator.free(exact_listing);
            try testing.expectEqualStrings(listing, exact_listing);
            try testing.expectError(error.FileTooLarge, Client.readLimit(testing.allocator, w.dial, "/pane", "/pane", listing.len - 1));
            try Client.write(testing.allocator, w.dial, w.body_path, w.replacement);
            const after = try Client.read(testing.allocator, w.dial, w.body_path, w.body_path);
            defer testing.allocator.free(after);
            try testing.expectEqualStrings(w.replacement, after);
            const screen = try Client.read(testing.allocator, w.dial, "/screen", "/screen");
            defer testing.allocator.free(screen);
            const parsed = try std.json.parseFromSlice(struct { cols: u16, rows: u16 }, testing.allocator, screen, .{ .ignore_unknown_fields = true });
            defer parsed.deinit();
            try testing.expectEqual(@as(u16, 40), parsed.value.cols);
            try testing.expectEqual(@as(u16, 12), parsed.value.rows);
        }
    };

    const protocols: []const []const u8 = if (quic_enabled) &.{ "tcp", "quic" } else &.{"tcp"};
    for (protocols) |protocol| for ([_][]const u8{ "127.0.0.1", "::1" }) |host| {
        const p = try pardes.Pardes.init(gpa, .{ .tty_only = true, .cols = 40, .rows = 12 });
        defer p.deinit();
        const pane = try p.setTestFile("initial\n");
        while (p.nextEffect()) |_| {}
        var bind_buf: [64]u8 = undefined;
        const bind = try std.fmt.bufPrint(&bind_buf, "{s}!{s}!0", .{ protocol, host });
        const tcp = std.mem.eql(u8, protocol, "tcp");
        const l = listen(testing.io, gpa, p, "roundtrip", "", if (tcp) bind else null, if (tcp) null else bind) orelse return error.ListenFailed;
        defer {
            l.reset(p);
            l.deinit(gpa);
        }
        try testing.expect(l.path().len != 0);
        try testing.expectEqual(tcp, l.tcp_address != null);
        try testing.expectEqual(@as(usize, max_conns), l.runner.conns.len);
        try testing.expect(l.watcher == null);
        const port = (if (tcp) l.tcp_address else l.quic_address).?.getPort();
        try testing.expect(port != 0);
        var dial_buf: [64]u8 = undefined;
        const network_dial = try std.fmt.bufPrint(&dial_buf, "{s}!{s}!{d}", .{ protocol, host, port });
        var body_buf: [64]u8 = undefined;
        const body = try std.fmt.bufPrint(&body_buf, "/pane/{d}/body", .{pane.serial});
        for ([_][]const u8{ network_dial, l.path(), network_dial }, 0..) |dial, attempt| {
            var worker: Worker = .{ .dial = dial, .body_path = body, .expected = if (attempt == 0) "initial\n" else replacement, .replacement = replacement };
            const thread = try std.Thread.spawn(.{}, Worker.run, .{&worker});
            defer thread.join();
            // The editor rests while the client works: its requests are
            // answered on the runner's tasks, not by this thread.
            const deadline = Client.nowMs() + 3 * Client.budget_ms;
            pardes.turn.rest();
            while (!worker.done.load(.acquire) and Client.nowMs() < deadline) {
                if (quic_enabled) {
                    pardes.turn.wake();
                    _ = l.tick();
                    pardes.turn.rest();
                }
                Client.nap(1);
            }
            // The client has hung up, but its connection's task may not have
            // seen that yet. The releases it then owes are refused once
            // `reset` starts cutting (they name the old editor's opens), and
            // this test's "replacement" is the same editor, so they would
            // stay in `p.fs.opens`. Let the hangup pay them first, resting
            // so the task can take the turn: that is what is under test here.
            const settle_by = Client.nowMs() + 3 * Client.budget_ms;
            while (l.runner.count() != 0 and Client.nowMs() < settle_by) Client.nap(1);
            pardes.turn.wake();
            try testing.expect(worker.done.load(.acquire));
            if (worker.failure) |err| return err;
            try testing.expectEqual(@as(usize, 0), l.runner.count());
            l.reset(p);
            try testing.expectEqual(@as(usize, 0), l.runner.count());
            for (&l.conns, 0..) |conn, i| try testing.expect(!l.live(@intCast(i)) and !conn.draining);
            for (p.fs.opens) |o| try testing.expect(o.node == 0);
        }
    };
}

test "a change waits while the editor is out mid-step, a read does not, and the change lands when the editor rests" {
    if (comptime !supported) return error.SkipZigTest;
    const gpa = testing.allocator;
    var directory: [64:0]u8 = undefined;
    _ = try std.fmt.bufPrintSentinel(&directory, "/tmp/pardes-park-XXXXXX", .{}, 0);
    if (mkdtemp(&directory) == null) return error.TempDirectoryFailed;
    // What the listener posted and left inside goes with it.
    defer std.Io.Dir.cwd().deleteTree(testing.io, std.mem.sliceTo(&directory, 0)) catch |err| std.debug.print("deleteTree: {s}\n", .{@errorName(err)});
    const old_runtime = if (libc.getenv("XDG_RUNTIME_DIR")) |v| try gpa.dupeZ(u8, std.mem.span(v)) else null;
    defer {
        if (old_runtime) |v| {
            _ = setenv("XDG_RUNTIME_DIR", v, 1);
            gpa.free(v);
        } else _ = unsetenv("XDG_RUNTIME_DIR");
    }
    try testing.expectEqual(@as(c_int, 0), setenv("XDG_RUNTIME_DIR", &directory, 1));

    const p = try pardes.Pardes.init(gpa, .{ .tty_only = true, .cols = 40, .rows = 12 });
    defer p.deinit();
    const pane = try p.setTestFile("before\n");
    while (p.nextEffect()) |_| {}
    // This thread is the editor's: it has the turn from here on.
    const l = listen(testing.io, gpa, p, "park", "", null, null) orelse return error.ListenFailed;
    defer {
        l.reset(p);
        l.deinit(gpa);
    }
    var body_buf: [64]u8 = undefined;
    const body = try std.fmt.bufPrint(&body_buf, "/pane/{d}/body", .{pane.serial});

    const Writer = struct {
        dial: []const u8,
        path: []const u8,
        done: std.atomic.Value(bool) = .init(false),
        failure: ?anyerror = null,
        fn run(w: *@This()) void {
            defer w.done.store(true, .release);
            Client.write(testing.allocator, w.dial, w.path, "after\n") catch |err| {
                w.failure = err;
            };
        }
    };
    var writer: Writer = .{ .dial = l.path(), .path = body };

    // The editor goes out mid-step -- a Look resolving a path, say -- and a
    // write arrives meanwhile. It must not land: the step still holds
    // pointers into the pane. A read is answered all the same.
    pardes.turn.yield();
    const thread = try std.Thread.spawn(.{}, Writer.run, .{&writer});
    defer thread.join();
    Client.nap(150);
    try testing.expect(!writer.done.load(.acquire));
    const seen = try Client.readLimit(gpa, l.path(), body, body, 64);
    defer gpa.free(seen);
    try testing.expectEqualStrings("before\n", seen);
    try testing.expect(!writer.done.load(.acquire));
    pardes.turn.back();
    try testing.expectEqualStrings("before\n", pane.file.?.content);

    // Resting is what lets it in: the parked write is retried, served, and
    // the client hears back.
    pardes.turn.rest();
    const deadline = Client.nowMs() + Client.budget_ms;
    while (!writer.done.load(.acquire) and Client.nowMs() < deadline) Client.nap(1);
    pardes.turn.wake();
    try testing.expect(writer.done.load(.acquire));
    if (writer.failure) |err| return err;
    try testing.expectEqualStrings("after\n", pane.file.?.content);
}

test "a held read is answered when the log has news, and a flushed one spends nothing" {
    if (comptime !supported) return error.SkipZigTest;
    const gpa = testing.allocator;
    var directory: [64:0]u8 = undefined;
    _ = try std.fmt.bufPrintSentinel(&directory, "/tmp/pardes-held-XXXXXX", .{}, 0);
    if (mkdtemp(&directory) == null) return error.TempDirectoryFailed;
    // What the listener posted and left inside goes with it.
    defer std.Io.Dir.cwd().deleteTree(testing.io, std.mem.sliceTo(&directory, 0)) catch |err| std.debug.print("deleteTree: {s}\n", .{@errorName(err)});
    const old_runtime = if (libc.getenv("XDG_RUNTIME_DIR")) |v| try gpa.dupeZ(u8, std.mem.span(v)) else null;
    defer {
        if (old_runtime) |v| {
            _ = setenv("XDG_RUNTIME_DIR", v, 1);
            gpa.free(v);
        } else _ = unsetenv("XDG_RUNTIME_DIR");
    }
    try testing.expectEqual(@as(c_int, 0), setenv("XDG_RUNTIME_DIR", &directory, 1));

    const p = try pardes.Pardes.init(gpa, .{ .tty_only = true, .cols = 40, .rows = 12 });
    defer p.deinit();
    _ = try p.setTestFile("held\n");
    p.update(.tick); // the pane is logged, before the log is opened
    while (p.nextEffect()) |_| {}
    const l = listen(testing.io, gpa, p, "held", "", null, null) orelse return error.ListenFailed;
    defer {
        l.reset(p);
        l.deinit(gpa);
    }
    // This thread is the editor's and plays the client too, resting while
    // it does so that the runner's tasks can take the turn and answer.
    const s = try gpa.create(Client.Session);
    defer gpa.destroy(s);
    s.* = .{ .fd = -1, .deadline = Client.nowMs() + 3 * Client.budget_ms, .display_path = "/log" };
    var sock_buf: [sun_path_len]u8 = undefined;
    pardes.turn.rest();
    defer pardes.turn.wake();
    s.fd = try Client.connect(try Client.resolve(&sock_buf, l.path()), s.deadline);
    defer _ = libc.close(s.fd);
    s.cl = .init(.{ .in = &s.in, .out = &s.out });
    var remote: Client.RemoteError = .{};
    _ = try s.ask(.{ .version = .{} }, &remote);
    _ = try s.ask(.{ .attach = .{ .fid = 0, .uname = "held" } }, &remote);
    _ = try s.ask(.{ .walk = .{ .fid = 0, .newfid = 1, .names = &.{"log"} } }, &remote);
    _ = try s.ask(.{ .open = .{ .fid = 1, .mode = ninep.ordwr } }, &remote);
    var frozen: u64 = 0;
    while (true) {
        const n = (try s.ask(.{ .read = .{ .fid = 1, .offset = frozen, .count = 4096 } }, &remote)).read.len;
        if (n == 0) break;
        frozen += n;
    }
    _ = try s.ask(.{ .write = .{ .fid = 1, .offset = 0, .data = "follow\n" } }, &remote);

    const heldNow = struct {
        /// Waits until the core holds `n` reads.
        fn count(core: *pardes.Pardes, n: usize) !void {
            const deadline = Client.nowMs() + Client.budget_ms;
            while (Client.nowMs() < deadline) {
                pardes.turn.wake();
                var got: usize = 0;
                for (core.fs.opens) |o| got += @intFromBool(o.held != null);
                pardes.turn.rest();
                if (got == n) return;
                Client.nap(1);
            }
            return error.Timeout;
        }
    };
    const serial = p.panes[0].?.serial;
    var event_path: [32]u8 = undefined;
    var event_names: [3][]const u8 = .{ "pane", try std.fmt.bufPrint(&event_path, "{d}", .{serial}), "event" };
    _ = try s.ask(.{ .walk = .{ .fid = 0, .newfid = 2, .names = &event_names } }, &remote);
    _ = try s.ask(.{ .open = .{ .fid = 2, .mode = ninep.oread } }, &remote);

    // Flushed while held, a read is gone from the engine without the core
    // hearing of it. Its tag, asked again at once for a read of the pane's
    // event, must not be answered with the log's next record, and that
    // record must reach the log's next read.
    const flushed = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
    try s.flush();
    try heldNow.count(p, 1);
    _ = try s.cl.submit(.{ .flush = .{ .oldtag = flushed } });
    const interrupted = try s.settle();
    try testing.expectEqual(flushed, interrupted.tag);
    try testing.expect(interrupted.result == .fail);
    try testing.expect((try s.settle()).result == .flush);
    const reused = try s.cl.submit(.{ .read = .{ .fid = 2, .offset = 0, .count = 4096 } });
    try testing.expectEqual(flushed, reused);
    try s.flush();
    try heldNow.count(p, 2);
    pardes.turn.wake();
    p.setMessage(0, "after the flush");
    pardes.turn.rest();
    try heldNow.count(p, 1);
    pardes.turn.wake();
    p.fs.origin = 'K';
    _ = pardes.ctlfs.events.noteAction(p, 0, .body_exec, 0, 4, 0, "held");
    pardes.turn.rest();
    const clicked = try s.settle();
    try testing.expectEqual(reused, clicked.tag);
    try testing.expectEqualStrings("KX0 4 0 4 held\n", clicked.result.read);
    try testing.expect(std.mem.indexOf(u8, (try s.ask(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } }, &remote)).read, "after the flush") != null);

    // Held, a read is answered by the record that arrives, on its own
    // connection, with no retry of every parked request asked for; a
    // second read on the same open meanwhile is refused, not orphaned.
    const waiting = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
    try s.flush();
    try heldNow.count(p, 1);
    const second = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
    const refused = try s.settle();
    try testing.expectEqual(second, refused.tag);
    try testing.expectEqualStrings(pardes.ctlfs.e_in_use, refused.result.fail);
    pardes.turn.wake();
    p.setMessage(0, "while held");
    const retry_all = pardes.turn.parked;
    pardes.turn.rest();
    try testing.expect(!retry_all);
    const answered = try s.settle();
    try testing.expectEqual(waiting, answered.tag);
    try testing.expect(std.mem.indexOf(u8, answered.result.read, "while held") != null);

    // Clunked, the log's held read is interrupted by the engine and the
    // record it would have had is spent on nobody.
    _ = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
    try s.flush();
    try heldNow.count(p, 1);
    s.drop(1);
    s.drop(2);
    try heldNow.count(p, 0);
}

extern "c" fn setenv(name: [*:0]const u8, value: [*:0]const u8, overwrite: c_int) c_int;
extern "c" fn unsetenv(name: [*:0]const u8) c_int;

pub fn start(io: std.Io, gpa: std.mem.Allocator, core: *pardes.Pardes) ?*Listener {
    var name: [16]u8 = undefined;
    // unreachable: a u32 is at most 10 digits
    const fallback = std.fmt.bufPrint(&name, "{d}", .{@as(u32, @intCast(libc.getpid()))}) catch unreachable;
    return listen(io, gpa, core, core.opts.ninep_name, fallback, core.opts.ninep_tcp, core.opts.ninep_quic) orelse {
        core.reportError(0, "9p listener", error.ListenFailed);
        return null;
    };
}

/// What a pane shell is told about the editor above it, set into this
/// process's environment just before `forkpty` so the child inherits it.
///
/// Two independent facts, and they are separate variables because they answer
/// separate questions. `PARDES_PID` says "you are inside this editor, and it
/// will take a Look from you": a session whose 9P listener never came up still
/// owns its children, so the child says so rather than looking, from its own
/// side, exactly like no pardes at all. `PARDES_9P` and `PARDES_PANE` say how
/// to reach it, and a `--nested` session exports them too — its socket stays
/// open to scripts and to mounted shells — while withholding `PARDES_PID`,
/// which is the whole of what `--nested` means.
pub fn exportPaneEnv(listener: ?*const Listener, serial: u32, adopts: bool) void {
    var announced = false;
    if (adopts) announcing: {
        var buf: [16]u8 = undefined;
        const text = std.fmt.bufPrintSentinel(&buf, "{d}", .{@as(u32, @intCast(libc.getpid()))}, 0) catch
            break :announcing;
        announced = setenv("PARDES_PID", text, 1) == 0;
    }
    if (!announced) _ = unsetenv("PARDES_PID");

    if (listener) |l| exporting: {
        var sock: [sun_path_len]u8 = undefined;
        const path = std.fmt.bufPrintSentinel(&sock, "{s}", .{l.path()}, 0) catch break :exporting;
        var buf: [16]u8 = undefined;
        const id = std.fmt.bufPrintSentinel(&buf, "{d}", .{serial}, 0) catch break :exporting;
        if (setenv("PARDES_9P", path, 1) != 0) break :exporting;
        if (setenv("PARDES_PANE", id, 1) == 0) return;
    }
    _ = unsetenv("PARDES_9P");
    _ = unsetenv("PARDES_PANE");
}

test "9P shell environment states being inside pardes apart from how to reach it" {
    const names = [_][*:0]const u8{ "XDG_RUNTIME_DIR", "HOME", "PARDES_PID", "PARDES_9P", "PARDES_PANE" };
    var saved: [names.len]?[:0]u8 = @splat(null);
    for (names, &saved) |name, *value| {
        if (libc.getenv(name)) |old| value.* = try testing.allocator.dupeZ(u8, std.mem.span(old));
    }
    defer for (names, saved) |name, value| {
        if (value) |old| {
            _ = setenv(name, old, 1);
            testing.allocator.free(old);
        } else _ = unsetenv(name);
    };

    var dir: [sun_path_len:0]u8 = undefined;
    _ = setenv("XDG_RUNTIME_DIR", "/run/user/1000", 1);
    try testing.expectEqualStrings("/run/user/1000", socketDir(&dir).?);
    _ = unsetenv("XDG_RUNTIME_DIR");
    _ = setenv("HOME", "/home/example", 1);
    try testing.expectEqualStrings("/home/example/.local/state/pardes", socketDir(&dir).?);
    _ = unsetenv("HOME");
    try testing.expect(socketDir(&dir) == null);

    const listener = try testing.allocator.create(Listener);
    defer testing.allocator.destroy(listener);
    listener.* = .{ .io = testing.io, .core = undefined }; // only its path is read
    const path = "/tmp/pardes-example.sock";
    @memcpy(listener.path_buf[0..path.len], path);
    listener.path_len = path.len;
    var own: [16]u8 = undefined;
    const own_pid = try std.fmt.bufPrint(&own, "{d}", .{@as(u32, @intCast(libc.getpid()))});

    // A `--nested` session: reachable for scripts and mounts, but nobody's
    // parent, so a pardes started in one of its shells runs a session of
    // its own instead of handing its argument over.
    exportPaneEnv(listener, 7, false);
    try testing.expectEqualStrings(path, std.mem.span(libc.getenv("PARDES_9P").?));
    try testing.expectEqualStrings("7", std.mem.span(libc.getenv("PARDES_PANE").?));
    try testing.expect(libc.getenv("PARDES_PID") == null);

    exportPaneEnv(listener, 8, true);
    try testing.expectEqualStrings("8", std.mem.span(libc.getenv("PARDES_PANE").?));
    try testing.expectEqualStrings(own_pid, std.mem.span(libc.getenv("PARDES_PID").?));

    // A session whose listener never came up is still the session this shell
    // is inside: the pid stands on its own, and only the address is missing.
    exportPaneEnv(null, 0, true);
    try testing.expect(libc.getenv("PARDES_9P") == null);
    try testing.expect(libc.getenv("PARDES_PANE") == null);
    try testing.expectEqualStrings(own_pid, std.mem.span(libc.getenv("PARDES_PID").?));

    exportPaneEnv(null, 0, false);
    try testing.expect(libc.getenv("PARDES_9P") == null);
    try testing.expect(libc.getenv("PARDES_PANE") == null);
    try testing.expect(libc.getenv("PARDES_PID") == null);
}

pub const Client = struct {
    pub const budget_ms: i64 = 2000;

    pub const max_depth: usize = 2 * ninep.max_welem;

    const uname = "pardes";

    const Dial = union(enum) { unix: [:0]const u8, tcp: std.Io.net.IpAddress, quic: std.Io.net.IpAddress };

    pub const Error = error{
        PathTooDeep,
        BadDial,
        Dial,
        Hangup,
        Timeout,
        Botch,
        Remote,
        IsDirectory,
        NotFound,
        FileTooLarge,
        QuicUnavailable,
    };

    pub fn read(gpa: std.mem.Allocator, dial: []const u8, path: []const u8, display_path: []const u8) ![]u8 {
        return readLimit(gpa, dial, path, display_path, limits.max_file_bytes);
    }

    pub fn readLimit(gpa: std.mem.Allocator, dial: []const u8, path: []const u8, display_path: []const u8, max_bytes: usize) ![]u8 {
        if (comptime !supported) return error.Unsupported;
        var names: [max_depth][]const u8 = undefined;
        const n = try elements(path, &names);
        var sock_buf: [sun_path_len]u8 = undefined;
        const sock = try resolve(&sock_buf, dial);
        var remote: RemoteError = .{};
        return fetchBytes(gpa, sock, names[0..n], &remote, null, display_path, @min(max_bytes, limits.max_file_bytes));
    }

    /// Whether a peer answers at `dial`: a version and attach, and its root
    /// read. Any answer from the peer, an error of its own included, is one.
    pub fn probe(gpa: std.mem.Allocator, dial: []const u8) !void {
        const got = readLimit(gpa, dial, "", "", 0) catch |err| switch (err) {
            error.IsDirectory, error.NotFound, error.Remote, error.FileTooLarge => return,
            else => return err,
        };
        gpa.free(got);
    }

    pub fn write(gpa: std.mem.Allocator, dial: []const u8, path: []const u8, bytes: []const u8) !void {
        if (comptime !supported) return error.Unsupported;
        var names: [max_depth][]const u8 = undefined;
        const n = try elements(path, &names);
        var sock_buf: [sun_path_len]u8 = undefined;
        const sock = try resolve(&sock_buf, dial);
        var remote: RemoteError = .{};
        const result = try fetchBytes(gpa, sock, names[0..n], &remote, bytes, path, limits.max_file_bytes);
        gpa.free(result);
    }

    const RemoteError = struct {
        buf: [ninep.errmax]u8 = undefined,
        len: usize = 0,

        fn set(r: *RemoteError, msg: []const u8) error{Remote} {
            r.len = @min(msg.len, r.buf.len);
            @memcpy(r.buf[0..r.len], msg[0..r.len]);
            return error.Remote;
        }
    };

    fn elements(path: []const u8, out: *[max_depth][]const u8) Error!usize {
        var n: usize = 0;
        var it = std.mem.tokenizeScalar(u8, path, '/');
        while (it.next()) |name| {
            if (n == out.len) return Error.PathTooDeep;
            out[n] = name;
            n += 1;
        }
        return n;
    }

    fn resolve(buf: *[sun_path_len]u8, dial: []const u8) error{ BadDial, QuicUnavailable }!Dial {
        if (dial.len == 0) return error.BadDial;
        if (std.mem.startsWith(u8, dial, "tcp!")) return .{ .tcp = try networkAddress(dial, false) };
        if (std.mem.startsWith(u8, dial, "quic!")) {
            if (comptime !quic_enabled) return error.QuicUnavailable;
            return .{ .quic = try networkAddress(dial, false) };
        }
        const explicit_unix = std.mem.startsWith(u8, dial, "unix!");
        const path = if (explicit_unix) dial[5..] else dial;
        if (explicit_unix and !std.mem.startsWith(u8, path, "/")) return error.BadDial;
        if (std.mem.indexOfScalar(u8, path, '/') != null) {
            if (std.mem.indexOfScalar(u8, path, 0) != null) return error.BadDial;
            return .{ .unix = std.fmt.bufPrintSentinel(buf, "{s}", .{path}, 0) catch return error.BadDial };
        }
        var dir_buf: [sun_path_len:0]u8 = undefined;
        const dir = socketDir(&dir_buf) orelse return error.BadDial;
        return .{ .unix = socketPath(buf, dir, dial) orelse return error.BadDial };
    }

    pub fn validateDial(dial: []const u8) error{ BadDial, QuicUnavailable }!void {
        var buf: [sun_path_len]u8 = undefined;
        _ = try resolve(&buf, dial);
    }

    const Session = struct {
        fd: c_int,
        quic: if (quic_enabled) ?quic.Connection else void = if (quic_enabled) null else {},
        deadline: i64,
        display_path: []const u8,
        cl: ninep.Client = undefined,
        in: [msize]u8 = undefined,
        out: [msize]u8 = undefined,
        stage: [msize]u8 = undefined,

        fn wait(s: *Session, events: i16) Error!void {
            while (true) {
                const left = s.deadline - nowMs();
                if (left <= 0) return Error.Timeout;
                if (comptime quic_enabled) {
                    if (s.quic) |*connection| {
                        var fds = [1]libc.pollfd{connection.poll().?};
                        const timeout = @min(left, connection.nextDue() orelse budget_ms);
                        const ready = libc.poll(&fds, 1, @intCast(timeout));
                        if (ready < 0) {
                            if (libc.errno(ready) == .INTR) continue;
                            return Error.Hangup;
                        }
                        if (nowMs() >= s.deadline) return Error.Timeout;
                        if (fds[0].revents & @as(i16, @intCast(libc.POLL.NVAL)) != 0) return Error.Hangup;
                        connection.events() catch return Error.Hangup;
                        return;
                    }
                }
                return transport.wait(s.fd, events, s.deadline) catch |err| switch (err) {
                    error.Timeout => Error.Timeout,
                    else => Error.Hangup,
                };
            }
        }

        fn flush(s: *Session) Error!void {
            while (s.cl.output().len != 0) {
                if (nowMs() >= s.deadline) return Error.Timeout;
                const bytes = s.cl.output();
                if (comptime quic_enabled) {
                    if (s.quic) |*connection| {
                        const sent = connection.write(bytes) catch return Error.Hangup;
                        if (sent == 0) {
                            try s.wait(poll_out);
                            continue;
                        }
                        s.cl.wrote(sent);
                        continue;
                    }
                }
                try s.wait(poll_out);
                const sent = (transport.write(s.fd, bytes) catch return Error.Hangup) orelse continue;
                s.cl.wrote(@intCast(sent));
            }
        }

        fn settle(s: *Session) Error!ninep.Client.Done {
            while (true) {
                if (nowMs() >= s.deadline) return Error.Timeout;
                try s.flush();
                if (s.cl.take()) |done| return done;
                if (s.cl.dead) return Error.Botch;
                const room = s.cl.in.len - s.cl.in_len;
                if (room == 0) return Error.Botch;
                if (comptime quic_enabled) {
                    if (s.quic) |*connection| {
                        const got = (connection.read(s.stage[0..@min(room, s.stage.len)]) catch return Error.Hangup) orelse {
                            try s.wait(poll_in);
                            continue;
                        };
                        if (got == 0) return Error.Hangup;
                        const n = s.cl.push(s.stage[0..got]);
                        std.debug.assert(n == got);
                        continue;
                    }
                }
                try s.wait(poll_in);
                const got = (transport.read(s.fd, s.stage[0..@min(room, s.stage.len)]) catch return Error.Hangup) orelse continue;
                if (got == 0) return Error.Hangup;
                const n = s.cl.push(s.stage[0..@intCast(got)]);
                std.debug.assert(n == @as(usize, @intCast(got)));
            }
        }

        fn ask(s: *Session, req: ninep.Client.Request, remote: *RemoteError) Error!ninep.Client.Result {
            _ = s.cl.submit(req) catch return Error.Botch;
            const done = try s.settle();
            if (done.result == .fail) return remote.set(done.result.fail);
            if (std.mem.eql(u8, @tagName(done.result), @tagName(std.meta.activeTag(req)))) return done.result;
            return Error.Botch;
        }

        fn drop(s: *Session, fid: u32) void {
            _ = s.cl.submit(.{ .clunk = .{ .fid = fid } }) catch return;
            _ = s.settle() catch {};
        }

        fn dropNoWait(s: *Session, fid: u32) void {
            _ = s.cl.submit(.{ .clunk = .{ .fid = fid } }) catch return;
            s.flush() catch {};
        }
    };

    /// Version, attach and a walk to `names`: the fid there and its qid.
    fn walkTo(s: *Session, names: []const []const u8, remote: *RemoteError) !struct { fid: u32, qid: ninep.Qid } {
        _ = try s.ask(.{ .version = .{} }, remote);
        if (s.cl.msize == 0) return Error.Botch;

        const root: u32 = 0;
        var here = (try s.ask(.{ .attach = .{ .fid = root, .uname = uname } }, remote)).attach;
        var cur: u32 = root;
        var next: u32 = 1;

        var i: usize = 0;
        while (i < names.len) {
            const n = @min(ninep.max_welem, names.len - i);
            const w = (try s.ask(.{ .walk = .{
                .fid = cur,
                .newfid = next,
                .names = names[i..][0..n],
            } }, remote)).walk;
            if (w.nwqid != n) return Error.NotFound;
            here = w.wqid[n - 1];
            if (cur != root) s.drop(cur);
            cur = next;
            next = if (next == 1) 2 else 1;
            i += n;
        }
        return .{ .fid = cur, .qid = here };
    }

    fn transact(
        s: *Session,
        names: []const []const u8,
        out: *std.Io.Writer.Allocating,
        remote: *RemoteError,
        write_bytes: ?[]const u8,
        read_limit: usize,
    ) !void {
        const at = try walkTo(s, names, remote);
        const cur = at.fid;
        const here = at.qid;
        defer s.dropNoWait(cur);

        const directory = here.type & ninep.qtdir != 0;

        const mode: u8 = if (write_bytes != null) ninep.owrite else ninep.oread;
        const truncate = write_bytes != null and names.len > 0 and
            (std.mem.eql(u8, names[0], "os") or std.mem.eql(u8, names[names.len - 1], "body"));
        _ = try s.ask(.{ .open = .{ .fid = cur, .mode = mode | if (truncate) ninep.otrunc else 0 } }, remote);
        if (write_bytes) |bytes| {
            if (directory) return Error.IsDirectory;
            var written: usize = 0;
            while (written < bytes.len) {
                const chunk = bytes[written..][0..@min(bytes.len - written, s.cl.maxWrite())];
                const count = (try s.ask(.{ .write = .{ .fid = cur, .offset = written, .data = chunk } }, remote)).write;
                if (count == 0 or count > chunk.len) return Error.Botch;
                written += count;
            }
            return;
        }

        const max_bytes: u64 = @min(read_limit, @as(usize, if (directory) limits.max_stream_bytes else limits.max_file_bytes));
        const wire_limit: u64 = if (directory) limits.max_stream_bytes else max_bytes;
        var off: u64 = 0;
        while (true) {
            const want: u32 = @intCast(@min(@as(u64, s.cl.maxRead()), wire_limit + 1 - off));
            const data = (try s.ask(.{ .read = .{ .fid = cur, .offset = off, .count = want } }, remote)).read;
            if (data.len == 0) return;
            if (off + data.len > wire_limit) return Error.FileTooLarge;
            if (directory) {
                var pos: usize = 0;
                while (pos < data.len) {
                    if (data.len - pos < 2) return Error.Botch;
                    const len: usize = 2 + @as(usize, std.mem.readInt(u16, data[pos..][0..2], .little));
                    if (len > data.len - pos) return Error.Botch;
                    const entry = ninep.Stat.decode(data[pos..][0..len]) catch return Error.Botch;
                    const display_dir = std.mem.trimEnd(u8, s.display_path, "/");
                    const row_len = display_dir.len + entry.name.len + 2 + @as(usize, @intFromBool(entry.qid.type & ninep.qtdir != 0));
                    if (row_len > max_bytes - out.written().len) return Error.FileTooLarge;
                    try out.writer.print("{s}/{s}", .{ display_dir, entry.name });
                    if (entry.qid.type & ninep.qtdir != 0) try out.writer.writeByte('/');
                    try out.writer.writeByte('\n');
                    pos += len;
                }
            } else try out.writer.writeAll(data);
            off += data.len;
        }
    }

    fn fetchBytes(
        gpa: std.mem.Allocator,
        sock: Dial,
        names: []const []const u8,
        remote: *RemoteError,
        write_bytes: ?[]const u8,
        display_path: []const u8,
        read_limit: usize,
    ) ![]u8 {
        if (comptime !supported) return Error.Dial;
        const s = try startSession(gpa, sock, display_path);
        defer endSession(gpa, s);
        var out: std.Io.Writer.Allocating = .init(gpa);
        errdefer out.deinit();
        try transact(s, names, &out, remote, write_bytes, read_limit);
        return out.toOwnedSlice();
    }

    /// A connected session whose requests have the usual deadline.
    fn startSession(gpa: std.mem.Allocator, sock: Dial, display_path: []const u8) !*Session {
        const deadline = nowMs() +| budget_ms;
        const s = try gpa.create(Session);
        s.* = .{ .fd = -1, .deadline = deadline, .display_path = display_path };
        errdefer endSession(gpa, s);
        if (sock == .quic) {
            if (comptime quic_enabled) {
                s.quic = quic.Connection.dial(sock.quic) catch return Error.Dial;
                s.fd = s.quic.?.fd;
            } else return Error.QuicUnavailable;
        } else s.fd = try connect(sock, deadline);
        s.cl = .init(.{ .in = &s.in, .out = &s.out });
        return s;
    }

    fn endSession(gpa: std.mem.Allocator, s: *Session) void {
        if (quic_enabled and s.quic != null) {
            s.quic.?.deinit();
        } else if (s.fd >= 0) _ = libc.close(s.fd);
        gpa.destroy(s);
    }

    /// Opens `path` on one connection and writes `first` to it (/log's
    /// `follow new`), all with the usual deadline; then, once `ready(ctx)`
    /// has said it is not done already, reads it with no deadline, a read at
    /// a time, until `record(ctx, bytes)` says done. An end of file or a
    /// dropped connection is Hangup. What `pardes --wait` blocks on.
    pub fn follow(
        gpa: std.mem.Allocator,
        dial: []const u8,
        path: []const u8,
        first: []const u8,
        ctx: anytype,
        comptime ready: fn (@TypeOf(ctx)) bool,
        comptime record: fn (@TypeOf(ctx), []const u8) bool,
    ) !void {
        if (comptime !supported) return error.Unsupported;
        var names: [max_depth][]const u8 = undefined;
        const n = try elements(path, &names);
        var sock_buf: [sun_path_len]u8 = undefined;
        const sock = try resolve(&sock_buf, dial);
        var remote: RemoteError = .{};
        const s = try startSession(gpa, sock, path);
        defer endSession(gpa, s);
        const fid = (try walkTo(s, names[0..n], &remote)).fid;
        _ = try s.ask(.{ .open = .{ .fid = fid, .mode = ninep.ordwr } }, &remote);
        _ = try s.ask(.{ .write = .{ .fid = fid, .offset = 0, .data = first } }, &remote);
        if (ready(ctx)) return;
        s.deadline = std.math.maxInt(i64);
        while (true) {
            const data = (try s.ask(.{ .read = .{ .fid = fid, .offset = 0, .count = s.cl.maxRead() } }, &remote)).read;
            if (data.len == 0) return Error.Hangup;
            if (record(ctx, data)) return;
        }
    }

    fn connect(sock: Dial, deadline: i64) Error!c_int {
        const address: transport.Address = switch (sock) {
            .unix => |path| .{ .unix = path },
            .tcp => |ip| .{ .tcp = ip },
            .quic => return Error.QuicUnavailable,
        };
        return transport.connectFd(address, deadline) catch return Error.Dial;
    }

    const nowMs = transport.nowMs;

    const poll_in: i16 = @intCast(libc.POLL.IN);
    const poll_out: i16 = @intCast(libc.POLL.OUT);

    fn nap(ms: c_int) void {
        _ = libc.poll(&[0]libc.pollfd{}, 0, ms);
    }

    test "a path becomes walk elements, normalised the way a shell would" {
        var out: [max_depth][]const u8 = undefined;
        try testing.expectEqual(@as(usize, 3), try elements("/pane/1/body", &out));
        try testing.expectEqualStrings("pane", out[0]);
        try testing.expectEqualStrings("1", out[1]);
        try testing.expectEqualStrings("body", out[2]);

        try testing.expectEqual(@as(usize, 3), try elements("pane/1/body", &out));
        try testing.expectEqual(@as(usize, 3), try elements("//pane//1//body//", &out));
        try testing.expectEqual(@as(usize, 1), try elements("/index", &out));

        try testing.expectEqual(@as(usize, 0), try elements("/", &out));

        var deep: [8 * max_depth]u8 = @splat('/');
        for (0..max_depth + 1) |i| deep[i * 2 + 1] = 'a';
        try testing.expectError(Error.PathTooDeep, elements(deep[0 .. (max_depth + 1) * 2], &out));
    }

    test "a bare dial resolves to the socket --9p binds, and a path is taken as given" {
        if (comptime !supported) return error.SkipZigTest;
        var buf: [sun_path_len]u8 = undefined;

        const named = (try resolve(&buf, "work")).unix;
        try testing.expect(std.mem.endsWith(u8, named, "/pardes-9p-work.sock"));
        var expect: [sun_path_len]u8 = undefined;
        var dir_buf: [sun_path_len:0]u8 = undefined;
        const dir = socketDir(&dir_buf).?;
        try testing.expectEqualStrings(socketPath(&expect, dir, "work").?, named);

        const path = (try resolve(&buf, "/tmp/somewhere.sock")).unix;
        try testing.expectEqualStrings("/tmp/somewhere.sock", path);
        try testing.expectEqualStrings("/tmp/somewhere.sock", (try resolve(&buf, "unix!/tmp/somewhere.sock")).unix);

        try testing.expectError(error.BadDial, resolve(&buf, ""));
        try testing.expectError(error.BadDial, resolve(&buf, "/tmp/a\x00b"));
    }

    test "a dial with nothing listening is one error and not a wait" {
        if (comptime !supported) return error.SkipZigTest;
        var names: [max_depth][]const u8 = undefined;
        const n = try elements("/pane/1/body", &names);
        var remote: RemoteError = .{};
        const before = nowMs();
        try testing.expectError(
            Error.Dial,
            fetchBytes(testing.allocator, .{ .unix = "/tmp/pardes-9p-no-such-socket.sock" }, names[0..n], &remote, null, "/pane/1/body", limits.max_file_bytes),
        );
        try testing.expect(nowMs() - before < budget_ms);
    }

    test "one fetch has three msize buffers and bounded transport metadata" {
        const transport_bytes = if (quic_enabled) @sizeOf(?quic.Connection) else 0;
        try testing.expectEqual(@as(usize, msize), @as(usize, (Session{ .fd = -1, .deadline = 0, .display_path = "" }).in.len));
        try testing.expect(transport_bytes <= 64);
        try testing.expect(@sizeOf(Session) <= 3 * msize + @sizeOf(ninep.Client) + 128 + transport_bytes);
        try testing.expect(@sizeOf(ninep.Client) <= 512);
    }

    test "expired sessions do not send or consume buffered protocol work" {
        var session: Session = .{ .fd = -1, .deadline = 0, .display_path = "" };
        session.cl = .init(.{ .in = &session.in, .out = &session.out });
        _ = try session.cl.submit(.{ .version = .{} });
        const queued = session.cl.output().len;
        try testing.expect(queued > 0);
        try testing.expectError(Error.Timeout, session.flush());
        try testing.expectEqual(queued, session.cl.output().len);
        try testing.expectError(Error.Timeout, session.settle());
        try testing.expectError(Error.Timeout, session.wait(poll_in));
    }
};