path: root/9proc/src/linux/probe.zig

//! The Linux platform layer: one background thread runs a `poll()` loop over
//! a listener and every client connection, feeding each connection's core
//! `Conn` with `push`/`step`/`output`/`wrote`. No per-connection threads, no
//! allocation after `init`; every buffer lives in a caller-placed `Storage`.
//!
//! Also home of the debug facilities (`debug`, `provider`) and the /runtime
//! generators (`runtime`). See docs/LIBRARY.md.
//!
//! Client admission: a new connection takes a free slot. When every slot is
//! taken, the connection that has held a slot without any fid (never
//! attached, or fully clunked) for longer than `evict_idle_ms` is dropped in
//! its favour; if there is none, the new connection is closed ("refused").
//! Nothing that holds a fid is ever evicted.
//!
//! `sleepServing(ms)` lets a request handler (a ctl command, say) wait
//! without stalling the other clients: called on the probe thread from inside
//! a request it keeps running the poll loop for every client whose request
//! is not in progress until the time is up. Requests served from inside such
//! a wait may wait themselves, up to `max_nested_sleeps` deep (each level is a
//! different client, so the depth is bounded by the client table anyway); the
//! level past that, and any call off the probe thread, is a plain sleep.
const std = @import("std");
const builtin = @import("builtin");
const linux = std.os.linux;
const cloud9 = @import("cloud9");
const core = @import("../core.zig");

pub const debug = @import("debug.zig");
pub const provider = @import("provider.zig");
pub const runtime = @import("runtime.zig");
pub const DebugProvider = provider.DebugProvider;

pub const Listen = union(enum) {
    /// A unix socket path (< 108 bytes); a stale socket file is unlinked first.
    unix: []const u8,
    /// An IPv4 literal "a.b.c.d:port".
    tcp: []const u8,
    /// An already listening socket, owned by the caller.
    fd: i32,
    /// One pre-connected client on these descriptors (stdio: 0 and 1). Nothing
    /// is accepted and the loop ends when the client hangs up.
    client: struct { in: i32, out: i32 },
};

pub const Options = struct {
    /// For `std.debug` symbolization.
    io: std.Io,
    listen: Listen,
    /// Largest msize offered to clients (clamped to the server's `cfg.msize`).
    msize: u32 = 64 * 1024,
    /// Hold a panicking thread until /panic/ctl says "continue".
    hold_on_panic: bool = true,
    /// Real-time signal used to snapshot other threads.
    capture_signal: u8 = debug.default_capture_signal,
    /// Install the SIGTRAP handler so `@breakpoint()` parks the thread.
    breakpoints: bool = true,
    /// Mount /threads, /addr, /mem, /hex, /breakpoints, /panic (six provider slots).
    mount_debug: bool = true,
};

pub const Error = error{
    /// Another `Debug` (another probe) exists in this process.
    AlreadyInitialized,
    /// No register capture on this architecture.
    Unsupported,
    /// `Shared` has fewer than six free provider slots.
    TooManyProviders,
    PathTooLong,
    BadAddress,
    /// A syscall failed; `last_errno` says which error.
    Syscall,
};

/// Idle time without fids after which a slot holder may be evicted.
pub const evict_idle_ms: i64 = 500;
/// Largest number of connections accepted per poll wakeup.
const accept_burst = 64;
/// How deep `sleepServing` may nest (each level keeps a poll round on the stack).
pub const max_nested_sleeps = 8;

/// Static per-client storage: `max_clients` core `Storage`s and `Conn`s, the
/// poll table, the debug text arena and the debug provider's snapshot pool.
pub fn Storage(comptime max_clients: u8, comptime Srv: type) type {
    return Probe(Srv).Storage(max_clients);
}

pub fn Probe(comptime Srv: type) type {
    return struct {
        const Self = @This();

        pub fn Storage(comptime max_clients: u8) type {
            comptime std.debug.assert(max_clients > 0);
            return struct {
                pub const capacity = max_clients;
                conns: [max_clients]Srv.Storage,
                clients: [max_clients]Client,
                /// [0] wake eventfd, [1] listener, [2..] one per client slot.
                pollfds: [max_clients + 2]linux.pollfd,
                text_buf: [16 * 1024]u8,
                dp: DebugProvider,
            };
        }

        pub const Client = struct {
            conn: Srv.Conn,
            in: i32 = -1,
            out: i32 = -1,
            used: bool = false,
            /// Descriptors we opened (accepted) are closed on drop; borrowed ones are not.
            owned: bool = false,
            /// Send with MSG_NOSIGNAL; falls back to write(2) on ENOTSOCK.
            is_socket: bool = true,
            /// Monotonic ms of the last byte received.
            last_active: i64 = 0,
        };

        shared: *Srv.Shared,
        clients: []Client,
        conns: []Srv.Storage,
        pollfds: []linux.pollfd,
        dbg: debug.Debug,
        dp: *DebugProvider,
        msize: u32,
        listen_fd: i32 = -1,
        own_listener: bool = false,
        is_tcp: bool = false,
        single: bool = false,
        wake_fd: i32 = -1,
        unix_path: [108]u8 = undefined,
        unix_len: usize = 0,
        thread: ?std.Thread = null,
        thread_tid: std.atomic.Value(u32) = .init(0),
        nclients: std.atomic.Value(u32) = .init(0),
        /// Connections closed because no slot was free.
        refused: u64 = 0,
        stopping: std.atomic.Value(bool) = .init(false),
        /// Slots whose request is being handled (excluded from nested servicing and eviction).
        serving: std.StaticBitSet(256) = .initEmpty(),
        /// Current `sleepServing` nesting depth.
        nested: u8 = 0,
        debug_ready: bool = false,
        last_errno: linux.E = .SUCCESS,

        // -- lifecycle -------------------------------------------------------

        /// Installs the debug facilities, mounts the debug providers into
        /// `shared` and opens the listener. `storage` is a `*Storage(n)`. On
        /// failure `shared` may already hold the debug providers and must be
        /// discarded.
        pub fn init(p: *Self, shared: *Srv.Shared, storage: anytype, opts: Options) Error!void {
            p.* = .{
                .shared = shared,
                .clients = &storage.clients,
                .conns = &storage.conns,
                .pollfds = &storage.pollfds,
                .dbg = undefined,
                .dp = &storage.dp,
                .msize = opts.msize,
            };
            for (p.clients) |*c| c.used = false;
            shared.hash_seed = randomSeed();
            debug.hold_on_panic = opts.hold_on_panic;
            p.dbg.init(.{ .io = opts.io, .text_buf = &storage.text_buf, .capture_signal = opts.capture_signal }) catch |e| return switch (e) {
                error.AlreadyInitialized => error.AlreadyInitialized,
                error.Unsupported => error.Unsupported,
                else => error.Syscall,
            };
            p.debug_ready = true;
            errdefer {
                p.dbg.deinit();
                p.debug_ready = false;
            }
            if (opts.breakpoints) p.dbg.enableBreakpoints() catch |e| switch (e) {
                // No breakpoint support on this architecture: everything else still works.
                error.Unsupported => {},
                else => return error.Syscall,
            };
            if (opts.mount_debug) {
                p.dp.init(&p.dbg);
                p.dp.mountAll(shared) catch return error.TooManyProviders;
            }
            const efd = linux.eventfd(0, linux.EFD.CLOEXEC | linux.EFD.NONBLOCK);
            try p.check(efd);
            p.wake_fd = @intCast(efd);
            errdefer {
                _ = linux.close(p.wake_fd);
                p.wake_fd = -1;
            }
            switch (opts.listen) {
                .unix => |path| try p.listenUnix(path),
                .tcp => |text| try p.listenTcp(text),
                .fd => |fd| {
                    try p.setNonblock(fd);
                    p.listen_fd = fd;
                },
                .client => |c| {
                    p.single = true;
                    try p.setNonblock(c.in);
                    if (c.out != c.in) try p.setNonblock(c.out);
                    _ = p.addClient(c.in, c.out, false);
                },
            }
        }

        /// Spawns the poll thread.
        pub fn start(p: *Self) std.Thread.SpawnError!void {
            std.debug.assert(p.thread == null);
            p.stopping.store(false, .release);
            p.thread = try std.Thread.spawn(.{}, run, .{p});
        }

        /// Waits for the poll thread to end (only happens by itself in
        /// `.client` mode, when the client hangs up).
        pub fn wait(p: *Self) void {
            if (p.thread) |t| {
                t.join();
                p.thread = null;
            }
        }

        /// Stops the poll thread, drops every client, closes what `init`
        /// opened and restores the signal dispositions.
        pub fn stop(p: *Self) void {
            // Joining the poll thread from itself would hang forever; a
            // request handler that wants the server gone uses `requestStop`.
            std.debug.assert(p.thread_tid.load(.acquire) != @as(u32, @intCast(linux.gettid())));
            p.stopping.store(true, .release);
            p.wakeLoop();
            p.wait();
            for (p.clients, 0..) |*c, i| if (c.used) p.dropClient(i);
            if (p.listen_fd >= 0) {
                if (p.own_listener) _ = linux.close(p.listen_fd);
                p.listen_fd = -1;
            }
            if (p.unix_len > 0) {
                _ = linux.unlink(@ptrCast(&p.unix_path));
                p.unix_len = 0;
            }
            if (p.wake_fd >= 0) {
                _ = linux.close(p.wake_fd);
                p.wake_fd = -1;
            }
            if (p.debug_ready) {
                p.dbg.deinit();
                p.debug_ready = false;
            }
        }

        /// Live client count (for /runtime/clients).
        pub fn clientCount(p: *const Self) u32 {
            return p.nclients.load(.acquire);
        }

        pub fn clientCounter(p: *const Self) *const std.atomic.Value(u32) {
            return &p.nclients;
        }

        /// Waits `ms` while keeping the other clients served (see the file comment).
        pub fn sleepServing(p: *Self, ms: u64) void {
            const on_thread = p.thread_tid.load(.acquire) == @as(u32, @intCast(linux.gettid()));
            if (!on_thread or p.serving.count() == 0 or p.nested >= max_nested_sleeps) return sleepMs(ms);
            p.nested += 1;
            defer p.nested -= 1;
            const deadline = monotonicMs() + @as(i64, @intCast(@min(ms, std.math.maxInt(i32))));
            while (!p.stopping.load(.acquire)) {
                const now = monotonicMs();
                if (now >= deadline) break;
                p.pollOnce(@intCast(deadline - now));
            }
        }

        /// Asks the poll thread to stop; safe to call from a signal handler
        /// (an atomic store and one write to the wake eventfd). `stop` (or
        /// `wait`) still has to run afterwards to release everything.
        pub fn requestStop(p: *Self) void {
            p.stopping.store(true, .release);
            p.wakeLoop();
        }

        // -- the loop --------------------------------------------------------

        fn run(p: *Self) void {
            const tid: u32 = @intCast(linux.gettid());
            p.thread_tid.store(tid, .release);
            // A breakpoint or panic on this thread must never park it (see debug.zig).
            debug.server_tid.store(tid, .release);
            setThreadName("9proc");
            while (!p.stopping.load(.acquire)) {
                if (p.single and p.clientCount() == 0) break;
                p.pollOnce(-1);
            }
            debug.server_tid.store(0, .release);
            p.thread_tid.store(0, .release);
        }

        fn wakeLoop(p: *Self) void {
            if (p.wake_fd < 0) return;
            const one: u64 = 1;
            _ = linux.write(p.wake_fd, @ptrCast(&one), 8);
        }

        /// One `poll()` round: accept, read, step, write. Slots whose request
        /// is in progress (`serving`, only inside `sleepServing`) are left untouched.
        fn pollOnce(p: *Self, timeout_ms: i32) void {
            p.pollfds[0] = .{ .fd = p.wake_fd, .events = linux.POLL.IN, .revents = 0 };
            p.pollfds[1] = .{ .fd = p.listen_fd, .events = linux.POLL.IN, .revents = 0 };
            for (p.clients, 0..) |*c, i| {
                var fd: i32 = -1;
                var events: i16 = 0;
                if (c.used and !p.serving.isSet(i)) {
                    fd = c.in;
                    if (c.conn.output().len > 0) {
                        fd = c.out;
                        events = linux.POLL.OUT;
                    } else if (inputRoom(&c.conn) > 0) {
                        events = linux.POLL.IN;
                    }
                }
                p.pollfds[2 + i] = .{ .fd = fd, .events = events, .revents = 0 };
            }
            const rc = linux.poll(p.pollfds.ptr, p.pollfds.len, timeout_ms);
            switch (linux.errno(rc)) {
                .SUCCESS => {},
                .INTR => return,
                else => {
                    sleepMs(10);
                    return;
                },
            }
            if (p.pollfds[0].revents != 0) {
                var v: u64 = 0;
                _ = linux.read(p.wake_fd, @ptrCast(&v), 8);
            }
            if (p.stopping.load(.acquire)) return;
            if (p.pollfds[1].revents != 0) p.acceptSome();
            for (p.clients, 0..) |*c, i| {
                const re = p.pollfds[2 + i].revents;
                if (re == 0 or !c.used or p.serving.isSet(i)) continue;
                if (re & (linux.POLL.IN | linux.POLL.HUP | linux.POLL.ERR | linux.POLL.NVAL) != 0) {
                    p.readClient(i, re & linux.POLL.HUP != 0);
                } else if (re & linux.POLL.OUT != 0) {
                    p.service(i);
                }
                if (p.stopping.load(.acquire)) return;
            }
        }

        fn acceptSome(p: *Self) void {
            var n: usize = 0;
            while (n < accept_burst) : (n += 1) {
                const rc = linux.accept4(p.listen_fd, null, null, linux.SOCK.NONBLOCK | linux.SOCK.CLOEXEC);
                switch (linux.errno(rc)) {
                    .SUCCESS => {},
                    .AGAIN => return,
                    .INTR, .CONNABORTED => continue,
                    // Descriptor/memory exhaustion is transient (clients hang up);
                    // back off instead of spinning on a readable listener.
                    .MFILE, .NFILE, .NOBUFS, .NOMEM, .PERM => {
                        sleepMs(100);
                        return;
                    },
                    else => return,
                }
                const cfd: i32 = @intCast(rc);
                if (p.is_tcp) {
                    const one: u32 = 1;
                    _ = linux.setsockopt(cfd, linux.IPPROTO.TCP, linux.TCP.NODELAY, @ptrCast(&one), @sizeOf(u32));
                }
                if (p.addClient(cfd, cfd, true) != null) continue;
                if (p.evictable()) |victim| {
                    p.dropClient(victim);
                    _ = p.addClient(cfd, cfd, true);
                    continue;
                }
                p.refused += 1;
                // A flood must not flood stderr.
                if (p.refused == 1 or p.refused % 1000 == 0)
                    std.debug.print("9proc: refused connection ({d} clients open, {d} refused so far)\n", .{ p.clients.len, p.refused });
                _ = linux.close(cfd);
            }
        }

        /// The longest-idle slot holder without fids, if idle long enough.
        fn evictable(p: *Self) ?usize {
            const now = monotonicMs();
            var best: ?usize = null;
            for (p.clients, 0..) |*c, i| {
                if (!c.used or !c.owned) continue;
                if (p.serving.isSet(i)) continue;
                if (c.conn.fidCount() != 0) continue;
                if (now - c.last_active < evict_idle_ms) continue;
                if (best == null or c.last_active < p.clients[best.?].last_active) best = i;
            }
            return best;
        }

        fn addClient(p: *Self, in: i32, out: i32, owned: bool) ?usize {
            for (p.clients, 0..) |*c, i| {
                if (c.used) continue;
                c.conn = Srv.Conn.init(p.shared, &p.conns[i], p.msize);
                c.in = in;
                c.out = out;
                c.used = true;
                c.owned = owned;
                c.is_socket = true;
                c.last_active = monotonicMs();
                _ = p.nclients.fetchAdd(1, .acq_rel);
                return i;
            }
            return null;
        }

        fn dropClient(p: *Self, i: usize) void {
            const c = &p.clients[i];
            if (!c.used) return;
            c.conn.hangup();
            if (c.owned) {
                _ = linux.close(c.in);
                if (c.out != c.in) _ = linux.close(c.out);
            }
            c.used = false;
            c.in = -1;
            c.out = -1;
            _ = p.nclients.fetchSub(1, .acq_rel);
        }

        /// Free space in the connection's input buffer (cloud9 keeps one
        /// msize-sized frame; `push` copies at most this much).
        fn inputRoom(conn: *const Srv.Conn) usize {
            return conn.server.in.len - conn.server.in_len;
        }

        fn readClient(p: *Self, i: usize, hup: bool) void {
            const c = &p.clients[i];
            var buf: [64 * 1024]u8 = undefined;
            const room = inputRoom(&c.conn);
            if (room == 0) return p.service(i);
            const want = @min(room, buf.len);
            while (true) {
                const rc = linux.read(c.in, &buf, want);
                switch (linux.errno(rc)) {
                    .SUCCESS => {
                        if (rc == 0) return p.dropClient(i);
                        const taken = c.conn.push(buf[0..rc]);
                        std.debug.assert(taken == rc);
                        c.last_active = monotonicMs();
                        return p.service(i);
                    },
                    .INTR => continue,
                    .AGAIN => {
                        if (hup) p.dropClient(i);
                        return;
                    },
                    else => return p.dropClient(i),
                }
            }
        }

        /// Runs requests and drains output until nothing moves.
        fn service(p: *Self, i: usize) void {
            const c = &p.clients[i];
            std.debug.assert(!p.serving.isSet(i));
            p.serving.set(i);
            defer p.serving.unset(i);
            while (c.used) {
                var moved = false;
                while (true) {
                    const more = c.conn.step() catch return p.dropClient(i);
                    if (!more) break;
                    moved = true;
                }
                const before = c.conn.output().len;
                p.flush(c) catch return p.dropClient(i);
                if (c.conn.output().len != before) moved = true;
                if (!moved) return;
            }
        }

        fn flush(p: *Self, c: *Client) error{Closed}!void {
            _ = p;
            while (c.conn.output().len > 0) {
                const chunk = c.conn.output();
                const rc = if (c.is_socket)
                    linux.sendto(c.out, chunk.ptr, chunk.len, linux.MSG.NOSIGNAL, null, 0)
                else
                    linux.write(c.out, chunk.ptr, chunk.len);
                switch (linux.errno(rc)) {
                    .SUCCESS => {
                        if (rc == 0) return;
                        c.conn.wrote(rc);
                    },
                    .INTR => continue,
                    .AGAIN => return,
                    .NOTSOCK => c.is_socket = false,
                    else => return error.Closed,
                }
            }
        }

        // -- listeners -------------------------------------------------------

        fn check(p: *Self, rc: usize) Error!void {
            const e = linux.errno(rc);
            if (e != .SUCCESS) {
                p.last_errno = e;
                return error.Syscall;
            }
        }

        fn setNonblock(p: *Self, fd: i32) Error!void {
            const rc = linux.fcntl(fd, linux.F.GETFL, 0);
            try p.check(rc);
            const nonblock: u32 = @bitCast(linux.O{ .NONBLOCK = true });
            try p.check(linux.fcntl(fd, linux.F.SETFL, rc | nonblock));
        }

        fn listenUnix(p: *Self, path: []const u8) Error!void {
            var sa: linux.sockaddr.un = .{ .path = @splat(0) };
            if (path.len == 0 or path.len >= sa.path.len) return error.PathTooLong;
            @memcpy(sa.path[0..path.len], path);
            const rc = linux.socket(linux.AF.UNIX, linux.SOCK.STREAM | linux.SOCK.CLOEXEC | linux.SOCK.NONBLOCK, 0);
            try p.check(rc);
            const lfd: i32 = @intCast(rc);
            errdefer _ = linux.close(lfd);
            // No libc, so no "is it still listening" probe: unlink a stale socket and bind.
            _ = linux.unlink(@ptrCast(&sa.path));
            try p.check(linux.bind(lfd, @ptrCast(&sa), @sizeOf(linux.sockaddr.un)));
            try p.check(linux.listen(lfd, 128));
            p.listen_fd = lfd;
            p.own_listener = true;
            p.unix_path = sa.path;
            p.unix_len = path.len;
        }

        fn listenTcp(p: *Self, text: []const u8) Error!void {
            const sa = parseIpv4(text) orelse return error.BadAddress;
            const rc = linux.socket(linux.AF.INET, linux.SOCK.STREAM | linux.SOCK.CLOEXEC | linux.SOCK.NONBLOCK, 0);
            try p.check(rc);
            const lfd: i32 = @intCast(rc);
            errdefer _ = linux.close(lfd);
            const one: u32 = 1;
            _ = linux.setsockopt(lfd, linux.SOL.SOCKET, linux.SO.REUSEADDR, @ptrCast(&one), @sizeOf(u32));
            try p.check(linux.bind(lfd, @ptrCast(&sa), @sizeOf(linux.sockaddr.in)));
            try p.check(linux.listen(lfd, 128));
            p.listen_fd = lfd;
            p.own_listener = true;
            p.is_tcp = true;
        }
    };
}

/// Entropy for the core's fid hash (so fid numbers cannot be chosen to
/// collide); falls back to the clock if getrandom fails.
fn randomSeed() u32 {
    var b: [4]u8 = undefined;
    if (linux.errno(linux.getrandom(&b, b.len, 0)) == .SUCCESS) return std.mem.readInt(u32, &b, .little);
    var ts: linux.timespec = undefined;
    _ = linux.clock_gettime(.MONOTONIC, &ts);
    return @truncate(@as(u64, @bitCast(ts.nsec)) ^ (@as(u64, @bitCast(ts.sec)) << 20));
}

/// "a.b.c.d:port" as a socket address, or null.
pub fn parseIpv4(text: []const u8) ?linux.sockaddr.in {
    const colon = std.mem.lastIndexOfScalar(u8, text, ':') orelse return null;
    const port = std.fmt.parseInt(u16, text[colon + 1 ..], 10) catch return null;
    var octets: [4]u8 = undefined;
    var it = std.mem.splitScalar(u8, text[0..colon], '.');
    for (&octets) |*o| o.* = std.fmt.parseInt(u8, it.next() orelse return null, 10) catch return null;
    if (it.next() != null) return null;
    return .{ .port = std.mem.nativeToBig(u16, port), .addr = @bitCast(octets) };
}

/// Names the calling thread (comm, at most 15 bytes) via prctl.
pub fn setThreadName(name: []const u8) void {
    var buf: [16]u8 = @splat(0);
    const n = @min(name.len, 15);
    @memcpy(buf[0..n], name[0..n]);
    _ = linux.prctl(@intFromEnum(linux.PR.SET_NAME), @intFromPtr(&buf), 0, 0, 0);
}

pub fn sleepMs(ms: u64) void {
    var req: linux.timespec = .{ .sec = @intCast(ms / 1000), .nsec = @intCast((ms % 1000) * 1_000_000) };
    var rem: linux.timespec = undefined;
    while (linux.errno(linux.nanosleep(&req, &rem)) == .INTR) req = rem;
}

pub fn monotonicMs() i64 {
    var ts: linux.timespec = undefined;
    _ = linux.clock_gettime(.MONOTONIC, &ts);
    return ts.sec * 1000 + @divTrunc(ts.nsec, 1_000_000);
}

// ---------------------------------------------------------------------------
// Tests: a real unix socket, a cloud9.Client on the other end.
// ---------------------------------------------------------------------------

const testing = std.testing;

test {
    _ = debug;
    _ = provider;
    _ = runtime;
}

const TestBuild = struct {
    pub const zig_version: []const u8 = builtin.zig_version_string;
    pub const target: []const u8 = "test";
    pub const optimize: []const u8 = "Debug";
    pub const time: []const u8 = "2024-01-01T00:00:00Z";
    pub const change: []const u8 = "none";
};

const test_cfg: core.Config = .{
    .name = "probetest",
    .build = TestBuild,
    .msize = 8192,
    .max_fids = 16,
    .max_providers = 6,
    .snapshot_slots = 2,
    .snapshot_bytes = 1024,
};
const TS = core.Server(test_cfg);
const TP = Probe(TS);

/// A blocking client over a connected socket.
const SockClient = struct {
    fd: i32,
    client: cloud9.Client,
    cin: [8192]u8 = undefined,
    cout: [8192]u8 = undefined,

    fn connect(sc: *SockClient, path: []const u8) !void {
        var sa: linux.sockaddr.un = .{ .path = @splat(0) };
        @memcpy(sa.path[0..path.len], path);
        const rc = linux.socket(linux.AF.UNIX, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, 0);
        if (linux.errno(rc) != .SUCCESS) return error.Socket;
        sc.fd = @intCast(rc);
        if (linux.errno(linux.connect(sc.fd, &sa, @sizeOf(linux.sockaddr.un))) != .SUCCESS) return error.Connect;
        sc.client = .init(.{ .in = &sc.cin, .out = &sc.cout });
    }

    fn close(sc: *SockClient) void {
        _ = linux.close(sc.fd);
    }

    /// One round trip; null when the server closed the connection.
    fn rpc(sc: *SockClient, req: cloud9.Client.Request) !?cloud9.Client.Result {
        _ = try sc.client.submit(req);
        while (sc.client.output().len > 0) {
            const out = sc.client.output();
            const rc = linux.write(sc.fd, out.ptr, out.len);
            switch (linux.errno(rc)) {
                .SUCCESS => sc.client.wrote(rc),
                .PIPE, .CONNRESET => return null,
                else => return error.Write,
            }
        }
        var buf: [8192]u8 = undefined;
        while (true) {
            if (sc.client.take()) |done| return done.result;
            const rc = linux.read(sc.fd, &buf, buf.len);
            switch (linux.errno(rc)) {
                .SUCCESS => {},
                .CONNRESET => return null,
                else => return error.Read,
            }
            if (rc == 0) return null;
            var rest: []const u8 = buf[0..rc];
            while (rest.len > 0) rest = rest[sc.client.push(rest)..];
        }
    }

    fn session(sc: *SockClient) !void {
        const v = (try sc.rpc(.{ .version = .{ .msize = 8192 } })) orelse return error.Closed;
        try testing.expectEqual(@as(u32, 8192), v.version.msize);
        const a = (try sc.rpc(.{ .attach = .{ .fid = 0, .uname = "t" } })) orelse return error.Closed;
        try testing.expect(a == .attach);
    }

    fn readFile(sc: *SockClient, names: []const []const u8, out: []u8) ![]u8 {
        const w = (try sc.rpc(.{ .walk = .{ .fid = 0, .newfid = 1, .names = names } })) orelse return error.Closed;
        try testing.expectEqual(@as(u16, @intCast(names.len)), w.walk.nwqid);
        _ = (try sc.rpc(.{ .open = .{ .fid = 1, .mode = cloud9.oread } })) orelse return error.Closed;
        const r = (try sc.rpc(.{ .read = .{ .fid = 1, .offset = 0, .count = @intCast(out.len) } })) orelse return error.Closed;
        const n = r.read.len;
        @memcpy(out[0..n], r.read);
        _ = (try sc.rpc(.{ .clunk = .{ .fid = 1 } })) orelse return error.Closed;
        return out[0..n];
    }
};

fn testSockPath(buf: []u8, tag: []const u8) ![]const u8 {
    return std.fmt.bufPrint(buf, "/tmp/9proc-probe-{d}-{s}.sock", .{ linux.getpid(), tag });
}

const TestCtx = struct { info: runtime.Info };

test "probe: start, serve a client over a unix socket, stop" {
    var ctx: TestCtx = .{ .info = .now() };
    var shared: TS.Shared = .init(&ctx);
    const storage = try testing.allocator.create(TP.Storage(2));
    defer testing.allocator.destroy(storage);
    var probe: TP = undefined;
    var path_buf: [64]u8 = undefined;
    const path = try testSockPath(&path_buf, "basic");
    try probe.init(&shared, storage, .{ .io = testing.io, .listen = .{ .unix = path } });
    defer probe.stop();
    try probe.start();
    ctx.info.clients = probe.clientCounter();

    var sc: SockClient = undefined;
    try sc.connect(path);
    defer sc.close();
    try sc.session();
    var buf: [1024]u8 = undefined;
    const zv = try sc.readFile(&.{ "build", "zig_version" }, &buf);
    try testing.expectEqualStrings(builtin.zig_version_string, zv);
    try testing.expectEqual(@as(u32, 1), probe.clientCount());

    // The debug providers are mounted: /threads lists the probe thread by name.
    const names = (try sc.rpc(.{ .walk = .{ .fid = 0, .newfid = 2, .names = &.{"threads"} } })) orelse return error.Closed;
    try testing.expectEqual(@as(u16, 1), names.walk.nwqid);
    _ = (try sc.rpc(.{ .open = .{ .fid = 2, .mode = cloud9.oread } })) orelse return error.Closed;
    const dir = (try sc.rpc(.{ .read = .{ .fid = 2, .offset = 0, .count = 4096 } })) orelse return error.Closed;
    try testing.expect(dir.read.len > 0);
    _ = (try sc.rpc(.{ .clunk = .{ .fid = 2 } })) orelse return error.Closed;

    // A missing file is the Plan 9 error string.
    const bad = (try sc.rpc(.{ .walk = .{ .fid = 0, .newfid = 3, .names = &.{"nope"} } })) orelse return error.Closed;
    try testing.expect(bad == .fail);
    try testing.expectEqualStrings("file does not exist", bad.fail);

    probe.stop();
    // stop() is idempotent and the socket file is gone.
    probe.stop();
    var gone: SockClient = undefined;
    try testing.expectError(error.Connect, gone.connect(path));
    // The debug facilities can be set up again after stop.
    var probe2: TP = undefined;
    var shared2: TS.Shared = .init(&ctx);
    try probe2.init(&shared2, storage, .{ .io = testing.io, .listen = .{ .unix = path } });
    probe2.stop();
}

test "probe: max_clients refusal and idle eviction" {
    var ctx: TestCtx = .{ .info = .now() };
    var shared: TS.Shared = .init(&ctx);
    const storage = try testing.allocator.create(TP.Storage(2));
    defer testing.allocator.destroy(storage);
    var probe: TP = undefined;
    var path_buf: [64]u8 = undefined;
    const path = try testSockPath(&path_buf, "limit");
    try probe.init(&shared, storage, .{ .io = testing.io, .listen = .{ .unix = path } });
    defer probe.stop();
    try probe.start();

    // Two attached clients fill the table; a third is accepted then closed.
    var a: SockClient = undefined;
    try a.connect(path);
    defer a.close();
    try a.session();
    var b: SockClient = undefined;
    try b.connect(path);
    defer b.close();
    try b.session();
    var c: SockClient = undefined;
    try c.connect(path);
    defer c.close();
    try testing.expectEqual(@as(?cloud9.Client.Result, null), try c.rpc(.{ .version = .{ .msize = 8192 } }));
    try testing.expectEqual(@as(u64, 1), probe.refused);
    // Attached clients are never evicted, even when idle for long.
    sleepMs(evict_idle_ms + 100);
    var d: SockClient = undefined;
    try d.connect(path);
    defer d.close();
    try testing.expectEqual(@as(?cloud9.Client.Result, null), try d.rpc(.{ .version = .{ .msize = 8192 } }));
    var buf: [256]u8 = undefined;
    _ = try a.readFile(&.{"README"}, &buf);

    // A client without fids that has been idle long enough gives way.
    _ = (try b.rpc(.{ .clunk = .{ .fid = 0 } })) orelse return error.Closed;
    sleepMs(evict_idle_ms + 100);
    var e: SockClient = undefined;
    try e.connect(path);
    defer e.close();
    try e.session();
    try testing.expectEqual(@as(?cloud9.Client.Result, null), try b.rpc(.{ .version = .{ .msize = 8192 } }));
    try testing.expectEqual(@as(u32, 2), probe.clientCount());
}

test "probe: single pre-connected client mode ends when the client hangs up" {
    var ctx: TestCtx = .{ .info = .now() };
    var shared: TS.Shared = .init(&ctx);
    const storage = try testing.allocator.create(TP.Storage(1));
    defer testing.allocator.destroy(storage);
    var sv: [2]i32 = undefined;
    try testing.expectEqual(linux.E.SUCCESS, linux.errno(linux.socketpair(linux.AF.UNIX, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, 0, &sv)));
    var probe: TP = undefined;
    try probe.init(&shared, storage, .{ .io = testing.io, .listen = .{ .client = .{ .in = sv[1], .out = sv[1] } } });
    defer probe.stop();
    try probe.start();
    var sc: SockClient = .{ .fd = sv[0], .client = undefined };
    sc.client = .init(.{ .in = &sc.cin, .out = &sc.cout });
    try sc.session();
    var buf: [256]u8 = undefined;
    try testing.expect((try sc.readFile(&.{"README"}, &buf)).len > 0);
    try testing.expectEqual(@as(u32, 1), probe.clientCount());
    _ = linux.close(sv[0]);
    probe.wait();
    try testing.expectEqual(@as(u32, 0), probe.clientCount());
    _ = linux.close(sv[1]);
}

test "parseIpv4 and sleepServing off the probe thread" {
    const sa = parseIpv4("127.0.0.1:564").?;
    try testing.expectEqual(std.mem.nativeToBig(u16, 564), sa.port);
    try testing.expectEqual(@as(u32, @bitCast([4]u8{ 127, 0, 0, 1 })), sa.addr);
    try testing.expect(parseIpv4("localhost:1") == null);
    try testing.expect(parseIpv4("1.2.3:1") == null);
    try testing.expect(parseIpv4("1.2.3.4") == null);
    try testing.expect(parseIpv4("1.2.3.4:70000") == null);
    const t0 = monotonicMs();
    var probe: TP = undefined;
    probe.thread_tid = .init(0);
    probe.serving = .initEmpty();
    probe.nested = 0;
    probe.sleepServing(20);
    try testing.expect(monotonicMs() - t0 >= 20);
}