sx/library/modules/std/thread.sx

// std.thread — OS threads, Mutex/Cond, and a bounded worker Pool over
// pthreads (PLAN-HTTPZ S6).
//
// THE RE-ENTRY CONTRACT (pinned by examples/1636): a thread entry is a
// `callconv(.c)` function — it has NO implicit context — and enters
// the sx world by fabricating one: `push Context.{ allocator = xx gpa }`
// around the default-conv code it runs. Pool workers do exactly that,
// each with its own malloc-backed GPA, so tasks allocate freely and
// never share allocator state across threads.
//
// ALLOCATOR DISCIPLINE: GPA is malloc/free — thread-safe. Arena and
// every other context-flowing allocator is NOT; never share one across
// threads. Pool buffers come from the creating context's allocator and
// are touched only under the pool mutex.
//
// MOVE SEMANTICS: a pthread mutex/cond is address-sensitive once
// initialized — POSIX leaves moving one undefined. Mutex/Cond therefore
// initialize IN PLACE (`m.setup()`) and Pool lives behind a pointer
// (`Pool.create`), never by-value copy.
//
// PER-OS: sizes below are darwin's (pthread_mutex_t 64 bytes,
// pthread_cond_t 48, pthread_t a pointer). glibc differs (40/48);
// PLAN-HTTPZ C3 selects per-OS when the linux target activates.

#import "modules/std.sx";

tlib :: #library "c";

pthread_create  :: (thread: *usize, attr: *void, start: (*void) -> *void callconv(.c), arg: *void) -> i32 extern tlib;
pthread_join    :: (thread: usize, retval: **void) -> i32 extern tlib;
pthread_detach  :: (thread: usize) -> i32 extern tlib;

pthread_mutex_init    :: (m: *MutexBuf, attr: *void) -> i32 extern tlib;
pthread_mutex_lock    :: (m: *MutexBuf) -> i32 extern tlib;
pthread_mutex_unlock  :: (m: *MutexBuf) -> i32 extern tlib;
pthread_mutex_destroy :: (m: *MutexBuf) -> i32 extern tlib;

pthread_cond_init      :: (c: *CondBuf, attr: *void) -> i32 extern tlib;
pthread_cond_wait      :: (c: *CondBuf, m: *MutexBuf) -> i32 extern tlib;
pthread_cond_signal    :: (c: *CondBuf) -> i32 extern tlib;
pthread_cond_broadcast :: (c: *CondBuf) -> i32 extern tlib;
pthread_cond_destroy   :: (c: *CondBuf) -> i32 extern tlib;

// darwin pthread_mutex_t: { long __sig; char __opaque[56]; } — 64 bytes.
MutexBuf :: struct {
    sig: i64 = 0;
    o0: i64 = 0; o1: i64 = 0; o2: i64 = 0; o3: i64 = 0;
    o4: i64 = 0; o5: i64 = 0; o6: i64 = 0;
}

// darwin pthread_cond_t: { long __sig; char __opaque[40]; } — 48 bytes.
CondBuf :: struct {
    sig: i64 = 0;
    o0: i64 = 0; o1: i64 = 0; o2: i64 = 0; o3: i64 = 0; o4: i64 = 0;
}

ThreadErr :: error {
    Spawn,    // pthread_create refused
    Init,     // mutex/cond/pool initialization failed
}

// ── Mutex / Cond (in-place; see MOVE SEMANTICS above) ────────────────

Mutex :: struct {
    buf: MutexBuf = .{};

    setup :: (self: *Mutex) -> bool {
        return pthread_mutex_init(@self.buf, null) == 0;
    }
    lock :: (self: *Mutex) {
        pthread_mutex_lock(@self.buf);
    }
    unlock :: (self: *Mutex) {
        pthread_mutex_unlock(@self.buf);
    }
    destroy :: (self: *Mutex) {
        pthread_mutex_destroy(@self.buf);
    }
}

Cond :: struct {
    buf: CondBuf = .{};

    setup :: (self: *Cond) -> bool {
        return pthread_cond_init(@self.buf, null) == 0;
    }
    // Atomically releases `m` and sleeps; reacquires `m` before returning.
    wait :: (self: *Cond, m: *Mutex) {
        pthread_cond_wait(@self.buf, @m.buf);
    }
    signal :: (self: *Cond) {
        pthread_cond_signal(@self.buf);
    }
    broadcast :: (self: *Cond) {
        pthread_cond_broadcast(@self.buf);
    }
    destroy :: (self: *Cond) {
        pthread_cond_destroy(@self.buf);
    }
}

// ── Thread ───────────────────────────────────────────────────────────

Thread :: struct {
    handle: usize = 0;

    // `entry` is the C->sx boundary: callconv(.c), fabricates its own
    // Context before touching default-conv sx code (examples/1636).
    spawn :: (entry: (*void) -> *void callconv(.c), arg: *void) -> (Thread, !ThreadErr) {
        t : Thread = .{};
        if pthread_create(@t.handle, null, entry, arg) != 0 { raise error.Spawn; }
        return t;
    }
    join :: (self: *Thread) {
        pthread_join(self.handle, null);
    }
    detach :: (self: *Thread) {
        pthread_detach(self.handle);
    }
}

// ── Pool: fixed workers, bounded queue, backpressure ────────────────
//
// httpz's thread_pool shape: `submit` enqueues a task for the next free
// worker and returns false when the backlog is full (the caller sheds —
// reject, retry, or run inline). `shutdown` drains nothing: queued
// tasks still run, then workers exit and join.

PoolTask :: struct {
    f: (usize) -> void;   // default-conv: runs inside the worker's context
    arg: usize = 0;
}

Pool :: struct {
    mu: Mutex = .{};
    nonempty: Cond = .{};
    tasks: [*]PoolTask = null;
    cap: i64 = 0;
    head: i64 = 0;
    len: i64 = 0;
    stop: bool = false;
    threads: [*]usize = null;
    count: i64 = 0;

    // Heap-allocate (the pool must never move: workers hold its address,
    // and it embeds a live mutex), init in place, spawn the workers.
    create :: (workers: i64, backlog: i64) -> (*Pool, !ThreadErr) {
        alloc := context.allocator;
        p : *Pool = xx alloc.alloc_bytes(size_of(Pool));
        p.* = Pool.{};
        if !p.mu.setup() { raise error.Init; }
        if !p.nonempty.setup() { raise error.Init; }
        p.tasks = xx alloc.alloc_bytes(backlog * size_of(PoolTask));
        p.cap = backlog;
        p.threads = xx alloc.alloc_bytes(workers * size_of(usize));
        p.count = workers;
        i : i64 = 0;
        while i < workers {
            if pthread_create(@p.threads[i], null, pool_worker, xx p) != 0 {
                // join what started, then fail loudly
                p.count = i;
                p.shutdown();
                raise error.Spawn;
            }
            i += 1;
        }
        return p;
    }

    // False = backlog full (backpressure); the task did not enqueue.
    submit :: (self: *Pool, f: (usize) -> void, arg: usize) -> bool {
        self.mu.lock();
        if self.len == self.cap or self.stop {
            self.mu.unlock();
            return false;
        }
        slot := (self.head + self.len) % self.cap;
        self.tasks[slot] = PoolTask.{ f = f, arg = arg };
        self.len += 1;
        self.nonempty.signal();
        self.mu.unlock();
        return true;
    }

    // Stop accepting work, let queued tasks finish, join every worker.
    shutdown :: (self: *Pool) {
        self.mu.lock();
        self.stop = true;
        self.nonempty.broadcast();
        self.mu.unlock();
        i : i64 = 0;
        while i < self.count {
            pthread_join(self.threads[i], null);
            i += 1;
        }
        self.mu.destroy();
        self.nonempty.destroy();
    }
}

// The worker loop: C entry, own fabricated Context, then
// pop-task/run-task until stop with an empty queue.
pool_worker :: (arg: *void) -> *void callconv(.c) {
    p : *Pool = xx arg;
    gpa := GPA.init();
    push Context.{ allocator = xx gpa } {
        while true {
            p.mu.lock();
            while !p.stop and p.len == 0 {
                p.nonempty.wait(@p.mu);
            }
            if p.len == 0 {   // stop, and nothing queued
                p.mu.unlock();
                break;
            }
            t := p.tasks[p.head];
            p.head = (p.head + 1) % p.cap;
            p.len -= 1;
            p.mu.unlock();
            f := t.f;
            f(t.arg);
        }
    }
    return null;
}