extract ThreadPool and WaitGroup from compiler to std lib

lib/std/Thread/Pool.zig

@@ -0,0 +1,152 @@
+const std = @import("std");
+const builtin = @import("builtin");
+const Pool = @This();
+const WaitGroup = @import("WaitGroup.zig");
+
+mutex: std.Thread.Mutex = .{},
+cond: std.Thread.Condition = .{},
+run_queue: RunQueue = .{},
+is_running: bool = true,
+allocator: std.mem.Allocator,
+threads: []std.Thread,
+
+const RunQueue = std.SinglyLinkedList(Runnable);
+const Runnable = struct {
+    runFn: RunProto,
+};
+
+const RunProto = *const fn (*Runnable) void;
+
+pub fn init(pool: *Pool, allocator: std.mem.Allocator) !void {
+    pool.* = .{
+        .allocator = allocator,
+        .threads = &[_]std.Thread{},
+    };
+
+    if (builtin.single_threaded) {
+        return;
+    }
+
+    const thread_count = std.math.max(1, std.Thread.getCpuCount() catch 1);
+    pool.threads = try allocator.alloc(std.Thread, thread_count);
+    errdefer allocator.free(pool.threads);
+
+    // kill and join any threads we spawned previously on error.
+    var spawned: usize = 0;
+    errdefer pool.join(spawned);
+
+    for (pool.threads) |*thread| {
+        thread.* = try std.Thread.spawn(.{}, worker, .{pool});
+        spawned += 1;
+    }
+}
+
+pub fn deinit(pool: *Pool) void {
+    pool.join(pool.threads.len); // kill and join all threads.
+    pool.* = undefined;
+}
+
+fn join(pool: *Pool, spawned: usize) void {
+    if (builtin.single_threaded) {
+        return;
+    }
+
+    {
+        pool.mutex.lock();
+        defer pool.mutex.unlock();
+
+        // ensure future worker threads exit the dequeue loop
+        pool.is_running = false;
+    }
+
+    // wake up any sleeping threads (this can be done outside the mutex)
+    // then wait for all the threads we know are spawned to complete.
+    pool.cond.broadcast();
+    for (pool.threads[0..spawned]) |thread| {
+        thread.join();
+    }
+
+    pool.allocator.free(pool.threads);
+}
+
+pub fn spawn(pool: *Pool, comptime func: anytype, args: anytype) !void {
+    if (builtin.single_threaded) {
+        @call(.auto, func, args);
+        return;
+    }
+
+    const Args = @TypeOf(args);
+    const Closure = struct {
+        arguments: Args,
+        pool: *Pool,
+        run_node: RunQueue.Node = .{ .data = .{ .runFn = runFn } },
+
+        fn runFn(runnable: *Runnable) void {
+            const run_node = @fieldParentPtr(RunQueue.Node, "data", runnable);
+            const closure = @fieldParentPtr(@This(), "run_node", run_node);
+            @call(.auto, func, closure.arguments);
+
+            // The thread pool's allocator is protected by the mutex.
+            const mutex = &closure.pool.mutex;
+            mutex.lock();
+            defer mutex.unlock();
+
+            closure.pool.allocator.destroy(closure);
+        }
+    };
+
+    {
+        pool.mutex.lock();
+        defer pool.mutex.unlock();
+
+        const closure = try pool.allocator.create(Closure);
+        closure.* = .{
+            .arguments = args,
+            .pool = pool,
+        };
+
+        pool.run_queue.prepend(&closure.run_node);
+    }
+
+    // Notify waiting threads outside the lock to try and keep the critical section small.
+    pool.cond.signal();
+}
+
+fn worker(pool: *Pool) void {
+    pool.mutex.lock();
+    defer pool.mutex.unlock();
+
+    while (true) {
+        while (pool.run_queue.popFirst()) |run_node| {
+            // Temporarily unlock the mutex in order to execute the run_node
+            pool.mutex.unlock();
+            defer pool.mutex.lock();
+
+            const runFn = run_node.data.runFn;
+            runFn(&run_node.data);
+        }
+
+        // Stop executing instead of waiting if the thread pool is no longer running.
+        if (pool.is_running) {
+            pool.cond.wait(&pool.mutex);
+        } else {
+            break;
+        }
+    }
+}
+
+pub fn waitAndWork(pool: *Pool, wait_group: *WaitGroup) void {
+    while (!wait_group.isDone()) {
+        if (blk: {
+            pool.mutex.lock();
+            defer pool.mutex.unlock();
+            break :blk pool.run_queue.popFirst();
+        }) |run_node| {
+            run_node.data.runFn(&run_node.data);
+            continue;
+        }
+
+        wait_group.wait();
+        return;
+    }
+}