1 files changed, 194 insertions, 0 deletions
diff --git a/lib/std/Thread/Condition.zig b/lib/std/Thread/Condition.zig
new file mode 100644
index 0000000000..a14b57f6b4
--- /dev/null
+++ b/lib/std/Thread/Condition.zig
@@ -0,0 +1,194 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! A condition provides a way for a kernel thread to block until it is signaled
+//! to wake up. Spurious wakeups are possible.
+//! This API supports static initialization and does not require deinitialization.
+
+impl: Impl = .{},
+
+const std = @import("../std.zig");
+const Condition = @This();
+const windows = std.os.windows;
+const linux = std.os.linux;
+const Mutex = std.Thread.Mutex;
+const assert = std.debug.assert;
+
+pub fn wait(cond: *Condition, mutex: *Mutex) void {
+    cond.impl.wait(mutex);
+}
+
+pub fn signal(cond: *Condition) void {
+    cond.impl.signal();
+}
+
+pub fn broadcast(cond: *Condition) void {
+    cond.impl.broadcast();
+}
+
+const Impl = if (std.builtin.single_threaded)
+    SingleThreadedCondition
+else if (std.Target.current.os.tag == .windows)
+    WindowsCondition
+else if (std.Thread.use_pthreads)
+    PthreadCondition
+else
+    AtomicCondition;
+
+pub const SingleThreadedCondition = struct {
+    pub fn wait(cond: *SingleThreadedCondition, mutex: *Mutex) void {
+        unreachable; // deadlock detected
+    }
+
+    pub fn signal(cond: *SingleThreadedCondition) void {}
+
+    pub fn broadcast(cond: *SingleThreadedCondition) void {}
+};
+
+pub const WindowsCondition = struct {
+    cond: windows.CONDITION_VARIABLE = windows.CONDITION_VARIABLE_INIT,
+
+    pub fn wait(cond: *WindowsCondition, mutex: *Mutex) void {
+        const rc = windows.kernel32.SleepConditionVariableSRW(
+            &cond.cond,
+            &mutex.srwlock,
+            windows.INFINITE,
+            @as(windows.ULONG, 0),
+        );
+        assert(rc != windows.FALSE);
+    }
+
+    pub fn signal(cond: *WindowsCondition) void {
+        windows.kernel32.WakeConditionVariable(&cond.cond);
+    }
+
+    pub fn broadcast(cond: *WindowsCondition) void {
+        windows.kernel32.WakeAllConditionVariable(&cond.cond);
+    }
+};
+
+pub const PthreadCondition = struct {
+    cond: std.c.pthread_cond_t = .{},
+
+    pub fn wait(cond: *PthreadCondition, mutex: *Mutex) void {
+        const rc = std.c.pthread_cond_wait(&cond.cond, &mutex.impl.pthread_mutex);
+        assert(rc == 0);
+    }
+
+    pub fn signal(cond: *PthreadCondition) void {
+        const rc = std.c.pthread_cond_signal(&cond.cond);
+        assert(rc == 0);
+    }
+
+    pub fn broadcast(cond: *PthreadCondition) void {
+        const rc = std.c.pthread_cond_broadcast(&cond.cond);
+        assert(rc == 0);
+    }
+};
+
+pub const AtomicCondition = struct {
+    pending: bool = false,
+    queue_mutex: Mutex = .{},
+    queue_list: QueueList = .{},
+
+    pub const QueueList = std.SinglyLinkedList(QueueItem);
+
+    pub const QueueItem = struct {
+        futex: i32 = 0,
+
+        fn wait(cond: *@This()) void {
+            while (@atomicLoad(i32, &cond.futex, .Acquire) == 0) {
+                switch (std.Target.current.os.tag) {
+                    .linux => {
+                        switch (linux.getErrno(linux.futex_wait(
+                            &cond.futex,
+                            linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAIT,
+                            0,
+                            null,
+                        ))) {
+                            0 => {},
+                            std.os.EINTR => {},
+                            std.os.EAGAIN => {},
+                            else => unreachable,
+                        }
+                    },
+                    else => spinLoopHint(),
+                }
+            }
+        }
+
+        fn notify(cond: *@This()) void {
+            @atomicStore(i32, &cond.futex, 1, .Release);
+
+            switch (std.Target.current.os.tag) {
+                .linux => {
+                    switch (linux.getErrno(linux.futex_wake(
+                        &cond.futex,
+                        linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAKE,
+                        1,
+                    ))) {
+                        0 => {},
+                        std.os.EFAULT => {},
+                        else => unreachable,
+                    }
+                },
+                else => {},
+            }
+        }
+    };
+
+    pub fn wait(cond: *AtomicCondition, mutex: *Mutex) void {
+        var waiter = QueueList.Node{ .data = .{} };
+
+        {
+            const held = cond.queue_mutex.acquire();
+            defer held.release();
+
+            cond.queue_list.prepend(&waiter);
+            @atomicStore(bool, &cond.pending, true, .SeqCst);
+        }
+
+        mutex.unlock();
+        waiter.data.wait();
+        mutex.lock();
+    }
+
+    pub fn signal(cond: *AtomicCondition) void {
+        if (@atomicLoad(bool, &cond.pending, .SeqCst) == false)
+            return;
+
+        const maybe_waiter = blk: {
+            const held = cond.queue_mutex.acquire();
+            defer held.release();
+
+            const maybe_waiter = cond.queue_list.popFirst();
+            @atomicStore(bool, &cond.pending, cond.queue_list.first != null, .SeqCst);
+            break :blk maybe_waiter;
+        };
+
+        if (maybe_waiter) |waiter|
+            waiter.data.notify();
+    }
+
+    pub fn broadcast(cond: *AtomicCondition) void {
+        if (@atomicLoad(bool, &cond.pending, .SeqCst) == false)
+            return;
+
+        @atomicStore(bool, &cond.pending, false, .SeqCst);
+
+        var waiters = blk: {
+            const held = cond.queue_mutex.acquire();
+            defer held.release();
+
+            const waiters = cond.queue_list;
+            cond.queue_list = .{};
+            break :blk waiters;
+        };
+
+        while (waiters.popFirst()) |waiter|
+            waiter.data.notify();
+    }
+};