Waiter

    const Waiter = struct {
        sleeper: Sleeper = undefined,
        cancelable: Cancelable,
        mutex: *Mutex,
        node: std.DoublyLinkedList.Node = .{},

        fn add(context: ?*anyopaque) callconv(.c) void {
            const waiter: *Waiter = @ptrCast(@alignCast(context));
            waiter.cancelable.enter(waiter.sleeper.fiber) catch |err| switch (err) {
                error.CancelRequested => return waiter.wake(),
            };
            var state = @atomicRmw(State, &waiter.mutex.state, .Add, .{
                .locked = false,
                .num_waiters = 1,
            }, .monotonic);
            state.num_waiters += 1;
            while (!state.locked) {
                @branchHint(.unlikely);
                state = @cmpxchgWeak(State, &waiter.mutex.state, state, .{
                    .locked = true,
                    .num_waiters = state.num_waiters - 1,
                }, .acquire, .monotonic) orelse break;
            } else return waiter.mutex.waiters.append(&waiter.node);
            waiter.cancelable.leave(waiter.sleeper.fiber) catch |err| switch (err) {
                error.CancelRequested => {
                    waiter.node.next = &waiter.node;
                    return;
                },
            };
            waiter.wake();
        }

        fn canceled(context: ?*anyopaque) callconv(.c) void {
            const cancelable: *Cancelable = @ptrCast(@alignCast(context));
            const waiter: *Waiter = @fieldParentPtr("cancelable", cancelable);
            cancelable.requested(waiter.sleeper.fiber);
            const mutex = waiter.mutex;
            if (waiter.node.next != &waiter.node) {
                @branchHint(.likely);
                mutex.waiters.remove(&waiter.node);
                assert(@atomicRmw(State, &mutex.state, .Sub, .{
                    .locked = false,
                    .num_waiters = 1,
                }, .monotonic).num_waiters >= 1);
            }
            waiter.node = undefined;
            waiter.wake();
        }

        fn remove(context: ?*anyopaque) callconv(.c) void {
            const mutex: *Mutex = @ptrCast(@alignCast(context));
            var state = @atomicLoad(State, &mutex.state, .monotonic);
            while (!state.locked and state.num_waiters > 0) {
                @branchHint(.likely);
                state = @cmpxchgWeak(State, &mutex.state, state, .{
                    .locked = true,
                    .num_waiters = state.num_waiters - 1,
                }, .acquire, .monotonic) orelse break;
            } else return;
            var num_removed: State.NumWaiters = 0;
            while (mutex.waiters.popFirst()) |node| {
                @branchHint(.likely);
                const waiter: *Waiter = @fieldParentPtr("node", node);
                node.* = undefined;
                waiter.cancelable.leave(waiter.sleeper.fiber) catch |err| switch (err) {
                    error.CancelRequested => {
                        num_removed += 1;
                        node.next = node;
                        continue;
                    },
                };
                break;
            }
            if (num_removed > 0) {
                @branchHint(.unlikely);
                assert(@atomicRmw(State, &mutex.state, .Sub, .{
                    .locked = false,
                    .num_waiters = num_removed,
                }, .monotonic).num_waiters >= num_removed);
            }
        }

        fn wake(waiter: *Waiter) void {
            Sleeper.wake(&waiter.sleeper);
        }
    }