canceled

        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();
        }