unix: ensure done_cb gets called after uv_cancel()

Wake up the event loop with uv_async_send() when a request is cancelled.
Ensures the done_cb is run on the next tick of the event loop.

Not sending a wakeup signal results in the done_cb not getting called until
another request completes, which may be a long time coming when it's the only
request in the queue or when other requests are executing long-running jobs.

Fixes #669.

src/unix/threadpool.c

@@ -158,6 +158,7 @@ int uv__work_cancel(uv_loop_t* loop, uv_req_t* req, struct uv__work* w) {
   w->work = uv__cancelled;
   uv_mutex_lock(&loop->wq_mutex);
   ngx_queue_insert_tail(&loop->wq, &w->wq);
+  uv_async_send(&loop->wq_async);
   uv_mutex_unlock(&loop->wq_mutex);
 
   return 0;

test/test-list.h

@@ -193,6 +193,7 @@ TEST_DECLARE   (threadpool_multiple_event_loops)
 TEST_DECLARE   (threadpool_cancel_getaddrinfo)
 TEST_DECLARE   (threadpool_cancel_work)
 TEST_DECLARE   (threadpool_cancel_fs)
+TEST_DECLARE   (threadpool_cancel_single)
 TEST_DECLARE   (thread_mutex)
 TEST_DECLARE   (thread_rwlock)
 TEST_DECLARE   (thread_create)
@@ -468,6 +469,7 @@ TASK_LIST_START
   TEST_ENTRY  (threadpool_cancel_getaddrinfo)
   TEST_ENTRY  (threadpool_cancel_work)
   TEST_ENTRY  (threadpool_cancel_fs)
+  TEST_ENTRY  (threadpool_cancel_single)
   TEST_ENTRY  (thread_mutex)
   TEST_ENTRY  (thread_rwlock)
   TEST_ENTRY  (thread_create)

test/test-threadpool-cancel.c

@@ -156,6 +156,15 @@ static void timer_cb(uv_timer_t* handle, int status) {
 }
 
 
+static void nop_work_cb(uv_work_t* req) {
+}
+
+
+static void nop_done_cb(uv_work_t* req, int status) {
+  req->data = "OK";
+}
+
+
 TEST_IMPL(threadpool_cancel_getaddrinfo) {
   uv_getaddrinfo_t reqs[4];
   struct cancel_info ci;
@@ -264,3 +273,35 @@ TEST_IMPL(threadpool_cancel_fs) {
 
   return 0;
 }
+
+
+TEST_IMPL(threadpool_cancel_single) {
+  uv_loop_t* loop;
+  uv_work_t req;
+  int cancelled;
+  int i;
+
+  loop = uv_default_loop();
+  for (i = 0; i < 5000; i++) {
+    req.data = NULL;
+    ASSERT(0 == uv_queue_work(loop, &req, nop_work_cb, nop_done_cb));
+
+    cancelled = uv_cancel((uv_req_t*) &req);
+    if (cancelled == 0)
+      break;
+
+    ASSERT(0 == uv_run(loop));
+  }
+
+  if (cancelled != 0) {
+    fputs("Failed to cancel a work req in 5,000 iterations, giving up.\n",
+          stderr);
+    return 1;
+  }
+
+  ASSERT(req.data == NULL);
+  ASSERT(0 == uv_run(loop));
+  ASSERT(req.data != NULL);  /* Should have been updated by nop_done_cb(). */
+
+  return 0;
+}