unix,win: fix threadpool race condition

90891b4 introduced a race
condition when accessing `slow_io_work_running` – it is being
increased and later decreased as part of the worker thread loop,
but was accessed with different mutexes during these operations.

This fixes the race condition by making sure both accesses
are protected through the global `mutex` of `threadpool.c`.

This fixes a number of flaky Node.js tests.

Refs: libuv#1845
Refs: nodejs/reliability#18
Refs: nodejs/node#23089
Refs: nodejs/node#23067
Refs: nodejs/node#23066
Refs: nodejs/node#23219

src/threadpool.c

@@ -62,10 +62,10 @@ static void worker(void* arg) {
   uv_sem_post((uv_sem_t*) arg);
   arg = NULL;
 
+  uv_mutex_lock(&mutex);
   for (;;) {
-    uv_mutex_lock(&mutex);
+    /* `mutex` should always be locked at this point. */
 
-  wait_for_work:
     /* Keep waiting while either no work is present or only slow I/O
        and we're at the threshold for that. */
     while (QUEUE_EMPTY(&wq) ||
@@ -93,13 +93,13 @@ static void worker(void* arg) {
          other work in the queue is done. */
       if (slow_io_work_running >= slow_work_thread_threshold()) {
         QUEUE_INSERT_TAIL(&wq, q);
-        goto wait_for_work;
+        continue;
       }
 
       /* If we encountered a request to run slow I/O work but there is none
          to run, that means it's cancelled => Start over. */
       if (QUEUE_EMPTY(&slow_io_pending_wq))
-        goto wait_for_work;
+        continue;
 
       is_slow_work = 1;
       slow_io_work_running++;
@@ -122,13 +122,19 @@ static void worker(void* arg) {
     w->work(w);
 
     uv_mutex_lock(&w->loop->wq_mutex);
-    if (is_slow_work)
-      slow_io_work_running--;
     w->work = NULL;  /* Signal uv_cancel() that the work req is done
                         executing. */
     QUEUE_INSERT_TAIL(&w->loop->wq, &w->wq);
     uv_async_send(&w->loop->wq_async);
     uv_mutex_unlock(&w->loop->wq_mutex);
+
+    /* Lock `mutex` since that is expected at the start of the next
+     * iteration. */
+    uv_mutex_lock(&mutex);
+    if (is_slow_work) {
+      /* `slow_io_work_running` is protected by `mutex`. */
+      slow_io_work_running--;
+    }
   }
 }