unix: signal done to last thread barrier waiter

Libuv's own thread barrier implementation signaled completion to the
first waiter that saw the threshold being reached, contrary to what
some native pthreads barrier implementations do, which is to signal
it to the _last_ waiter.

Libuv's behavior is not strictly non-conforming but it's inconvenient
because it means this snippet (that appears in the libuv documentation)
has a race condition in it:

    if (uv_barrier_wait(&barrier) > 0)
      uv_barrier_destroy(&barrier);  // can still have waiters

This issue was discovered and fixed by Ali Ijaz Sheikh, a.k.a @ofrobots,
but some refactoring introduced conflicts in his pull request and I
didn't have the heart to ask him to redo it from scratch. :-)

PR-URL: libuv#2019
Refs: libuv#2003
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

Author: bnoordhuis

src/unix/thread.c

@@ -86,6 +86,7 @@ int uv_barrier_init(uv_barrier_t* barrier, unsigned int count) {
 
 int uv_barrier_wait(uv_barrier_t* barrier) {
   struct _uv_barrier* b;
+  int last;
 
   if (barrier == NULL || barrier->b == NULL)
     return UV_EINVAL;
@@ -95,23 +96,20 @@ int uv_barrier_wait(uv_barrier_t* barrier) {
 
   if (++b->in == b->threshold) {
     b->in = 0;
-    b->out = b->threshold - 1;
+    b->out = b->threshold;
     uv_cond_signal(&b->cond);
-    uv_mutex_unlock(&b->mutex);
-    return 1;  /* This is the first thread to reach the threshold. */
+  } else {
+    do
+      uv_cond_wait(&b->cond, &b->mutex);
+    while (b->in != 0);
   }
 
-  /* Otherwise, wait for other threads until in is set to 0,
-     then return 0 to indicate this is not the first thread. */
-  do
-    uv_cond_wait(&b->cond, &b->mutex);
-  while (b->in != 0);
+  last = (--b->out == 0);
+  if (!last)
+    uv_cond_signal(&b->cond);  /* Not needed for last thread. */
 
-  /* mark thread exit */
-  b->out--;
-  uv_cond_signal(&b->cond);
   uv_mutex_unlock(&b->mutex);
-  return 0;
+  return last;
 }