unix, windows: make uv_barrier_wait() return bool

Make uv_barrier_wait() return a boolean value indicating whether this
thread was the "serializer".

From `man pthread_barrier_wait`:

    Upon successful completion, the pthread_barrier_wait() function
    shall return PTHREAD_BARRIER_SERIAL_THREAD for a single (arbitrary)
    thread synchronized at the barrier and zero for each of the other
    threads.

Exposing that information from libuv is useful because it can make
cleanup significantly easier:

    if (uv_barrier_wait(&barrier) > 0)
      uv_barrier_destroy(&barrier);

Signed-off-by: Fedor Indutny <fedor@indutny.com>

include/uv.h

@@ -2219,7 +2219,7 @@ UV_EXTERN int uv_cond_timedwait(uv_cond_t* cond, uv_mutex_t* mutex,
 
 UV_EXTERN int uv_barrier_init(uv_barrier_t* barrier, unsigned int count);
 UV_EXTERN void uv_barrier_destroy(uv_barrier_t* barrier);
-UV_EXTERN void uv_barrier_wait(uv_barrier_t* barrier);
+UV_EXTERN int uv_barrier_wait(uv_barrier_t* barrier);
 
 /* Runs a function once and only once. Concurrent calls to uv_once() with the
  * same guard will block all callers except one (it's unspecified which one).

src/unix/thread.c

@@ -399,7 +399,9 @@ void uv_barrier_destroy(uv_barrier_t* barrier) {
 }
 
 
-void uv_barrier_wait(uv_barrier_t* barrier) {
+int uv_barrier_wait(uv_barrier_t* barrier) {
+  int serial_thread;
+
   uv_mutex_lock(&barrier->mutex);
   if (++barrier->count == barrier->n) {
     uv_sem_wait(&barrier->turnstile2);
@@ -411,7 +413,8 @@ void uv_barrier_wait(uv_barrier_t* barrier) {
   uv_sem_post(&barrier->turnstile1);
 
   uv_mutex_lock(&barrier->mutex);
-  if (--barrier->count == 0) {
+  serial_thread = (--barrier->count == 0);
+  if (serial_thread) {
     uv_sem_wait(&barrier->turnstile1);
     uv_sem_post(&barrier->turnstile2);
   }
@@ -419,6 +422,7 @@ void uv_barrier_wait(uv_barrier_t* barrier) {
 
   uv_sem_wait(&barrier->turnstile2);
   uv_sem_post(&barrier->turnstile2);
+  return serial_thread;
 }
 
 #else /* !(defined(__APPLE__) && defined(__MACH__)) */
@@ -434,10 +438,11 @@ void uv_barrier_destroy(uv_barrier_t* barrier) {
 }
 
 
-void uv_barrier_wait(uv_barrier_t* barrier) {
+int uv_barrier_wait(uv_barrier_t* barrier) {
   int r = pthread_barrier_wait(barrier);
   if (r && r != PTHREAD_BARRIER_SERIAL_THREAD)
     abort();
+  return r == PTHREAD_BARRIER_SERIAL_THREAD;
 }
 
 #endif /* defined(__APPLE__) && defined(__MACH__) */

src/win/thread.c

@@ -660,7 +660,9 @@ void uv_barrier_destroy(uv_barrier_t* barrier) {
 }
 
 
-void uv_barrier_wait(uv_barrier_t* barrier) {
+int uv_barrier_wait(uv_barrier_t* barrier) {
+  int serial_thread;
+
   uv_mutex_lock(&barrier->mutex);
   if (++barrier->count == barrier->n) {
     uv_sem_wait(&barrier->turnstile2);
@@ -672,7 +674,8 @@ void uv_barrier_wait(uv_barrier_t* barrier) {
   uv_sem_post(&barrier->turnstile1);
 
   uv_mutex_lock(&barrier->mutex);
-  if (--barrier->count == 0) {
+  serial_thread = (--barrier->count == 0);
+  if (serial_thread) {
     uv_sem_wait(&barrier->turnstile1);
     uv_sem_post(&barrier->turnstile2);
   }
@@ -680,6 +683,7 @@ void uv_barrier_wait(uv_barrier_t* barrier) {
 
   uv_sem_wait(&barrier->turnstile2);
   uv_sem_post(&barrier->turnstile2);
+  return serial_thread;
 }
 
 

test/test-barrier.c

@@ -29,6 +29,8 @@ typedef struct {
   uv_barrier_t barrier;
   int delay;
   volatile int posted;
+  int main_barrier_wait_rval;
+  int worker_barrier_wait_rval;
 } worker_config;
 
 
@@ -38,7 +40,7 @@ static void worker(void* arg) {
   if (c->delay)
     uv_sleep(c->delay);
 
-  uv_barrier_wait(&c->barrier);
+  c->worker_barrier_wait_rval = uv_barrier_wait(&c->barrier);
 }
 
 
@@ -52,11 +54,13 @@ TEST_IMPL(barrier_1) {
   ASSERT(0 == uv_thread_create(&thread, worker, &wc));
 
   uv_sleep(100);
-  uv_barrier_wait(&wc.barrier);
+  wc.main_barrier_wait_rval = uv_barrier_wait(&wc.barrier);
 
   ASSERT(0 == uv_thread_join(&thread));
   uv_barrier_destroy(&wc.barrier);
 
+  ASSERT(1 == (wc.main_barrier_wait_rval ^ wc.worker_barrier_wait_rval));
+
   return 0;
 }
 
@@ -71,11 +75,13 @@ TEST_IMPL(barrier_2) {
   ASSERT(0 == uv_barrier_init(&wc.barrier, 2));
   ASSERT(0 == uv_thread_create(&thread, worker, &wc));
 
-  uv_barrier_wait(&wc.barrier);
+  wc.main_barrier_wait_rval = uv_barrier_wait(&wc.barrier);
 
   ASSERT(0 == uv_thread_join(&thread));
   uv_barrier_destroy(&wc.barrier);
 
+  ASSERT(1 == (wc.main_barrier_wait_rval ^ wc.worker_barrier_wait_rval));
+
   return 0;
 }
 
@@ -89,10 +95,12 @@ TEST_IMPL(barrier_3) {
   ASSERT(0 == uv_barrier_init(&wc.barrier, 2));
   ASSERT(0 == uv_thread_create(&thread, worker, &wc));
 
-  uv_barrier_wait(&wc.barrier);
+  wc.main_barrier_wait_rval = uv_barrier_wait(&wc.barrier);
 
   ASSERT(0 == uv_thread_join(&thread));
   uv_barrier_destroy(&wc.barrier);
 
+  ASSERT(1 == (wc.main_barrier_wait_rval ^ wc.worker_barrier_wait_rval));
+
   return 0;
 }