Test CRYPTO_once_t collisions.

The existing tests never actually tested this case.

Change-Id: Idb9cf0cbbe32fdf5cd353656a95fbedbaac09376
Reviewed-on: https://boringssl-review.googlesource.com/7612
Reviewed-by: Emily Stark (Dunn) <estark@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/thread_test.c

@@ -53,6 +53,7 @@ static int wait_for_thread(thread_t thread) {
 #else
 
 #include <pthread.h>
+#include <unistd.h>
 
 typedef pthread_t thread_t;
 
@@ -77,6 +78,14 @@ static unsigned g_once_init_called = 0;
 
 static void once_init(void) {
   g_once_init_called++;
+
+  /* Sleep briefly so one |call_once_thread| instance will call |CRYPTO_once|
+   * while the other is running this function. */
+#if defined(OPENSSL_WINDOWS)
+  Sleep(1 /* milliseconds */);
+#else
+  usleep(1000 /* microseconds */);
+#endif
 }
 
 static CRYPTO_once_t g_test_once = CRYPTO_ONCE_INIT;
@@ -91,9 +100,11 @@ static int test_once(void) {
     return 0;
   }
 
-  thread_t thread;
-  if (!run_thread(&thread, call_once_thread) ||
-      !wait_for_thread(thread)) {
+  thread_t thread1, thread2;
+  if (!run_thread(&thread1, call_once_thread) ||
+      !run_thread(&thread2, call_once_thread) ||
+      !wait_for_thread(thread1) ||
+      !wait_for_thread(thread2)) {
     fprintf(stderr, "thread failed.\n");
     return 0;
   }