Handles spurious wake-ups in pthread_join()

Removed 'join_count' from pthread_internal_t and switched to using the flag
PTHREAD_ATTR_FLAG_JOINED to indicate if a thread is being joined. Combined with
a switch to a while loop in pthread_join, this fixes spurious wake-ups but
prevents a thread from being joined multiple times. This is fine for
two reasons:

1) The pthread_join specification allows for undefined behavior when multiple
   threads try to join a single thread.

2) There is no thread safe way to allow multiple threads to join a single
   thread with the pthread interface.  The second thread calling pthread_join
   could be pre-empted until the thread is destroyed and its handle reused for
   a different thread.  Therefore multi-join is always an error.

Bug: https://code.google.com/p/android/issues/detail?id=52255
Change-Id: I8b6784d47620ffdcdbfb14524e7402e21d46c5f7

tests/pthread_test.cpp

@@ -317,3 +317,25 @@ TEST(pthread, pthread_kill__no_such_thread) {
 
   ASSERT_EQ(ESRCH, pthread_kill(dead_thread, 0));
 }
+
+TEST(pthread, pthread_join__multijoin) {
+  bool done = false;
+
+  pthread_t t1;
+  ASSERT_EQ(0, pthread_create(&t1, NULL, SpinFn, &done));
+
+  pthread_t t2;
+  ASSERT_EQ(0, pthread_create(&t2, NULL, JoinFn, reinterpret_cast<void*>(t1)));
+
+  sleep(1); // (Give t2 a chance to call pthread_join.)
+
+  // Multiple joins to the same thread should fail.
+  ASSERT_EQ(EINVAL, pthread_join(t1, NULL));
+
+  done = true;
+
+  // ...but t2's join on t1 still goes ahead (which we can tell because our join on t2 finishes).
+  void* join_result;
+  ASSERT_EQ(0, pthread_join(t2, &join_result));
+  ASSERT_EQ(0, reinterpret_cast<int>(join_result));
+}