Fix a race in fibers::BatchSemaphore

Summary: D66114255 fixed a data race in `fibers::Semaphore` occuring due to lack of 'happens-before' signal-wait synchronization. The very same issue exists in `fibers::BatchedSemaphore` which I came across today in a flaky unit test. Hence porting the fix.

Reviewed By: Gownta

Differential Revision: D68594868

fbshipit-source-id: 752a7b15198015928a7fe040fecc8b3c9e56a889

third-party/folly/src/folly/fibers/SemaphoreBase.cpp

@@ -34,7 +34,7 @@ bool SemaphoreBase::signalSlow(int64_t tokens) {
         // waitlist, ensure the token count increments. No need for CAS here as
         // we will always be under the mutex
         if (tokens_.compare_exchange_strong(
-                testVal, testVal + tokens, std::memory_order_relaxed)) {
+                testVal, testVal + tokens, std::memory_order_release)) {
           return true;
         }
         continue;

third-party/folly/src/folly/fibers/test/BatchSemaphoreTest.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/fibers/BatchSemaphore.h>
+#include <folly/portability/GTest.h>
+
+TEST(BatchSemaphoreTest, WaitSignalSynchronization) {
+  folly::fibers::BatchSemaphore sem{0};
+
+  int64_t data = 0;
+  folly::relaxed_atomic_bool signalled = false;
+
+  std::jthread t{[&]() {
+    while (!signalled) {
+      std::this_thread::yield();
+    }
+
+    sem.wait(1);
+    EXPECT_NE(data, 0);
+  }};
+
+  data = 1;
+  sem.signal(1);
+  signalled = true;
+}