fix synchronized/test/barrier condition variable predicate

Summary: From https://en.cppreference.com/w/cpp/thread/condition_variable/wait, predicate should return ​false if the waiting should be continued. In the current code, the logic is flipped.

Reviewed By: yfeldblum

Differential Revision: D18413606

fbshipit-source-id: 3b59efe49946a223204ab68fc9fbe3bb98e3b58e

folly/synchronization/test/Barrier.h

@@ -36,7 +36,7 @@ struct Barrier {
       gen_++;
       cv_.notify_all();
     } else {
-      cv_.wait(lockHeld, [&]() { return gen == gen_; });
+      cv_.wait(lockHeld, [&]() { return gen != gen_; });
     }
   }
 

folly/synchronization/test/BarrierTest.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its 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/synchronization/test/Barrier.h>
+
+#include <atomic>
+#include <thread>
+#include <vector>
+
+#include <folly/portability/GTest.h>
+
+using namespace folly;
+using namespace folly::test;
+
+class BarrierTest : public testing::Test {};
+
+TEST_F(BarrierTest, basic_barrier_test) {
+  constexpr unsigned kThreadCount = 10;
+
+  std::vector<std::thread> threads{kThreadCount};
+  Barrier gate{kThreadCount + 1};
+  std::atomic<int> mismatchCount{0};
+  std::atomic<bool> flag{false};
+
+  for (auto& t : threads) {
+    t = std::thread([&] {
+      gate.wait();
+      if (!flag.load()) {
+        mismatchCount.fetch_add(1);
+      }
+    });
+  }
+
+  // Set flag here and make sure all the threads see it correctly after wait()
+  // unblocks
+  flag.store(true);
+  gate.wait();
+
+  for (auto& t : threads) {
+    t.join();
+  }
+
+  EXPECT_EQ(mismatchCount.load(), 0);
+}