Possible fix to flaky db_write_test (#7418)

Summary:
Make the test robust to spurious wakeups on condition variable,
and clear sync points to ensure no use-after-free.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7418

Test Plan: repeated runs on updated test, watch CircleCI for recurrence

Reviewed By: jay-zhuang

Differential Revision: D23828823

Pulled By: pdillinger

fbshipit-source-id: af85117d9c02602541a90252840e0e5a6996de5b

db/db_write_test.cc

@@ -49,6 +49,8 @@ TEST_P(DBWriteTest, WriteThreadHangOnWriteStall) {
   std::atomic<int> thread_num(0);
   port::Mutex mutex;
   port::CondVar cv(&mutex);
+  // Guarded by mutex
+  int writers = 0;
 
   Reopen(options);
 
@@ -68,6 +70,7 @@ TEST_P(DBWriteTest, WriteThreadHangOnWriteStall) {
   };
   std::function<void(void *)> unblock_main_thread_func = [&](void *) {
     mutex.Lock();
+    ++writers;
     cv.SignalAll();
     mutex.Unlock();
   };
@@ -106,18 +109,18 @@ TEST_P(DBWriteTest, WriteThreadHangOnWriteStall) {
   mutex.Lock();
   // First leader
   threads.emplace_back(write_slowdown_func);
+  while (writers != 1) {
     cv.Wait();
+  }
   // Second leader. Will stall writes
   threads.emplace_back(write_slowdown_func);
-  cv.Wait();
   threads.emplace_back(write_no_slowdown_func);
-  cv.Wait();
   threads.emplace_back(write_slowdown_func);
-  cv.Wait();
   threads.emplace_back(write_no_slowdown_func);
-  cv.Wait();
   threads.emplace_back(write_slowdown_func);
+  while (writers != 6) {
     cv.Wait();
+  }
   mutex.Unlock();
 
   TEST_SYNC_POINT("DBWriteTest::WriteThreadHangOnWriteStall:1");
@@ -131,6 +134,8 @@ TEST_P(DBWriteTest, WriteThreadHangOnWriteStall) {
   for (auto& t : threads) {
     t.join();
   }
+  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
+  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->ClearAllCallBacks();
 }
 
 TEST_P(DBWriteTest, IOErrorOnWALWritePropagateToWriteThreadFollower) {