Fix Flaky ColumnFamilyTest.FlushCloseWALFiles

Summary: In ColumnFamilyTest.FlushCloseWALFiles, there is a small window in which the flush has finished but the log writer is not yet closed, causing the assert failure. Fix it by explicitly waiting the flush job to finish.

Test Plan: Run the test many times in high parallelism.

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D63423

db/column_family_test.cc

@@ -2851,6 +2851,11 @@ TEST_F(ColumnFamilyTest, FlushCloseWALFiles) {
   ASSERT_OK(Put(0, "fodor", "mirko"));
   ASSERT_OK(Put(1, "fodor", "mirko"));
 
+  rocksdb::SyncPoint::GetInstance()->LoadDependency({
+      {"DBImpl::BGWorkFlush:done", "FlushCloseWALFiles:0"},
+  });
+  rocksdb::SyncPoint::GetInstance()->EnableProcessing();
+
   // Block flush jobs from running
   test::SleepingBackgroundTask sleeping_task;
   env_->Schedule(&test::SleepingBackgroundTask::DoSleepTask, &sleeping_task,
@@ -2864,7 +2869,8 @@ TEST_F(ColumnFamilyTest, FlushCloseWALFiles) {
 
   sleeping_task.WakeUp();
   sleeping_task.WaitUntilDone();
-  WaitForFlush(1);
+  TEST_SYNC_POINT("FlushCloseWALFiles:0");
+  rocksdb::SyncPoint::GetInstance()->DisableProcessing();
   ASSERT_EQ(1, env.num_open_wal_file_.load());
 
   Reopen();