Fix intermittent hang in ColumnFamilyTest.FlushAndDropRaceCondition

Summary:
ColumnFamilyTest.FlushAndDropRaceCondition sometimes
hangs because the sync point, "FlushJob::InstallResults", sleeps
holding the DB mutex. Fixing it by releasing the mutex before sleeping.

Test Plan:
seq 1000 |parallel --gnu --eta 't=/dev/shm/rdb-{}; rm -rf $t;
mkdir $t && export TEST_TMPDIR=$t; ./column_family_test
-gtest_filter=*FlushAndDropRaceCondition* > $t/log-{}'

Reviewers: IslamAbdelRahman, anthony, kradhakrishnan, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D53349

db/column_family_test.cc

@@ -1999,7 +1999,9 @@ TEST_F(ColumnFamilyTest, FlushAndDropRaceCondition) {
   Reopen({options, options});
 
   rocksdb::SyncPoint::GetInstance()->LoadDependency(
-      {{"VersionSet::LogAndApply::ColumnFamilyDrop:1",
+      {{"VersionSet::LogAndApply::ColumnFamilyDrop:0",
+        "FlushJob::WriteLevel0Table"},
+       {"VersionSet::LogAndApply::ColumnFamilyDrop:1",
         "FlushJob::InstallResults"},
        {"FlushJob::InstallResults",
         "VersionSet::LogAndApply::ColumnFamilyDrop:2"}});

db/flush_job.cc

@@ -270,6 +270,7 @@ Status FlushJob::WriteLevel0Table(const autovector<MemTable*>& mems,
     if (!db_options_.disableDataSync && output_file_directory_ != nullptr) {
       output_file_directory_->Fsync();
     }
+    TEST_SYNC_POINT("FlushJob::WriteLevel0Table");
     db_mutex_->Lock();
   }
   base->Unref();

db/version_set.cc

@@ -2218,6 +2218,7 @@ Status VersionSet::LogAndApply(ColumnFamilyData* column_family_data,
     }
 
     if (edit->is_column_family_drop_) {
+      TEST_SYNC_POINT("VersionSet::LogAndApply::ColumnFamilyDrop:0");
       TEST_SYNC_POINT("VersionSet::LogAndApply::ColumnFamilyDrop:1");
       TEST_SYNC_POINT("VersionSet::LogAndApply::ColumnFamilyDrop:2");
     }