Fix occasional failure of DBTest.DynamicCompactionOptions

Summary: DBTest.DynamicCompactionOptions ocasionally fails during valgrind run. We sent a sleeping task to block compaction thread pool but we don't wait it to run.

Test Plan: Run the test multiple times in an environment which can cause failure.

Reviewers: rven, kradhakrishnan, igor, IslamAbdelRahman, anthony, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D51687

db/db_test.cc

@@ -7704,7 +7704,7 @@ TEST_F(DBTest, DynamicCompactionOptions) {
   test::SleepingBackgroundTask sleeping_task_low;
   env_->Schedule(&test::SleepingBackgroundTask::DoSleepTask, &sleeping_task_low,
                  Env::Priority::LOW);
-
+  sleeping_task_low.WaitUntilSleeping();
   ASSERT_EQ(NumTableFilesAtLevel(0), 0);
   int count = 0;
   Random rnd(301);
@@ -7737,6 +7737,7 @@ TEST_F(DBTest, DynamicCompactionOptions) {
   sleeping_task_low.Reset();
   env_->Schedule(&test::SleepingBackgroundTask::DoSleepTask, &sleeping_task_low,
                  Env::Priority::LOW);
+  sleeping_task_low.WaitUntilSleeping();
   count = 0;
   while (count < 64) {
     ASSERT_OK(Put(Key(count), RandomString(&rnd, 1024), wo));

util/testutil.h

@@ -306,6 +306,7 @@ class SleepingBackgroundTask {
   void DoSleep() {
     MutexLock l(&mutex_);
     sleeping_ = true;
+    bg_cv_.SignalAll();
     while (should_sleep_) {
       bg_cv_.Wait();
     }
@@ -313,6 +314,12 @@ class SleepingBackgroundTask {
     done_with_sleep_ = true;
     bg_cv_.SignalAll();
   }
+  void WaitUntilSleeping() {
+    MutexLock l(&mutex_);
+    while (!sleeping_) {
+      bg_cv_.Wait();
+    }
+  }
   void WakeUp() {
     MutexLock l(&mutex_);
     should_sleep_ = false;