Improve ThreadStatusSingleCompaction

Summary:
Improve ThreadStatusSingleCompaction in two ways:
1. Use SYNC_POINT to ensure compaction won't happen
   before the test finishes its "Put Phase" instead of
   using sleep.
2. In Put Phase, it continues until we have sufficient
   number of L0 files.  Note that during the put phase,
   there won't be any compaction that consumes L0 files
   because of item 1.

Test Plan: ./db_test  --gtest_filter="*ThreadStatusSingleCompaction*"

Reviewers: sdong, igor, rven

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D35727

db/db_impl.cc

@@ -1879,6 +1879,7 @@ void DBImpl::BGWorkFlush(void* db) {
 
 void DBImpl::BGWorkCompaction(void* db) {
   IOSTATS_SET_THREAD_POOL_ID(Env::Priority::LOW);
+  TEST_SYNC_POINT("DBImpl::BGWorkCompaction");
   reinterpret_cast<DBImpl*>(db)->BackgroundCallCompaction();
 }
 

db/db_test.cc

@@ -10216,7 +10216,10 @@ TEST_F(DBTest, ThreadStatusSingleCompaction) {
   const int kNumL0Files = 4;
   options.level0_file_num_compaction_trigger = kNumL0Files;
 
+
   rocksdb::SyncPoint::GetInstance()->LoadDependency({
+      {"DBTest::ThreadStatusSingleCompaction:0",
+       "DBImpl::BGWorkCompaction"},
       {"CompactionJob::Run():Start",
        "DBTest::ThreadStatusSingleCompaction:1"},
       {"DBTest::ThreadStatusSingleCompaction:2",
@@ -10228,6 +10231,7 @@ TEST_F(DBTest, ThreadStatusSingleCompaction) {
     DestroyAndReopen(options);
 
     Random rnd(301);
+    // The Put Phase.
     for (int file = 0; file < kNumL0Files; ++file) {
       for (int key = 0; key < kEntriesPerBuffer; ++key) {
         ASSERT_OK(Put(ToString(key + file * kEntriesPerBuffer),
@@ -10235,13 +10239,15 @@ TEST_F(DBTest, ThreadStatusSingleCompaction) {
       }
       Flush();
     }
+    // This makes sure a compaction won't be scheduled until
+    // we have done with the above Put Phase.
+    TEST_SYNC_POINT("DBTest::ThreadStatusSingleCompaction:0");
     ASSERT_GE(NumTableFilesAtLevel(0),
               options.level0_file_num_compaction_trigger);
 
-    // wait for compaction to be scheduled
-    env_->SleepForMicroseconds(250000);
-
+    // This makes sure at least one compaction is running.
     TEST_SYNC_POINT("DBTest::ThreadStatusSingleCompaction:1");
+
     if (options.enable_thread_tracking) {
       // expecting one single L0 to L1 compaction
       VerifyOperationCount(env_, ThreadStatus::OP_COMPACTION, 1);