Need to schedule compactions when manual compaction finishes

Summary: If there is an outstanding compaction scheduled but at the time a manual compaction is triggered, the manual compaction will preempt. In the end of the manual compaction, we should try to schedule compactions to make sure those preempted ones are not skipped.

Test Plan: make all check

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb, dhruba, igor

Differential Revision: https://reviews.facebook.net/D21321

db/db_impl.cc

@@ -1834,6 +1834,11 @@ Status DBImpl::RunManualCompaction(ColumnFamilyData* cfd, int input_level,
   assert(!manual.in_progress);
   assert(bg_manual_only_ > 0);
   --bg_manual_only_;
+  if (bg_manual_only_ == 0) {
+    // an automatic compaction should have been scheduled might have be
+    // preempted by the manual compactions. Need to schedule it back.
+    MaybeScheduleFlushOrCompaction();
+  }
   return manual.status;
 }