Prevent unnecessary calls to PurgeObsoleteFiles

Summary:
Split `JobContext::HaveSomethingToDelete` into two functions: itself and `JobContext::HaveSomethingToClean`. Now we won't call `DBImpl::PurgeObsoleteFiles` in cases where we really just need to call `JobContext::Clean`. The change is needed because I want to track pending calls to `PurgeObsoleteFiles` for a bug fix, which is much simpler if we only call it after `FindObsoleteFiles` finds files to delete.
Closes https://github.com/facebook/rocksdb/pull/3350

Differential Revision: D6690609

Pulled By: ajkr

fbshipit-source-id: 61502e7469288afe16a663a1b7df345baeaf246f

db/db_impl_compaction_flush.cc

@@ -441,7 +441,8 @@ Status DBImpl::CompactFiles(
   }  // release the mutex
 
   // delete unnecessary files if any, this is done outside the mutex
-  if (job_context.HaveSomethingToDelete() || !log_buffer.IsEmpty()) {
+  if (job_context.HaveSomethingToClean() ||
+      job_context.HaveSomethingToDelete() || !log_buffer.IsEmpty()) {
     // Have to flush the info logs before bg_compaction_scheduled_--
     // because if bg_flush_scheduled_ becomes 0 and the lock is
     // released, the deconstructor of DB can kick in and destroy all the
@@ -1303,7 +1304,8 @@ void DBImpl::BackgroundCallFlush() {
     // created. Thus, we force full scan in FindObsoleteFiles()
     FindObsoleteFiles(&job_context, !s.ok() && !s.IsShutdownInProgress());
     // delete unnecessary files if any, this is done outside the mutex
-    if (job_context.HaveSomethingToDelete() || !log_buffer.IsEmpty()) {
+    if (job_context.HaveSomethingToClean() ||
+        job_context.HaveSomethingToDelete() || !log_buffer.IsEmpty()) {
       mutex_.Unlock();
       // Have to flush the info logs before bg_flush_scheduled_--
       // because if bg_flush_scheduled_ becomes 0 and the lock is
@@ -1384,7 +1386,8 @@ void DBImpl::BackgroundCallCompaction(PrepickedCompaction* prepicked_compaction,
     FindObsoleteFiles(&job_context, !s.ok() && !s.IsShutdownInProgress());
 
     // delete unnecessary files if any, this is done outside the mutex
-    if (job_context.HaveSomethingToDelete() || !log_buffer.IsEmpty()) {
+    if (job_context.HaveSomethingToClean() ||
+        job_context.HaveSomethingToDelete() || !log_buffer.IsEmpty()) {
       mutex_.Unlock();
       // Have to flush the info logs before bg_compaction_scheduled_--
       // because if bg_flush_scheduled_ becomes 0 and the lock is

db/job_context.h

@@ -81,8 +81,11 @@ struct SuperVersionContext {
 struct JobContext {
   inline bool HaveSomethingToDelete() const {
     return full_scan_candidate_files.size() || sst_delete_files.size() ||
-           log_delete_files.size() || manifest_delete_files.size() ||
+           log_delete_files.size() || manifest_delete_files.size();
-           memtables_to_free.size() > 0 || logs_to_free.size() > 0 ||
+  }
+
+  inline bool HaveSomethingToClean() const {
+    return memtables_to_free.size() > 0 || logs_to_free.size() > 0 ||
            superversion_context.HaveSomethingToDelete();
   }