Fix data race in GetObsoleteFiles()

Summary:
GetObsoleteFiles() and LogAndApply() functions modify obsolete_manifests_ vector
we need to make sure that the mutex is held when we modify the obsolete_manifests_

Test Plan: run the test under TSAN

Reviewers: andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58011

db/version_set.cc

@@ -2322,10 +2322,6 @@ Status VersionSet::LogAndApply(ColumnFamilyData* column_family_data,
     if (s.ok() && new_descriptor_log) {
       s = SetCurrentFile(env_, dbname_, pending_manifest_file_number_,
                          db_options_->disableDataSync ? nullptr : db_directory);
-      // Leave the old file behind since PurgeObsoleteFiles will take care of it
-      // later. It's unsafe to delete now since file deletion may be disabled.
-      obsolete_manifests_.emplace_back(
-          DescriptorFileName("", manifest_file_number_));
     }
 
     if (s.ok()) {
@@ -2344,6 +2340,13 @@ Status VersionSet::LogAndApply(ColumnFamilyData* column_family_data,
     mu->Lock();
   }
 
+  // Append the old mainfest file to the obsolete_manifests_ list to be deleted
+  // by PurgeObsoleteFiles later.
+  if (s.ok() && new_descriptor_log) {
+    obsolete_manifests_.emplace_back(
+        DescriptorFileName("", manifest_file_number_));
+  }
+
   // Install the new version
   if (s.ok()) {
     if (edit->is_column_family_add_) {