Fix double deletes

Summary: While debugging clients compaction issues, I noticed bunch of delete bugs: P16329995. MakeTableName returns sst file with "/" prefix. We also need "/" prefix when we get the files though GetChildren(), so that we can properly dedup the files.

Test Plan: none

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23457

db/db_impl.cc

@@ -590,7 +590,8 @@ void DBImpl::FindObsoleteFiles(DeletionState& deletion_state,
       env_->GetChildren(db_options_.db_paths[path_id].path,
                         &files);  // Ignore errors
       for (std::string file : files) {
-        deletion_state.candidate_files.emplace_back(file, path_id);
+        // TODO(icanadi) clean up this mess to avoid having one-off "/" prefixes
+        deletion_state.candidate_files.emplace_back("/" + file, path_id);
       }
     }