Avoid populating live set if we don't need to

Summary: Also changed some comments

Test Plan: ./deletefile_test

Reviewers: haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14091

db/db_impl.cc

@@ -451,6 +451,22 @@ void DBImpl::FindObsoleteFiles(DeletionState& deletion_state,
     return;
   }
 
+  bool doing_the_full_scan = false;
+
+  // logic for figurint out if we're doing the full scan
+  if (no_full_scan) {
+    doing_the_full_scan = false;
+  } else if (force || options_.delete_obsolete_files_period_micros == 0) {
+    doing_the_full_scan = true;
+  } else {
+    const uint64_t now_micros = env_->NowMicros();
+    if (delete_obsolete_files_last_run_ +
+        options_.delete_obsolete_files_period_micros < now_micros) {
+      doing_the_full_scan = true;
+      delete_obsolete_files_last_run_ = now_micros;
+    }
+  }
+
   // get obsolete files
   versions_->GetObsoleteFiles(&deletion_state.sst_delete_files);
 
@@ -459,31 +475,19 @@ void DBImpl::FindObsoleteFiles(DeletionState& deletion_state,
   deletion_state.log_number = versions_->LogNumber();
   deletion_state.prev_log_number = versions_->PrevLogNumber();
 
-  // TODO we should not be catching live files here,
+  if (!doing_the_full_scan && !deletion_state.HaveSomethingToDelete()) {
-  // version_->GetObsoleteFiles() should tell us the truth, which
+    // avoid filling up sst_live if we're sure that we
-  // files are to be deleted. However, it does not, so we do
+    // are not going to do the full scan and that we don't have
-  // this to be safe, i.e. never delete files that could be
+    // anything to delete at the moment
-  // live
+    return;
+  }
+
+  // don't delete live files
   deletion_state.sst_live.assign(pending_outputs_.begin(),
                                  pending_outputs_.end());
   versions_->AddLiveFiles(&deletion_state.sst_live);
 
-  // if no_full_scan, never do the full scan
+  if (doing_the_full_scan) {
-  if (no_full_scan) {
-    return;
-  }
-  // if force == true, always fall through and do the full scan
-  // if force == false, do the full scan only every
-  //    options_.delete_obsolete_files_period_micros
-  if (!force && options_.delete_obsolete_files_period_micros != 0) {
-    const uint64_t now_micros = env_->NowMicros();
-    if (delete_obsolete_files_last_run_ +
-        options_.delete_obsolete_files_period_micros > now_micros) {
-      return;
-    }
-    delete_obsolete_files_last_run_ = now_micros;
-  }
-
     // set of all files in the directory
     env_->GetChildren(dbname_, &deletion_state.all_files); // Ignore errors
 
@@ -498,6 +502,7 @@ void DBImpl::FindObsoleteFiles(DeletionState& deletion_state,
       );
     }
   }
+}
 
 // Diffs the files listed in filenames and those that do not
 // belong to live files are posibly removed. Also, removes all the