Improve code structure and comment

src/dbnode/storage/bootstrap/bootstrapper/peers/source.go

@@ -485,44 +485,49 @@ func (s *peersSource) flush(
 		}
 	}
 
-	if seriesCachePolicy == series.CacheAll ||
+	// We only want to retain the series metadata in one of three cases:
+	// 	1) CacheAll caching policy (because we're expected to cache everything in memory)
+	// 	2) CacheAllMetadata caching policy (because we're expected to cache all metadata in memory)
+	// 	3) PersistConfig.FileSetType is set to FileSetSnapshotType because that means we're bootstrapping
+	//     an active block that we'll want to perform a flush on later, and we're only flushing here for
+	//     the sake of allowing the commit log bootstrapper to be able to recover this data if the node
+	//     goes down in-between this bootstrapper completing and the subsequent flush.
+	shouldRetainSeriesMetadata := seriesCachePolicy == series.CacheAll ||
 		seriesCachePolicy == series.CacheAllMetadata ||
-		persistConfig.FileSetType == persist.FileSetSnapshotType {
-		// In all three of these cases we still need to keep all of the metadata in memory
-		// so we can return early.
-		return nil
-	}
+		persistConfig.FileSetType == persist.FileSetSnapshotType
+
+	if !shouldRetainSeriesMetadata {
+		// If we're not going to keep all of the data, or at least all of the metadata in memory
+		// then we don't want to keep these series in the shard result. If we leave them in, then
+		// they will all get loaded into the shard object, and then immediately evicted on the next
+		// tick which causes unnecessary memory pressure.
+		numSeriesTriedToRemoveWithRemainingBlocks := 0
+		for _, entry := range shardResult.AllSeries().Iter() {
+			series := entry.Value()
+			numBlocksRemaining := len(series.Blocks.AllBlocks())
+			// Should never happen since we removed all the block in the previous loop and fetching
+			// bootstrap blocks should always be exclusive on the end side.
+			if numBlocksRemaining > 0 {
+				numSeriesTriedToRemoveWithRemainingBlocks++
+				continue
+			}
 
-	// If we're not going to keep all of the data, or at least all of the metadata in memory
-	// then we don't want to keep these series in the shard result. If we leave them in, then
-	// they will all get loaded into the shard object, and then immediately evicted on the next
-	// tick which causes unnecessary memory pressure.
-	numSeriesTriedToRemoveWithRemainingBlocks := 0
-	for _, entry := range shardResult.AllSeries().Iter() {
-		series := entry.Value()
-		numBlocksRemaining := len(series.Blocks.AllBlocks())
-		// Should never happen since we removed all the block in the previous loop and fetching
-		// bootstrap blocks should always be exclusive on the end side.
-		if numBlocksRemaining > 0 {
-			numSeriesTriedToRemoveWithRemainingBlocks++
-			continue
+			shardResult.RemoveSeries(series.ID)
+			series.Blocks.Close()
+			// Safe to finalize these IDs and Tags because the prepared object was the only other thing
+			// using them, and it has been closed.
+			series.ID.Finalize()
+			series.Tags.Finalize()
+		}
+		if numSeriesTriedToRemoveWithRemainingBlocks > 0 {
+			iOpts := s.opts.ResultOptions().InstrumentOptions()
+			instrument.EmitInvariantViolationAndGetLogger(iOpts).
+				WithFields(
+					xlog.NewField("start", tr.Start.Unix()),
+					xlog.NewField("end", tr.End.Unix()),
+					xlog.NewField("numTimes", numSeriesTriedToRemoveWithRemainingBlocks),
+				).Error("error tried to remove series that still has blocks")
 		}
-
-		shardResult.RemoveSeries(series.ID)
-		series.Blocks.Close()
-		// Safe to finalize these IDs and Tags because the prepared object was the only other thing
-		// using them, and it has been closed.
-		series.ID.Finalize()
-		series.Tags.Finalize()
-	}
-	if numSeriesTriedToRemoveWithRemainingBlocks > 0 {
-		iOpts := s.opts.ResultOptions().InstrumentOptions()
-		instrument.EmitInvariantViolationAndGetLogger(iOpts).
-			WithFields(
-				xlog.NewField("start", tr.Start.Unix()),
-				xlog.NewField("end", tr.End.Unix()),
-				xlog.NewField("numTimes", numSeriesTriedToRemoveWithRemainingBlocks),
-			).Error("error tried to remove series that still has blocks")
 	}
 
 	return nil