Fix Redis cache error when a query has no chunks to lookup (grafana#2700

)

* Fix Redis cache error when a query has no chunks to lookup

Signed-off-by: Marco Pracucci <marco@pracucci.com>

* Added CHANGELOG entry

Signed-off-by: Marco Pracucci <marco@pracucci.com>

* Fixed another case leading to 'wrong number of arguments for 'mget' command'

Signed-off-by: Marco Pracucci <marco@pracucci.com>

chunk_store.go

@@ -505,6 +505,11 @@ func (c *baseStore) lookupEntriesByQueries(ctx context.Context, queries []IndexQ
 	log, ctx := spanlogger.New(ctx, "store.lookupEntriesByQueries")
 	defer log.Span.Finish()
 
+	// Nothing to do if there are no queries.
+	if len(queries) == 0 {
+		return nil, nil
+	}
+
 	var lock sync.Mutex
 	var entries []IndexEntry
 	err := c.index.QueryPages(ctx, queries, func(query IndexQuery, resp ReadBatch) bool {
@@ -527,7 +532,12 @@ func (c *baseStore) lookupEntriesByQueries(ctx context.Context, queries []IndexQ
 	return entries, err
 }
 
-func (c *baseStore) parseIndexEntries(ctx context.Context, entries []IndexEntry, matcher *labels.Matcher) ([]string, error) {
+func (c *baseStore) parseIndexEntries(_ context.Context, entries []IndexEntry, matcher *labels.Matcher) ([]string, error) {
+	// Nothing to do if there are no entries.
+	if len(entries) == 0 {
+		return nil, nil
+	}
+
 	result := make([]string, 0, len(entries))
 	for _, entry := range entries {
 		chunkKey, labelValue, _, err := parseChunkTimeRangeValue(entry.RangeValue, entry.Value)

composite_store.go

@@ -162,6 +162,11 @@ func (c compositeStore) GetChunkRefs(ctx context.Context, userID string, from, t
 			return err
 		}
 
+		// Skip it if there are no chunks.
+		if len(ids) == 0 {
+			return nil
+		}
+
 		chunkIDs = append(chunkIDs, ids...)
 		fetchers = append(fetchers, fetcher...)
 		return nil

series_store.go

@@ -182,6 +182,11 @@ func (c *seriesStore) GetChunkRefs(ctx context.Context, userID string, from, thr
 	level.Debug(log).Log("chunks-post-filtering", len(chunks))
 	chunksPerQuery.Observe(float64(len(chunks)))
 
+	// We should return an empty chunks slice if there are no chunks.
+	if len(chunks) == 0 {
+		return [][]Chunk{}, []*Fetcher{}, nil
+	}
+
 	return [][]Chunk{chunks}, []*Fetcher{c.baseStore.Fetcher}, nil
 }