fix(blooms): Check length of tasks before accessing first element in slice (#14634)

This PR adds a length check for the partitioned series before accessing the first element of the resulting slice.

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

pkg/bloomgateway/bloomgateway.go

@@ -200,9 +200,7 @@ func (g *Gateway) FilterChunkRefs(ctx context.Context, req *logproto.FilterChunk
 	// Shortcut if request does not contain filters
 	if len(matchers) == 0 {
 		stats.Status = labelSuccess
-		return &logproto.FilterChunkRefResponse{
-			ChunkRefs: req.Refs,
-		}, nil
+		return &logproto.FilterChunkRefResponse{ChunkRefs: req.Refs}, nil
 	}
 
 	blocks := make([]bloomshipper.BlockRef, 0, len(req.Blocks))
@@ -218,9 +216,7 @@ func (g *Gateway) FilterChunkRefs(ctx context.Context, req *logproto.FilterChunk
 	// Shortcut if request does not contain blocks
 	if len(blocks) == 0 {
 		stats.Status = labelSuccess
-		return &logproto.FilterChunkRefResponse{
-			ChunkRefs: req.Refs,
-		}, nil
+		return &logproto.FilterChunkRefResponse{ChunkRefs: req.Refs}, nil
 	}
 
 	seriesByDay := partitionRequest(req)
@@ -233,6 +229,14 @@ func (g *Gateway) FilterChunkRefs(ctx context.Context, req *logproto.FilterChunk
 		"series_requested", len(req.Refs),
 	)
 
+	// len(seriesByDay) should never be 0
+	// Not sure how this can happen, but there was a bug report
+	// https://github.com/grafana/loki/issues/14623
+	if len(seriesByDay) == 0 {
+		stats.Status = labelSuccess
+		return &logproto.FilterChunkRefResponse{ChunkRefs: req.Refs}, nil
+	}
+
 	if len(seriesByDay) > 1 {
 		stats.Status = labelFailure
 		return nil, errors.New("request time range must span exactly one day")