Prometheus: Refactor error handling logic (#62254)

* Refactor error handling logic
- Enable query inspector to show error information
- Update naming

* Change fetch method return signature

* update the snapshot

* Revert "update the snapshot"

This reverts commit 28b26ce426.

pkg/tsdb/prometheus/querydata/request.go

@@ -97,10 +97,7 @@ func (s *QueryData) Execute(ctx context.Context, req *backend.QueryDataRequest)
 		if err != nil {
 			return &result, err
 		}
-		r, err := s.fetch(ctx, s.client, query, req.Headers)
-		if err != nil {
-			return &result, err
-		}
+		r := s.fetch(ctx, s.client, query, req.Headers)
 		if r == nil {
 			s.log.FromContext(ctx).Debug("Received nilresponse from runQuery", "query", query.Expr)
 			continue
@@ -111,69 +108,75 @@ func (s *QueryData) Execute(ctx context.Context, req *backend.QueryDataRequest)
 	return &result, nil
 }
 
-func (s *QueryData) fetch(ctx context.Context, client *client.Client, q *models.Query, headers map[string]string) (*backend.DataResponse, error) {
+func (s *QueryData) fetch(ctx context.Context, client *client.Client, q *models.Query, headers map[string]string) *backend.DataResponse {
 	traceCtx, end := s.trace(ctx, q)
 	defer end()
 
 	logger := s.log.FromContext(traceCtx)
 	logger.Debug("Sending query", "start", q.Start, "end", q.End, "step", q.Step, "query", q.Expr)
 
-	response := &backend.DataResponse{
+	dr := &backend.DataResponse{
 		Frames: data.Frames{},
 		Error:  nil,
 	}
 
 	if q.InstantQuery {
-		res, err := s.instantQuery(traceCtx, client, q, headers)
-		response.Error = err
-		response.Frames = res.Frames
+		res := s.instantQuery(traceCtx, client, q, headers)
+		dr.Error = res.Error
+		dr.Frames = res.Frames
 	}
 
 	if q.RangeQuery {
-		res, err := s.rangeQuery(traceCtx, client, q, headers)
-		if err != nil {
-			if response.Error == nil {
-				response.Error = err
+		res := s.rangeQuery(traceCtx, client, q, headers)
+		if res.Error != nil {
+			if dr.Error == nil {
+				dr.Error = res.Error
 			} else {
-				response.Error = fmt.Errorf("%v %w", response.Error, err)
+				dr.Error = fmt.Errorf("%v %w", dr.Error, res.Error)
 			}
 		}
-		response.Frames = append(response.Frames, res.Frames...)
+		dr.Frames = append(dr.Frames, res.Frames...)
 	}
 
 	if q.ExemplarQuery {
-		res, err := s.exemplarQuery(traceCtx, client, q, headers)
-		if err != nil {
+		res := s.exemplarQuery(traceCtx, client, q, headers)
+		if res.Error != nil {
 			// If exemplar query returns error, we want to only log it and
 			// continue with other results processing
-			logger.Error("Exemplar query failed", "query", q.Expr, "err", err)
+			logger.Error("Exemplar query failed", "query", q.Expr, "err", res.Error)
 		}
-		response.Frames = append(response.Frames, res.Frames...)
+		dr.Frames = append(dr.Frames, res.Frames...)
 	}
 
-	return response, nil
+	return dr
 }
 
-func (s *QueryData) rangeQuery(ctx context.Context, c *client.Client, q *models.Query, headers map[string]string) (backend.DataResponse, error) {
+func (s *QueryData) rangeQuery(ctx context.Context, c *client.Client, q *models.Query, headers map[string]string) backend.DataResponse {
 	res, err := c.QueryRange(ctx, q)
 	if err != nil {
-		return backend.DataResponse{}, err
+		return backend.DataResponse{
+			Error: err,
+		}
 	}
 	return s.parseResponse(ctx, q, res)
 }
 
-func (s *QueryData) instantQuery(ctx context.Context, c *client.Client, q *models.Query, headers map[string]string) (backend.DataResponse, error) {
+func (s *QueryData) instantQuery(ctx context.Context, c *client.Client, q *models.Query, headers map[string]string) backend.DataResponse {
 	res, err := c.QueryInstant(ctx, q)
 	if err != nil {
-		return backend.DataResponse{}, err
+		return backend.DataResponse{
+			Error: err,
+		}
 	}
 	return s.parseResponse(ctx, q, res)
 }
 
-func (s *QueryData) exemplarQuery(ctx context.Context, c *client.Client, q *models.Query, headers map[string]string) (backend.DataResponse, error) {
+func (s *QueryData) exemplarQuery(ctx context.Context, c *client.Client, q *models.Query, headers map[string]string) backend.DataResponse {
 	res, err := c.QueryExemplars(ctx, q)
 	if err != nil {
-		return backend.DataResponse{}, err
+		return backend.DataResponse{
+			Error: err,
+		}
 	}
 	return s.parseResponse(ctx, q, res)
 }

pkg/tsdb/prometheus/querydata/response.go

@@ -17,7 +17,7 @@ import (
 	"github.com/grafana/grafana/pkg/util/converter"
 )
 
-func (s *QueryData) parseResponse(ctx context.Context, q *models.Query, res *http.Response) (backend.DataResponse, error) {
+func (s *QueryData) parseResponse(ctx context.Context, q *models.Query, res *http.Response) backend.DataResponse {
 	defer func() {
 		if err := res.Body.Close(); err != nil {
 			s.log.FromContext(ctx).Error("Failed to close response body", "err", err)
@@ -30,6 +30,12 @@ func (s *QueryData) parseResponse(ctx context.Context, q *models.Query, res *htt
 		VectorWideSeries: s.enableWideSeries,
 	})
 
+	// Add frame to attach metadata
+	if len(r.Frames) == 0 && !q.ExemplarQuery {
+		r.Frames = append(r.Frames, data.NewFrame(""))
+	}
+
+	// The ExecutedQueryString can be viewed in QueryInspector in UI
 	for _, frame := range r.Frames {
 		if s.enableWideSeries {
 			addMetadataToWideFrame(q, frame)
@@ -38,12 +44,11 @@ func (s *QueryData) parseResponse(ctx context.Context, q *models.Query, res *htt
 		}
 	}
 
-	if r.Error != nil {
-		return r, r.Error
+	if r.Error == nil {
+		r = s.processExemplars(q, r)
 	}
 
-	r = s.processExemplars(q, r)
-	return r, nil
+	return r
 }
 
 func (s *QueryData) processExemplars(q *models.Query, dr backend.DataResponse) backend.DataResponse {