Return a 400 instead of a 500 for empty query (#98841)

pkg/tsdb/graphite/graphite.go

@@ -124,7 +124,14 @@ func (s *Service) QueryData(ctx context.Context, req *backend.QueryDataRequest)
 		logger.Warn("Found query models without targets", "models without targets", strings.Join(emptyQueries, "\n"))
 		// If no queries had a valid target, return an error; otherwise, attempt with the targets we have
 		if len(emptyQueries) == len(req.Queries) {
-			return &result, errors.New("no query target found for the alert rule")
+			if result.Responses == nil {
+				result.Responses = make(map[string]backend.DataResponse)
+			}
+			// marking this downstream error as it is a user error, but arguably this is a plugin error
+			// since the plugin should have frontend validation that prevents us from getting into this state
+			missingQueryResponse := backend.ErrDataResponseWithSource(400, backend.ErrorSourceDownstream, "no query target found for the alert rule")
+			result.Responses["A"] = missingQueryResponse
+			return &result, nil
 		}
 	}
 	formData["target"] = targetList

pkg/tsdb/graphite/graphite_test.go

@@ -125,7 +125,7 @@ func TestProcessQueries(t *testing.T) {
 		assert.Equal(t, expectedInvalid, invalids[0])
 	})
 
-	t.Run("QueryData with no valid queries returns an error", func(t *testing.T) {
+	t.Run("QueryData with no valid queries returns bad request response", func(t *testing.T) {
 		queries := []backend.DataQuery{
 			{
 				RefID: "A",
@@ -142,11 +142,12 @@ func TestProcessQueries(t *testing.T) {
 		}
 
 		service.im = fakeInstanceManager{}
-		_, err := service.QueryData(context.Background(), &backend.QueryDataRequest{
+		rsp, err := service.QueryData(context.Background(), &backend.QueryDataRequest{
 			Queries: queries,
 		})
-		assert.Error(t, err)
-		assert.Equal(t, err.Error(), "no query target found for the alert rule")
+		assert.NoError(t, err)
+		expectedResponse := backend.ErrDataResponseWithSource(400, backend.ErrorSourceDownstream, "no query target found for the alert rule")
+		assert.Equal(t, expectedResponse, rsp.Responses["A"])
 	})
 }