SSE: Return error messages instead of 500 on SSE command parse errors (#109480)

fixes #108897

---------

Co-authored-by: Kyle Brandt <kyle@grafana.com>

pkg/expr/commands.go

@@ -58,7 +58,7 @@ func UnmarshalMathCommand(rn *rawNode) (*MathCommand, error) {
 
 	gm, err := NewMathCommand(rn.RefID, exprString)
 	if err != nil {
-		return nil, fmt.Errorf("invalid math command type: %w", err)
+		return nil, fmt.Errorf("invalid math command: %w", err)
 	}
 	return gm, nil
 }

pkg/expr/errors.go

@@ -75,6 +75,25 @@ func MakeDependencyError(refID, depRefID string) error {
 	return DependencyError.Build(data)
 }
 
+var parsErrStr = "failed to parse expression [{{ .Public.refId }}]: {{.Public.error}}"
+
+var ParseError = errutil.NewBase(
+	errutil.StatusBadRequest, "sse.parseError").MustTemplate(
+	parsErrStr,
+	errutil.WithPublic(parsErrStr))
+
+func MakeParseError(refID string, err error) error {
+	data := errutil.TemplateData{
+		Public: map[string]interface{}{
+			"refId": refID,
+			"error": err.Error(),
+		},
+		Error: err,
+	}
+
+	return ParseError.Build(data)
+}
+
 var unexpectedNodeTypeErrString = "expected executable node type but got node type [{{ .Public.nodeType }} for refid [{{ .Public.refId}}]"
 
 var UnexpectedNodeTypeError = errutil.NewBase(

pkg/expr/nodes.go

@@ -154,7 +154,7 @@ func buildCMDNode(ctx context.Context, rn *rawNode, toggles featuremgmt.FeatureT
 		return nil, fmt.Errorf("expression command type '%v' in expression '%v' not implemented", commandType, rn.RefID)
 	}
 	if err != nil {
-		return nil, fmt.Errorf("failed to parse expression '%v': %w", rn.RefID, err)
+		return nil, MakeParseError(rn.RefID, err)
 	}
 
 	return node, nil

pkg/expr/service_test.go

@@ -148,6 +148,25 @@ func TestDSQueryError(t *testing.T) {
 	require.Equal(t, fp(42), res.Responses["C"].Frames[0].Fields[0].At(0))
 }
 
+func TestParseError(t *testing.T) {
+	resp := map[string]backend.DataResponse{}
+
+	queries := []Query{
+		{
+			RefID:      "A",
+			DataSource: dataSourceModel(),
+			JSON:       json.RawMessage(`{ "datasource": { "uid": "__expr__", "type": "__expr__"}, "type": "math", "expression": "asdf" }`),
+		},
+	}
+
+	s, req := newMockQueryService(resp, queries)
+
+	_, err := s.BuildPipeline(t.Context(), req)
+	require.ErrorContains(t, err, "parse")
+	require.ErrorContains(t, err, "math")
+	require.ErrorContains(t, err, "asdf")
+}
+
 func TestSQLExpressionCellLimitFromConfig(t *testing.T) {
 	tests := []struct {
 		name            string