Parse template when validating it (#49282) (#49284)

(cherry picked from commit e8b498fe8b)

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

pkg/services/ngalert/api/tooling/definitions/provisioning_templates.go

@@ -2,6 +2,7 @@ package definitions
 
 import (
 	"fmt"
+	"html/template"
 	"regexp"
 	"strings"
 
@@ -74,6 +75,11 @@ func (t *MessageTemplate) Validate() error {
 		return fmt.Errorf("template must have content")
 	}
 
+	_, err := template.New("").Parse(t.Template)
+	if err != nil {
+		return fmt.Errorf("invalid template: %w", err)
+	}
+
 	content := strings.TrimSpace(t.Template)
 	found, err := regexp.MatchString(`\{\{\s*define`, content)
 	if err != nil {

pkg/services/ngalert/provisioning/templates_test.go

@@ -234,6 +234,42 @@ func TestTemplateService(t *testing.T) {
 
 			require.Equal(t, tmpl.Template, result.Template)
 		})
+
+		t.Run("rejects syntactically invalid template", func(t *testing.T) {
+			sut := createTemplateServiceSut()
+			tmpl := definitions.MessageTemplate{
+				Name:     "name",
+				Template: "{{ .MyField }",
+			}
+			sut.config.(*MockAMConfigStore).EXPECT().
+				getsConfig(models.AlertConfiguration{
+					AlertmanagerConfiguration: defaultConfig,
+				})
+			sut.config.(*MockAMConfigStore).EXPECT().saveSucceeds()
+			sut.prov.(*MockProvisioningStore).EXPECT().saveSucceeds()
+
+			_, err := sut.SetTemplate(context.Background(), 1, tmpl)
+
+			require.ErrorIs(t, err, ErrValidation)
+		})
+
+		t.Run("does not reject template with unknown field", func(t *testing.T) {
+			sut := createTemplateServiceSut()
+			tmpl := definitions.MessageTemplate{
+				Name:     "name",
+				Template: "{{ .NotAField }}",
+			}
+			sut.config.(*MockAMConfigStore).EXPECT().
+				getsConfig(models.AlertConfiguration{
+					AlertmanagerConfiguration: defaultConfig,
+				})
+			sut.config.(*MockAMConfigStore).EXPECT().saveSucceeds()
+			sut.prov.(*MockProvisioningStore).EXPECT().saveSucceeds()
+
+			_, err := sut.SetTemplate(context.Background(), 1, tmpl)
+
+			require.NoError(t, err)
+		})
 	})
 
 	t.Run("deleting templates", func(t *testing.T) {