Templates: error on missing required values (#25932)

tests/vehicle-error.evcc.yaml

@@ -50,5 +50,6 @@ vehicles:
     type: template
     template: tesla # not optimal, since real communication with tesla server is happening
     title: Broken Tesla
+    clientId: test_client_id
     accessToken: A
     refreshToken: B

util/templates/template.go

@@ -7,6 +7,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"testing"
 	"text/template"
 
 	"github.com/Masterminds/sprig/v3"
@@ -354,6 +355,13 @@ func (t *Template) RenderResult(renderMode int, other map[string]any) ([]byte, m
 				if val != nil {
 					s = p.yamlQuote(fmt.Sprintf("%v", val))
 				}
+
+				// validate required fields from yaml
+				if s == "" && p.IsRequired() && (renderMode == RenderModeUnitTest ||
+					renderMode == RenderModeInstance && !testing.Testing()) {
+					return nil, nil, fmt.Errorf("missing required `%s`", p.Name)
+				}
+
 				res[out] = s
 			}
 		}

util/templates/template_test.go

@@ -32,3 +32,36 @@ func TestPresets(t *testing.T) {
 		{Name: "last"},
 	}, tmpl.Params)
 }
+
+func TestRequired(t *testing.T) {
+	tmpl := &Template{
+		TemplateDefinition: TemplateDefinition{
+			Params: []Param{
+				{
+					Name:     "param",
+					Required: true,
+				},
+			},
+		},
+	}
+
+	_, _, err := tmpl.RenderResult(RenderModeUnitTest, map[string]any{
+		"Param": "foo",
+	})
+	require.NoError(t, err)
+
+	_, _, err = tmpl.RenderResult(RenderModeUnitTest, map[string]any{
+		"Param": "",
+	})
+	require.Error(t, err)
+
+	_, _, err = tmpl.RenderResult(RenderModeUnitTest, map[string]any{
+		"Param": nil,
+	})
+	require.Error(t, err)
+
+	_, _, err = tmpl.RenderResult(RenderModeDocs, map[string]any{
+		"Param": nil,
+	})
+	require.NoError(t, err)
+}