Improve error message (rfc7807) (#13680)

* Add func formatErrorMessage

* Add unit test

* Commit generated code

* Fix indentation

* Using tabs

* Set error before model

* Commit generated code

* Fix tabs

* Commit generated code

* Fix tabs

* Fix tabs

* Commit generated code

Author: gcatanese

modules/openapi-generator/src/main/resources/go/api.mustache

@@ -385,7 +385,8 @@ func (a *{{{classname}}}Service) {{nickname}}Execute(r {{#structPrefix}}{{&class
 				newErr.error = err.Error()
 				return {{#returnType}}localVarReturnValue, {{/returnType}}localVarHTTPResponse, newErr
 			}
-			newErr.model = v
+            		newErr.error = formatErrorMessage(localVarHTTPResponse.Status, &v)
+            		newErr.model = v
 			{{^-last}}
 			return {{#returnType}}localVarReturnValue, {{/returnType}}localVarHTTPResponse, newErr
 			{{/-last}}

modules/openapi-generator/src/main/resources/go/client.mustache

@@ -616,3 +616,23 @@ func (e GenericOpenAPIError) Body() []byte {
 func (e GenericOpenAPIError) Model() interface{} {
 	return e.model
 }
+
+// format error message using title and detail when model implements rfc7807
+func formatErrorMessage(status string, v interface{}) string {
+
+    str := ""
+    metaValue := reflect.ValueOf(v).Elem()
+
+    field := metaValue.FieldByName("Title")
+    if field != (reflect.Value{}) {
+        str = fmt.Sprintf("%s", field.Interface())
+    }
+
+    field = metaValue.FieldByName("Detail")
+    if field != (reflect.Value{}) {
+        str = fmt.Sprintf("%s (%s)", str, field.Interface())
+    }
+
+    // status title (detail)
+    return fmt.Sprintf("%s %s", status, str)
+}

modules/openapi-generator/src/test/java/org/openapitools/codegen/go/GoClientCodegenTest.java

@@ -222,4 +222,22 @@ public void verifyTestFile() throws IOException {
                 "func Test_openapi_PetApiService(t *testing.T) {");
     }
 
+    @Test
+    public void verifyFormatErrorMessageInUse() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("go")
+                .setInputSpec("src/test/resources/3_0/go/petstore-with-problem-details.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        TestUtils.assertFileExists(Paths.get(output + "/api_pet.go"));
+        TestUtils.assertFileContains(Paths.get(output + "/api_pet.go"),
+                "newErr.error = formatErrorMessage(localVarHTTPResponse.Status, &v)");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/go/petstore-with-problem-details.yaml

@@ -0,0 +1,61 @@
+openapi: 3.0.0
+info:
+  description: >-
+    This spec is mainly for testing Petstore server and contains fake endpoints,
+    models. Please do not use this for any other purpose. Special characters: "
+    \
+  version: 1.0.0
+  title: OpenAPI Petstore
+  license:
+    name: Apache-2.0
+    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
+tags:
+  - name: pet
+    description: Everything about your Pets
+paths:
+  /foo:
+    get:
+      tags:
+        - pet
+      responses:
+        "200":
+          description: response
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Foo'
+        "404":
+          description: not found
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RestServiceError'
+        "422":
+          description: validation error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RestServiceError'
+components:
+  schemas:
+    Foo:
+      type: string
+      default: foo
+    # RFC7807 Problem Detail
+    RestServiceError:
+      properties:
+        type:
+          description: " A URI reference that identifies the problem type"
+          type: string
+        title:
+          description: "A short, human-readable summary of the problem type."
+          type: string
+        status:
+          description: "The HTTP Status Code"
+          type: integer
+        detail:
+          description: "A human-readable explanation specific to this occurrence of the problem."
+          type: string
+        instance:
+          description: "A unique URI that identifies the specific occurrence of the problem."
+          type: string