feat(openapi): Add flatten_oneofs plugin option

cmd/protoc-gen-openapi/examples/tests/oneof/openapi_flatten_oneofs.yaml

@@ -0,0 +1,100 @@
+# Generated with protoc-gen-openapi
+# https://github.com/fern-api/protoc-gen-openapi/tree/master/cmd/protoc-gen-openapi
+
+openapi: 3.0.3
+info:
+    title: OneOfService API
+    description: Service definition
+    version: 0.0.1
+paths:
+    /v1/payments:
+        post:
+            tags:
+                - OneOfService
+            operationId: OneOfService_CreatePayment
+            requestBody:
+                content:
+                    application/json:
+                        schema:
+                            $ref: '#/components/schemas/OneOfMessage'
+                required: true
+            responses:
+                "200":
+                    description: OK
+                    content:
+                        application/json:
+                            schema:
+                                $ref: '#/components/schemas/OneOfMessage'
+                default:
+                    description: Default error response
+                    content:
+                        application/json:
+                            schema:
+                                $ref: '#/components/schemas/Status'
+            x-fern-sdk-group-name: OneOfService
+            x-fern-sdk-method-name: CreatePayment
+components:
+    schemas:
+        EmptyOneOf:
+            type: object
+            properties: {}
+            x-fern-type-name: EmptyOneOf
+            source: tests/oneof/message.proto
+        GoogleProtobufAny:
+            type: object
+            properties:
+                '@type':
+                    type: string
+                    description: The type of the serialized message.
+            additionalProperties: true
+            description: Contains an arbitrary serialized message along with a @type that describes the type of the serialized message.
+            x-fern-encoding:
+                proto:
+                    type: google.protobuf.Any
+        OneOfMessage:
+            type: object
+            properties:
+                id:
+                    type: string
+                    description: Identifier for the message
+                    x-fern-encoding:
+                        proto:
+                            type: google.protobuf.StringValue
+                credit_card:
+                    type: string
+                    x-fern-encoding:
+                        proto:
+                            type: google.protobuf.StringValue
+                bank_transfer:
+                    type: string
+                    x-fern-encoding:
+                        proto:
+                            type: google.protobuf.StringValue
+                digital_wallet:
+                    type: string
+                    x-fern-encoding:
+                        proto:
+                            type: google.protobuf.StringValue
+                authentication_config:
+                    $ref: '#/components/schemas/EmptyOneOf'
+            description: Message demonstrating oneof field usage
+            x-fern-type-name: OneOfMessage
+            source: tests/oneof/message.proto
+        Status:
+            type: object
+            properties:
+                code:
+                    type: integer
+                    description: The status code, which should be an enum value of [google.rpc.Code][google.rpc.Code].
+                    format: int32
+                message:
+                    type: string
+                    description: A developer-facing error message, which should be in English. Any user-facing error message should be localized and sent in the [google.rpc.Status.details][google.rpc.Status.details] field, or localized by the client.
+                details:
+                    type: array
+                    items:
+                        $ref: '#/components/schemas/GoogleProtobufAny'
+                    description: A list of messages that carry the error details.  There is a common set of message types for APIs to use.
+            description: 'The `Status` type defines a logical error model that is suitable for different programming environments, including REST APIs and RPC APIs. It is used by [gRPC](https://github.com/grpc). Each `Status` message contains three pieces of data: error code, error message, and error details. You can find out more about this error model and how to work with it in the [API Design Guide](https://cloud.google.com/apis/design/errors).'
+tags:
+    - name: OneOfService

cmd/protoc-gen-openapi/generator/generator.go

@@ -47,6 +47,7 @@ type Configuration struct {
 	DefaultResponse *bool
 	OutputMode      *string
 	SourceRoot      *string
+	FlattenOneofs   *bool
 }
 
 const (
@@ -894,10 +895,10 @@ func (g *OpenAPIv3Generator) addSchemasForMessagesToDocumentV3(d *v3.Document, m
 		var required []string
 
 		for _, field := range message.Fields {
-			// Skip fields that are part of an explicit oneOf.
+			// Skip fields that are part of an explicit oneOf (unless flattening is enabled).
 			// Proto3 optional fields create synthetic oneofs that should be
 			// treated as regular optional fields, not as oneOf variants.
-			if field.Oneof != nil && !field.Oneof.Desc.IsSynthetic() {
+			if field.Oneof != nil && !field.Oneof.Desc.IsSynthetic() && !*g.conf.FlattenOneofs {
 				continue
 			}
 
@@ -969,8 +970,11 @@ func (g *OpenAPIv3Generator) addSchemasForMessagesToDocumentV3(d *v3.Document, m
 			Required:    required,
 		}
 
-		// Add oneOf fields to the schema
-		g.addOneOfFieldsToSchema(d, message.Oneofs, schema, schemaName, filename)
+		// Add oneOf fields to the schema (unless flattening is enabled,
+		// in which case they were already added as regular properties).
+		if !*g.conf.FlattenOneofs {
+			g.addOneOfFieldsToSchema(d, message.Oneofs, schema, schemaName, filename)
+		}
 
 		// Merge any `Schema` annotations with the current
 		extSchema := proto.GetExtension(message.Desc.Options(), v3.E_Schema)

cmd/protoc-gen-openapi/main.go

@@ -39,6 +39,7 @@ func main() {
 		DefaultResponse: flags.Bool("default_response", true, `add default response. If "true", automatically adds a default response to operations which use the google.rpc.Status message. Useful if you use envoy or grpc-gateway to transcode as they use this type for their default error responses.`),
 		OutputMode:      flags.String("output_mode", "merged", `output generation mode. By default, a single openapi.yaml is generated at the out folder. Use "source_relative' to generate a separate '[inputfile].openapi.yaml' next to each '[inputfile].proto'.`),
 		SourceRoot:      flags.String("source_root", "", `root directory of the source Protobuf files. This is used to add source information to the openapi.yaml file.`),
+		FlattenOneofs:   flags.Bool("flatten_oneofs", false, `flatten oneof fields as regular properties on the parent message instead of generating a oneOf union`),
 	}
 
 	opts := protogen.Options{

cmd/protoc-gen-openapi/plugin_test.go

@@ -270,6 +270,46 @@ func TestOpenAPIStringEnums(t *testing.T) {
 	}
 }
 
+func TestOpenAPIFlattenOneofs(t *testing.T) {
+	// Set PATH to include the protoc-gen-openapi plugin
+	os.Setenv("PATH", "../../:"+os.Getenv("PATH"))
+
+	for _, tt := range openapiTests {
+		fixture := path.Join(tt.path, "openapi_flatten_oneofs.yaml")
+		if _, err := os.Stat(fixture); errors.Is(err, os.ErrNotExist) {
+			if !GENERATE_FIXTURES {
+				continue
+			}
+		}
+		t.Run(tt.name, func(t *testing.T) {
+			err := exec.Command("protoc",
+				"-I", "../../",
+				"-I", "../../third_party",
+				"-I", "examples",
+				path.Join(tt.path, tt.protofile),
+				"--plugin=protoc-gen-openapi=./protoc-gen-openapi",
+				"--openapi_out=naming=proto,flatten_oneofs=true:.").Run()
+			if err != nil {
+				t.Fatalf("protoc failed: %+v", err)
+			}
+			if GENERATE_FIXTURES {
+				err := CopyFixture(TEMP_FILE, fixture)
+				if err != nil {
+					t.Fatalf("Can't generate fixture: %+v", err)
+				}
+			} else {
+				// Verify that the generated spec matches our expected version.
+				err = exec.Command("diff", TEMP_FILE, fixture).Run()
+				if err != nil {
+					t.Fatalf("Diff failed: %+v", err)
+				}
+			}
+			// if the test succeeded, clean up
+			os.Remove(TEMP_FILE)
+		})
+	}
+}
+
 func TestOpenAPIDefaultResponse(t *testing.T) {
 	// Set PATH to include the protoc-gen-openapi plugin
 	os.Setenv("PATH", "../../:"+os.Getenv("PATH"))