refactor(generate): split generating of code into two parts

Before both filtering of which services to generate AIP tests for
and the actual rendering of the code happened in one loop.

This commit separate its into two parts:
- Collect relevant services that tests can be generated for
- Generate code for the collected services

internal/plugin/generate.go

@@ -17,16 +17,32 @@ const (
 
 func Generate(plugin *protogen.Plugin) error {
 	plugin.SupportedFeatures |= 1 // proto3 optional
+	files, err := collectServices(plugin)
+	if err != nil {
+		return err
+	}
+	return generate(plugin, files)
+}
+
+type File struct {
+	*protogen.File
+	services []serviceGenerator
+}
+
+// collectServices collects valid services to generate AIP test code for.
+func collectServices(
+	plugin *protogen.Plugin,
+) ([]File, error) {
 	pkgResources := findResourcesPerPackage(plugin)
+	result := make([]File, 0, 10)
 	for _, file := range plugin.Files {
 		if len(file.Services) == 0 || !file.Generate {
 			continue
 		}
-		filename := fmt.Sprintf("%s_%s", file.GeneratedFilenamePrefix, fileSuffix)
-		f := plugin.NewGeneratedFile(filename, file.GoImportPath)
-		writeHeader(file, f)
-		f.Skip()
-
+		f := File{
+			File:     file,
+			services: make([]serviceGenerator, 0, 10),
+		}
 		for _, service := range file.Services {
 			resources := pkgResources[file.Desc.Package()]
 			if len(resources) == 0 {
@@ -48,7 +64,7 @@ func Generate(plugin *protogen.Plugin) error {
 				rs = append(rs, serviceResource.descriptor)
 				m, err := protogenMessage(plugin, serviceResource.message.FullName())
 				if err != nil {
-					return err
+					return nil, err
 				}
 				ms = append(ms, m)
 			}
@@ -57,6 +73,18 @@ func Generate(plugin *protogen.Plugin) error {
 				resources: rs,
 				messages:  ms,
 			}
+			f.services = append(f.services, generator)
+		}
+		result = append(result, f)
+	}
+	return result, nil
+}
+
+func generate(plugin *protogen.Plugin, files []File) error {
+	for _, file := range files {
+		f := createServiceTestFile(plugin, file)
+		f.Skip()
+		for _, generator := range file.services {
 			if err := generator.Generate(f); err != nil {
 				return err
 			}
@@ -66,6 +94,13 @@ func Generate(plugin *protogen.Plugin) error {
 	return nil
 }
 
+func createServiceTestFile(plugin *protogen.Plugin, file File) *protogen.GeneratedFile {
+	filename := fmt.Sprintf("%s_%s", file.GeneratedFilenamePrefix, fileSuffix)
+	f := plugin.NewGeneratedFile(filename, file.GoImportPath)
+	writeHeader(file.File, f)
+	return f
+}
+
 func writeHeader(file *protogen.File, f *protogen.GeneratedFile) {
 	f.P("// Code generated by protoc-gen-go-aip-test. DO NOT EDIT.")
 	f.P()