feat(internal/librarian/golang): run init and tidy on new client (#4343)

For new Go client, run `go mod init` and `go mod tidy` after generation.

For #3617

Author: JoeWang1127

internal/librarian/golang/generate.go

@@ -18,6 +18,7 @@ package golang
 import (
 	"context"
 	_ "embed"
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -128,6 +129,13 @@ func generate(ctx context.Context, library *config.Library, googleapisDir string
 	if err := updateSnippetMetadata(library, outdir); err != nil {
 		return err
 	}
+	if _, err := os.Stat(filepath.Join(absModuleRoot, "go.mod")); err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			// New client, init the module.
+			return initModule(ctx, absModuleRoot, modulePath(library))
+		}
+		return err
+	}
 	return nil
 }
 
@@ -384,6 +392,15 @@ func updateSnippetDirectory(baseDir, version string) error {
 	})
 }
 
+func initModule(ctx context.Context, dir, modPath string) error {
+	initArgs := []string{"go", "mod", "init", modPath}
+	if err := command.RunInDir(ctx, dir, initArgs[0], initArgs[1:]...); err != nil {
+		return err
+	}
+	tidyArgs := []string{"go", "mod", "tidy"}
+	return command.RunInDir(ctx, dir, tidyArgs[0], tidyArgs[1:]...)
+}
+
 // releaseLevel determines the release level for an API based on the API path and the library's current version.
 func releaseLevel(apiPath, version string) (string, error) {
 	apiVersion := filepath.Base(apiPath)

internal/librarian/golang/generate_test.go

@@ -16,6 +16,7 @@ package golang
 
 import (
 	"errors"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -229,7 +230,7 @@ func TestGenerate(t *testing.T) {
 			libraryName: "secretmanager",
 			apis:        []*config.API{{Path: "google/cloud/secretmanager/v1"}},
 			goModule: &config.GoModule{
-				DeleteGenerationOutputPaths: []string{"secretmanager/apiv1/secretmanagerpb"},
+				DeleteGenerationOutputPaths: []string{"secretmanager/apiv1/secret_manager_client.go"},
 				GoAPIs: []*config.GoAPI{
 					{
 						ClientPackage: "secretmanager",
@@ -239,10 +240,10 @@ func TestGenerate(t *testing.T) {
 				},
 			},
 			want: []string{
-				"secretmanager/apiv1/secret_manager_client.go",
+				"secretmanager/apiv1/secretmanagerpb/service.pb.go",
 			},
 			removed: []string{
-				"secretmanager/apiv1/secretmanagerpb",
+				"secretmanager/apiv1/secret_manager_client.go",
 			},
 		},
 		{
@@ -1079,3 +1080,22 @@ func TestBuildGAPICOpts(t *testing.T) {
 		})
 	}
 }
+
+func TestInitModule(t *testing.T) {
+	testhelper.RequireCommand(t, "go")
+	outDir := t.TempDir()
+	// Write an import so go mod tidy can generate a go.sum file.
+	content := []byte("package main\nimport _ \"golang.org/x/text\"\n")
+	if err := os.WriteFile(filepath.Join(outDir, "main.go"), content, 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := initModule(t.Context(), outDir, "example.com/testmod"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(filepath.Join(outDir, "go.mod")); errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("go.mod does not exist")
+	}
+	if _, err := os.Stat(filepath.Join(outDir, "go.sum")); errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("go.sum does not exist")
+	}
+}