feat(internal/librarian): allow adding APIs to existing Go libraries (#5124)

The `librarian add` command is updated to support adding new APIs to an
already existing library in `librarian.yaml`, specifically for Go.

For #4996

Author: JoeWang1127

internal/librarian/add.go

@@ -38,6 +38,8 @@ import (
 
 var (
 	errLibraryAlreadyExists      = errors.New("library already exists in config")
+	errAPIAlreadyExists          = errors.New("api already exists in library")
+	errAPIDuplicate              = errors.New("api duplicate in input")
 	errMissingAPI                = errors.New("must provide at least one API")
 	errMixedPreviewAndNonPreview = errors.New("cannot mix preview and non-preview APIs")
 	errPreviewRequiresLibrary    = errors.New("only APIs with an existing Library can have a Preview")
@@ -134,10 +136,15 @@ func addLibrary(cfg *config.Config, apis ...string) (string, *config.Config, err
 		return "", nil, errMixedPreviewAndNonPreview
 	}
 	paths := make([]*config.API, 0, len(apis))
+	seen := make(map[string]bool)
 	for _, a := range apis {
 		if isPreview {
 			a = strings.TrimPrefix(a, "preview/")
 		}
+		if seen[a] {
+			return "", nil, fmt.Errorf("%w: %s", errAPIDuplicate, a)
+		}
+		seen[a] = true
 		paths = append(paths, &config.API{Path: a})
 	}
 	name := deriveLibraryName(cfg.Language, paths[0].Path)
@@ -158,7 +165,10 @@ func addLibrary(cfg *config.Config, apis ...string) (string, *config.Config, err
 		return addPreviewLibrary(cfg, existingLib, paths, name)
 	}
 	if exists {
-		return "", nil, fmt.Errorf("%w: %s", errLibraryAlreadyExists, name)
+		if cfg.Language != config.LanguageGo {
+			return "", nil, fmt.Errorf("%w: %s", errLibraryAlreadyExists, name)
+		}
+		return updateExistingLibrary(cfg, existingLib, paths)
 	}
 	return addNewLibrary(cfg, paths, name)
 }
@@ -196,6 +206,16 @@ func addNewLibrary(cfg *config.Config, apis []*config.API, name string) (string,
 	return name, cfg, nil
 }
 
+func updateExistingLibrary(cfg *config.Config, existingLib *config.Library, apis []*config.API) (string, *config.Config, error) {
+	for _, api := range apis {
+		if slices.ContainsFunc(existingLib.APIs, func(a *config.API) bool { return api.Path == a.Path }) {
+			return "", nil, fmt.Errorf("%w: %s in library %s", errAPIAlreadyExists, api.Path, existingLib.Name)
+		}
+	}
+	existingLib.APIs = append(existingLib.APIs, apis...)
+	return existingLib.Name, cfg, nil
+}
+
 // syncToStateYAML updates the .librarian/state.yaml with any new libraries.
 func syncToStateYAML(repoDir string, cfg *config.Config) error {
 	stateFile := filepath.Join(repoDir, legacyconfig.LibrarianDir, legacyconfig.LibrarianStateFile)

internal/librarian/add_test.go

@@ -287,6 +287,131 @@ func TestAddLibrary(t *testing.T) {
 	}
 }
 
+func TestAddLibrary_ExistingLibrary(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		apis     []string
+		cfg      *config.Config
+		wantName string
+		wantCfg  *config.Config
+	}{
+		{
+			name: "update existing library",
+			apis: []string{"google/cloud/secretmanager/v1beta2"},
+			cfg: &config.Config{
+				Language: config.LanguageGo,
+				Libraries: []*config.Library{
+					{
+						Name:    "secretmanager",
+						Version: "1.2.3",
+						APIs: []*config.API{
+							{Path: "google/cloud/secretmanager/v1"},
+						},
+					},
+				},
+			},
+			wantName: "secretmanager",
+			wantCfg: &config.Config{
+				Language: config.LanguageGo,
+				Libraries: []*config.Library{
+					{
+						Name:    "secretmanager",
+						Version: "1.2.3",
+						APIs: []*config.API{
+							{Path: "google/cloud/secretmanager/v1"},
+							{Path: "google/cloud/secretmanager/v1beta2"},
+						},
+					},
+				},
+			},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			t.Chdir(tmpDir)
+			if err := yaml.Write(config.LibrarianYAML, test.cfg); err != nil {
+				t.Fatal(err)
+			}
+			gotName, gotCfg, err := addLibrary(test.cfg, test.apis...)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if diff := cmp.Diff(test.wantName, gotName); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+			if diff := cmp.Diff(test.wantCfg, gotCfg); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestAddLibrary_ExistingLibrary_Error(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		apis    []string
+		cfg     *config.Config
+		wantErr error
+	}{
+		{
+			name: "fail if api already exists",
+			apis: []string{"google/cloud/secretmanager/v1beta2"},
+			cfg: &config.Config{
+				Language: config.LanguageGo,
+				Libraries: []*config.Library{
+					{
+						Name:    "secretmanager",
+						Version: "1.2.3",
+						APIs: []*config.API{
+							{Path: "google/cloud/secretmanager/v1"},
+							{Path: "google/cloud/secretmanager/v1beta2"},
+						},
+					},
+				},
+			},
+			wantErr: errAPIAlreadyExists,
+		},
+		{
+			name: "fail if api duplicated",
+			apis: []string{
+				"google/cloud/secretmanager/v1beta2",
+				"google/cloud/secretmanager/v1beta2",
+			},
+			wantErr: errAPIDuplicate,
+		},
+		{
+			name: "python doesn't support updating existing library",
+			apis: []string{"google/cloud/secretmanager/v1beta2"},
+			cfg: &config.Config{
+				Language: config.LanguagePython,
+				Libraries: []*config.Library{
+					{
+						Name:    "google-cloud-secretmanager",
+						Version: "1.2.3",
+						APIs: []*config.API{
+							{Path: "google/cloud/secretmanager/v1"},
+							{Path: "google/cloud/secretmanager/v1beta2"},
+						},
+					},
+				},
+			},
+			wantErr: errLibraryAlreadyExists,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			t.Chdir(tmpDir)
+			if err := yaml.Write(config.LibrarianYAML, test.cfg); err != nil {
+				t.Fatal(err)
+			}
+			_, _, err := addLibrary(test.cfg, test.apis...)
+			if !errors.Is(err, test.wantErr) {
+				t.Fatalf("expected error %v, got %v", test.wantErr, err)
+			}
+		})
+	}
+}
+
 func TestAddLibrary_Preview(t *testing.T) {
 	for _, test := range []struct {
 		name             string