chore(tool/cmd/migrate): preserve existing libraries during ruby migration (#6970)

Existing library configurations from librarian.yaml are preserved during
Ruby library migration rather than being overwritten.

The `parseExistingLibraries` helper reads existing configurations from
`librarian.yaml` if present, and `mergeLibs` retains settings for
previously configured libraries while appending newly discovered
libraries.

For #6632
Fixes #6968

Author: JoeWang1127

tool/cmd/migrate/ruby.go

@@ -81,11 +81,15 @@ func runRubyMigration(ctx context.Context, repoPath string) error {
 			},
 		},
 	}
+	existingLibs, err := parseExistingLibraries(repoPath)
+	if err != nil {
+		return err
+	}
 	libs, err := findRubyLibraries(src.Dir, repoPath)
 	if err != nil {
 		return err
 	}
-	cfg.Libraries = libs
+	cfg.Libraries = mergeLibs(existingLibs, libs)
 	// The directory name in Googleapis is present for migration code to look
 	// up API details. It shouldn't be persisted.
 	cfg.Sources.Googleapis.Dir = ""
@@ -96,6 +100,21 @@ func runRubyMigration(ctx context.Context, repoPath string) error {
 	return nil
 }
 
+func parseExistingLibraries(repoPath string) ([]*config.Library, error) {
+	absConfigPath, err := filepath.Abs(filepath.Join(repoPath, config.LibrarianYAML))
+	if err != nil {
+		return nil, fmt.Errorf("getting absolute path to %s: %w", config.LibrarianYAML, err)
+	}
+	cfg, err := yaml.Read[config.Config](absConfigPath)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	return cfg.Libraries, nil
+}
+
 func findRubyLibraries(googleapisPath, repoPath string) ([]*config.Library, error) {
 	entries, err := os.ReadDir(repoPath)
 	if err != nil {
@@ -244,3 +263,25 @@ func parseVersionedBuild(googleapisDir, apiPath string) (*VersionedBuild, error)
 	}
 	return vb, nil
 }
+
+// mergeLibs merges existing libraries with discovered libraries. Existing libraries' configurations
+// are preserved and discovered libraries are appended to the result.
+func mergeLibs(existingLibs []*config.Library, libs []*config.Library) []*config.Library {
+	existingMap := toMap(existingLibs)
+	res := make([]*config.Library, len(existingLibs))
+	copy(res, existingLibs)
+	for _, lib := range libs {
+		if _, ok := existingMap[lib.Name]; !ok {
+			res = append(res, lib)
+		}
+	}
+	return res
+}
+
+func toMap(libs []*config.Library) map[string]*config.Library {
+	libraryMap := make(map[string]*config.Library)
+	for _, lib := range libs {
+		libraryMap[lib.Name] = lib
+	}
+	return libraryMap
+}

tool/cmd/migrate/ruby_test.go

@@ -295,3 +295,189 @@ func TestParseVersionedBuild(t *testing.T) {
 		})
 	}
 }
+
+func TestMergeLibs(t *testing.T) {
+	for _, test := range []struct {
+		name         string
+		existingLibs []*config.Library
+		libs         []*config.Library
+		want         []*config.Library
+	}{
+		{
+			name: "preserve existing library configuration",
+			existingLibs: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+					APIs: []*config.API{
+						{Path: "google/cloud/secretmanager/v1"},
+					},
+				},
+			},
+			libs: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "0.1.0",
+					APIs: []*config.API{
+						{Path: "google/cloud/secretmanager/v1"},
+					},
+				},
+			},
+			want: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+					APIs: []*config.API{
+						{Path: "google/cloud/secretmanager/v1"},
+					},
+				},
+			},
+		},
+		{
+			name: "append new discovered libraries",
+			existingLibs: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+				},
+			},
+			libs: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "0.1.0",
+				},
+				{
+					Name:    "google-cloud-compute-v1",
+					Version: "0.1.0",
+				},
+			},
+			want: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+				},
+				{
+					Name:    "google-cloud-compute-v1",
+					Version: "0.1.0",
+				},
+			},
+		},
+		{
+			name: "nil existing libraries returns discovered libraries",
+			libs: []*config.Library{
+				{
+					Name:    "google-cloud-compute-v1",
+					Version: "0.1.0",
+				},
+			},
+			want: []*config.Library{
+				{
+					Name:    "google-cloud-compute-v1",
+					Version: "0.1.0",
+				},
+			},
+		},
+		{
+			name: "preserve existing libraries not in discovered list",
+			existingLibs: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+				},
+				{
+					Name:    "google-cloud-recaptcha_enterprise-v1",
+					Version: "1.0.0",
+				},
+			},
+			libs: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "0.1.0",
+				},
+			},
+			want: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+				},
+				{
+					Name:    "google-cloud-recaptcha_enterprise-v1",
+					Version: "1.0.0",
+				},
+			},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got := mergeLibs(test.existingLibs, test.libs)
+			if diff := cmp.Diff(test.want, got); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestParseExistingLibraries(t *testing.T) {
+	for _, test := range []struct {
+		name  string
+		setup func(t *testing.T, dir string)
+		want  []*config.Library
+	}{
+		{
+			name: "valid librarian.yaml with libraries",
+			setup: func(t *testing.T, dir string) {
+				cfg := &config.Config{
+					Libraries: []*config.Library{
+						{
+							Name:    "google-cloud-secret_manager-v1",
+							Version: "1.2.0",
+							APIs: []*config.API{
+								{Path: "google/cloud/secretmanager/v1"},
+							},
+						},
+					},
+				}
+				if err := yaml.Write(filepath.Join(dir, config.LibrarianYAML), cfg); err != nil {
+					t.Fatal(err)
+				}
+			},
+			want: []*config.Library{
+				{
+					Name:    "google-cloud-secret_manager-v1",
+					Version: "1.2.0",
+					APIs: []*config.API{
+						{Path: "google/cloud/secretmanager/v1"},
+					},
+				},
+			},
+		},
+		{
+			name:  "librarian.yaml does not exist",
+			setup: func(t *testing.T, dir string) {},
+			want:  nil,
+		},
+		{
+			name: "librarian.yaml without libraries",
+			setup: func(t *testing.T, dir string) {
+				cfg := &config.Config{
+					Language: config.LanguageRuby,
+				}
+				if err := yaml.Write(filepath.Join(dir, config.LibrarianYAML), cfg); err != nil {
+					t.Fatal(err)
+				}
+			},
+			want: nil,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			dir := t.TempDir()
+			test.setup(t, dir)
+			got, err := parseExistingLibraries(dir)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if diff := cmp.Diff(test.want, got); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}