feat(librarian): remove channel from librarian.yaml if can be derived from library name (#3221)

Let `librarian tidy` remove redundant fields in Channel if can be
derived from library name. When generating, `prepareLibrary` should
derive channel path and service_config from defaults.

Also fix test setups for `TestGenerateCommand` and `TestGenerateSkip`
that Channel path and service config file should be present. This was
not an issue before because no `channel` was explicitly specified in
test config.

Rules used go derive Channel:
- path:  replacing all '-' in the library's name with '/'.
- service_config:  '[resolved_path]/[service_name]_[version].yaml'.
- If both the path and service_config fields of a channel are removed,
the entire channel entry will be removed from the configuration.

Assumption made that may need adjustments in the future:
- Rules for deriving service_config from path is the same across
language.
- Derivation is on "best-effort" , and for simplicity I assumed the
[service_name] and [version] are the last 2 parts from `path`.

For #3139

Author: zhumin8

internal/librarian/generate.go

@@ -21,6 +21,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/googleapis/librarian/internal/config"
 	"github.com/googleapis/librarian/internal/fetch"
@@ -168,6 +169,35 @@ func defaultOutput(language, channel, defaultOut string) string {
 	}
 }
 
+func deriveChannelPath(language string, lib *config.Library) string {
+	switch language {
+	case "rust":
+		return rust.DeriveChannelPath(lib.Name)
+	default:
+		return strings.ReplaceAll(lib.Name, "-", "/")
+	}
+}
+
+// deriveServiceConfig returns the conventionally derived service config path for a given channel.
+//
+// The final service config path is constructed using the pattern: "[resolved_path]/[service_name]_[version].yaml".
+//
+// For example, if resolved_path is "google/cloud/speech/v1", it derives to "google/cloud/speech/v1/speech_v1.yaml".
+//
+// It returns an empty string if the resolved path does not contain sufficient components
+// (e.g., missing version or service name) or if the version component does not start with 'v'.
+func deriveServiceConfig(resolvedPath string) string {
+	parts := strings.Split(resolvedPath, "/")
+	if len(parts) >= 2 {
+		version := parts[len(parts)-1]
+		service := parts[len(parts)-2]
+		if strings.HasPrefix(version, "v") {
+			return fmt.Sprintf("%s/%s_%s.yaml", resolvedPath, service, version)
+		}
+	}
+	return ""
+}
+
 func dirExists(path string) bool {
 	info, err := os.Stat(path)
 	if err != nil {
@@ -210,13 +240,23 @@ func generateLibrary(ctx context.Context, cfg *config.Config, libraryName string
 // For Rust libraries without an explicit output path, it derives the output
 // from the first channel path.
 func prepareLibrary(language string, lib *config.Library, defaults *config.Default) (*config.Library, error) {
+	if len(lib.Channels) == 0 {
+		// If no channels are specified, create an empty channel first
+		lib.Channels = append(lib.Channels, &config.Channel{})
+	}
+	for _, ch := range lib.Channels {
+		if ch.Path == "" {
+			ch.Path = deriveChannelPath(language, lib)
+		}
+		if ch.ServiceConfig == "" {
+			ch.ServiceConfig = deriveServiceConfig(ch.Path)
+		}
+	}
+
 	if lib.Output == "" {
 		if lib.Veneer {
 			return nil, fmt.Errorf("veneer %q requires an explicit output path", lib.Name)
 		}
-		if len(lib.Channels) == 0 {
-			return nil, fmt.Errorf("library %q has no channels, cannot determine default output", lib.Name)
-		}
 		lib.Output = defaultOutput(language, lib.Channels[0].Path, defaults.Output)
 	}
 	return fillDefaults(lib, defaults), nil

internal/librarian/generate_test.go

@@ -33,30 +33,28 @@ func TestGenerateCommand(t *testing.T) {
 		lib2       = "library-two"
 		lib2Output = "output2"
 	)
-	wd, err := os.Getwd()
-	if err != nil {
-		t.Fatal(err)
-	}
-	googleapisDir := filepath.Join(wd, "testdata", "googleapis")
-
 	tempDir := t.TempDir()
 	t.Chdir(tempDir)
+	googleapisDir := createGoogleapisServiceConfigs(t, tempDir, map[string]string{
+		"google/cloud/speech/v1":       "speech_v1.yaml",
+		"grafeas/v1":                   "grafeas_v1.yaml",
+		"google/cloud/texttospeech/v1": "texttospeech_v1.yaml",
+	})
 	configPath := filepath.Join(tempDir, librarianConfigPath)
-
 	configContent := fmt.Sprintf(`language: testhelper
 sources:
   googleapis:
     dir: %s
 libraries:
   - name: %s
     output: %s
-    apis:
+    channels:
       - path: google/cloud/speech/v1
-      - path: google/cloud/speech/v1p1beta1
-      - path: google/cloud/speech/v2
       - path: grafeas/v1
   - name: %s
     output: %s
+    channels:
+      - path: google/cloud/texttospeech/v1
 `, googleapisDir, lib1, lib1Output, lib2, lib2Output)
 	if err := os.WriteFile(configPath, []byte(configContent), 0644); err != nil {
 		t.Fatal(err)
@@ -147,11 +145,12 @@ func TestGenerateSkip(t *testing.T) {
 		lib2       = "library-two"
 		lib2Output = "output2"
 	)
-	wd, err := os.Getwd()
-	if err != nil {
-		t.Fatal(err)
-	}
-	googleapisDir := filepath.Join(wd, "testdata", "googleapis")
+	tempDir := t.TempDir()
+	t.Chdir(tempDir)
+	googleapisDir := createGoogleapisServiceConfigs(t, tempDir, map[string]string{
+		"google/cloud/speech/v1":       "speech_v1.yaml",
+		"google/cloud/texttospeech/v1": "texttospeech_v1.yaml",
+	})
 
 	allLibraries := map[string]string{
 		lib1: lib1Output,
@@ -184,8 +183,12 @@ libraries:
   - name: %s
     output: %s
     skip_generate: true
+    channels:
+      - path: google/cloud/speech/v1
   - name: %s
     output: %s
+    channels:
+      - path: google/cloud/texttospeech/v1
 `, googleapisDir, lib1, lib1Output, lib2, lib2Output)
 			if err := os.WriteFile(filepath.Join(tempDir, librarianConfigPath), []byte(configContent), 0644); err != nil {
 				t.Fatal(err)
@@ -218,54 +221,59 @@ libraries:
 
 func TestPrepareLibrary(t *testing.T) {
 	for _, test := range []struct {
-		name     string
-		language string
-		output   string
-		veneer   bool
-		channels []*config.Channel
-		want     string
-		wantErr  bool
+		name              string
+		language          string
+		output            string
+		veneer            bool
+		channels          []*config.Channel
+		wantOutput        string
+		wantErr           bool
+		wantChannelPath   string
+		wantServiceConfig string
 	}{
 		{
-			name:     "empty output derives path from channel",
-			language: "rust",
-			channels: []*config.Channel{{Path: "google/cloud/secretmanager/v1"}},
-			want:     "src/generated/cloud/secretmanager/v1",
+			name:       "empty output derives path from channel",
+			language:   "rust",
+			channels:   []*config.Channel{{Path: "google/cloud/secretmanager/v1"}},
+			wantOutput: "src/generated/cloud/secretmanager/v1",
 		},
 		{
-			name:     "explicit output keeps explicit path",
-			language: "rust",
-			output:   "custom/output",
-			channels: []*config.Channel{{Path: "google/cloud/secretmanager/v1"}},
-			want:     "custom/output",
+			name:       "explicit output keeps explicit path",
+			language:   "rust",
+			output:     "custom/output",
+			channels:   []*config.Channel{{Path: "google/cloud/secretmanager/v1"}},
+			wantOutput: "custom/output",
 		},
 		{
-			name:     "empty output uses default for non-rust",
-			language: "go",
-			channels: []*config.Channel{{Path: "google/cloud/secretmanager/v1"}},
-			want:     "src/generated",
+			name:       "empty output uses default for non-rust",
+			language:   "go",
+			channels:   []*config.Channel{{Path: "google/cloud/secretmanager/v1"}},
+			wantOutput: "src/generated",
 		},
 		{
-			name:     "rust with no channels returns error",
-			language: "rust",
-			channels: nil,
-			wantErr:  true,
+			name:              "rust with no channels creates default and derives path",
+			language:          "rust",
+			channels:          nil,
+			wantErr:           false,
+			wantOutput:        "src/generated/cloud/test/v1",
+			wantChannelPath:   "google/cloud/test/v1",
+			wantServiceConfig: "google/cloud/test/v1/test_v1.yaml",
 		},
 		{
 			name:    "veneer without output returns error",
 			veneer:  true,
 			wantErr: true,
 		},
 		{
-			name:   "veneer with explicit output succeeds",
-			veneer: true,
-			output: "src/storage",
-			want:   "src/storage",
+			name:       "veneer with explicit output succeeds",
+			veneer:     true,
+			output:     "src/storage",
+			wantOutput: "src/storage",
 		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			lib := &config.Library{
-				Name:     "test-lib",
+				Name:     "google-cloud-test-v1",
 				Output:   test.output,
 				Veneer:   test.veneer,
 				Channels: test.channels,
@@ -283,13 +291,45 @@ func TestPrepareLibrary(t *testing.T) {
 			if err != nil {
 				t.Fatal(err)
 			}
-			if got.Output != test.want {
-				t.Errorf("got output %q, want %q", got.Output, test.want)
+			if got.Output != test.wantOutput {
+				t.Errorf("got output %q, want %q", got.Output, test.wantOutput)
+			}
+			if test.wantChannelPath != "" || test.wantServiceConfig != "" {
+				if len(got.Channels) == 0 {
+					t.Fatalf("expected a channel, got none")
+				}
+				ch := got.Channels[0]
+				if test.wantChannelPath != "" && ch.Path != test.wantChannelPath {
+					t.Errorf("got channel path %q, want %q", ch.Path, test.wantChannelPath)
+				}
+				if test.wantServiceConfig != "" && ch.ServiceConfig != test.wantServiceConfig {
+					t.Errorf("got service config %q, want %q", ch.ServiceConfig, test.wantServiceConfig)
+				}
 			}
 		})
 	}
 }
 
+// createGoogleapisServiceConfigs creates a mock googleapis directory structure
+// with service config files for testing purposes.
+// The configs map keys are channel paths (e.g., "google/cloud/speech/v1")
+// and values are the service config filenames (e.g., "speech_v1.yaml").
+func createGoogleapisServiceConfigs(t *testing.T, tempDir string, configs map[string]string) string {
+	t.Helper()
+	googleapisDir := filepath.Join(tempDir, "googleapis")
+
+	for channelPath, filename := range configs {
+		dir := filepath.Join(googleapisDir, channelPath)
+		if err := os.MkdirAll(dir, 0755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(dir, filename), []byte(""), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return googleapisDir
+}
+
 func TestCleanOutput(t *testing.T) {
 	for _, test := range []struct {
 		name    string

internal/librarian/internal/rust/generate.go

@@ -153,6 +153,12 @@ func DefaultLibraryName(channel string) string {
 	return strings.ReplaceAll(channel, "/", "-")
 }
 
+// DeriveChannelPath derives a channel path from a library name.
+// For example: google-cloud-secretmanager-v1 -> google/cloud/secretmanager/v1.
+func DeriveChannelPath(name string) string {
+	return strings.ReplaceAll(name, "-", "/")
+}
+
 // DefaultOutput derives an output path from a channel path and default output.
 // For example: google/cloud/secretmanager/v1 with default src/generated/
 // returns src/generated/cloud/secretmanager/v1.

internal/librarian/tidy.go

@@ -55,6 +55,17 @@ func RunTidy() error {
 		if lib.Output != "" && len(lib.Channels) == 1 && isDerivableOutput(cfg, lib) {
 			lib.Output = ""
 		}
+		for _, ch := range lib.Channels {
+			if isDerivableChannelPath(cfg.Language, lib, ch) {
+				ch.Path = ""
+			}
+			if isDerivableServiceConfig(cfg.Language, lib, ch) {
+				ch.ServiceConfig = ""
+			}
+		}
+		lib.Channels = slices.DeleteFunc(lib.Channels, func(ch *config.Channel) bool {
+			return ch.Path == "" && ch.ServiceConfig == ""
+		})
 	}
 	return yaml.Write(librarianConfigPath, formatConfig(cfg))
 }
@@ -64,6 +75,18 @@ func isDerivableOutput(cfg *config.Config, lib *config.Library) bool {
 	return lib.Output == derivedOutput
 }
 
+func isDerivableChannelPath(language string, lib *config.Library, ch *config.Channel) bool {
+	return ch.Path == deriveChannelPath(language, lib)
+}
+
+func isDerivableServiceConfig(language string, lib *config.Library, ch *config.Channel) bool {
+	path := ch.Path
+	if path == "" {
+		path = deriveChannelPath(language, lib)
+	}
+	return ch.ServiceConfig != "" && ch.ServiceConfig == deriveServiceConfig(path)
+}
+
 func validateLibraries(cfg *config.Config) error {
 	var (
 		errs      []error