refactor(internal/librarian): parallelize rust source fetching (#3528)

The Rust source fetching logic is extracted into a dedicated
fetchRustSources function that fetches all source repositories
concurrently.

Author: julieqiu

go.mod

@@ -23,6 +23,7 @@ require (
 	github.com/yuin/goldmark v1.7.13
 	golang.org/x/exp v0.0.0-20250911091902-df9299821621
 	golang.org/x/mod v0.30.0
+	golang.org/x/sync v0.19.0
 	google.golang.org/genproto v0.0.0-20251103181224-f26f9409b101
 	google.golang.org/genproto/googleapis/api v0.0.0-20251029180050-ab9386a59fda
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20251029180050-ab9386a59fda
@@ -265,7 +266,6 @@ require (
 	golang.org/x/exp/typeparams v0.0.0-20251023183803-a4bb9ffd2546 // indirect
 	golang.org/x/net v0.47.0 // indirect
 	golang.org/x/oauth2 v0.31.0 // indirect
-	golang.org/x/sync v0.18.0 // indirect
 	golang.org/x/sys v0.38.0 // indirect
 	golang.org/x/telemetry v0.0.0-20251111182119-bc8e575c7b54 // indirect
 	golang.org/x/text v0.31.0 // indirect

go.sum

@@ -880,8 +880,8 @@ golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.3.0/go.mod h1:FU7BRWz2tNW+3quACPkgCx/L+uEAv1htQ0V83Z9Rj+Y=
 golang.org/x/sync v0.4.0/go.mod h1:FU7BRWz2tNW+3quACPkgCx/L+uEAv1htQ0V83Z9Rj+Y=
-golang.org/x/sync v0.18.0 h1:kr88TuHDroi+UVf+0hZnirlk8o8T+4MrK6mr60WkH/I=
-golang.org/x/sync v0.18.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
+golang.org/x/sync v0.19.0 h1:vV+1eWNmZ5geRlYjzm2adRgW2/mcpevXNg50YZtPCE4=
+golang.org/x/sync v0.19.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20181116152217-5ac8a444bdc5/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=

internal/librarian/generate.go

@@ -28,6 +28,7 @@ import (
 	"github.com/googleapis/librarian/internal/librarian/rust"
 	"github.com/googleapis/librarian/internal/yaml"
 	"github.com/urfave/cli/v3"
+	"golang.org/x/sync/errgroup"
 )
 
 const (
@@ -191,28 +192,11 @@ func generate(ctx context.Context, language string, library *config.Library, cfg
 			return nil, err
 		}
 	case languageRust:
-		sources := &rust.Sources{
-			Googleapis: googleapisDir,
-		}
-		sources.Discovery, err = fetchSource(ctx, cfgSources.Discovery, discoveryRepo)
-		if err != nil {
-			return nil, err
-		}
-		sources.Conformance, err = fetchSource(ctx, cfgSources.Conformance, protobufRepo)
-		if err != nil {
-			return nil, err
-		}
-		sources.Showcase, err = fetchSource(ctx, cfgSources.Showcase, showcaseRepo)
+		sources, err := fetchRustSources(ctx, cfgSources)
 		if err != nil {
 			return nil, err
 		}
-		if cfgSources.ProtobufSrc != nil {
-			dir, err := fetchSource(ctx, cfgSources.ProtobufSrc, protobufRepo)
-			if err != nil {
-				return nil, err
-			}
-			sources.ProtobufSrc = filepath.Join(dir, cfgSources.ProtobufSrc.Subpath)
-		}
+		sources.Googleapis = googleapisDir
 		keep, err := rust.Keep(library)
 		if err != nil {
 			return nil, fmt.Errorf("library %s: %w", library.Name, err)
@@ -230,6 +214,53 @@ func generate(ctx context.Context, language string, library *config.Library, cfg
 	return library, nil
 }
 
+// fetchRustSources fetches all source repositories needed for Rust generation
+// in parallel. It returns a rust.Sources struct with all directories populated.
+func fetchRustSources(ctx context.Context, cfgSources *config.Sources) (*rust.Sources, error) {
+	sources := &rust.Sources{}
+
+	g, ctx := errgroup.WithContext(ctx)
+	g.Go(func() error {
+		dir, err := fetchSource(ctx, cfgSources.Discovery, discoveryRepo)
+		if err != nil {
+			return err
+		}
+		sources.Discovery = dir
+		return nil
+	})
+	g.Go(func() error {
+		dir, err := fetchSource(ctx, cfgSources.Conformance, protobufRepo)
+		if err != nil {
+			return err
+		}
+		sources.Conformance = dir
+		return nil
+	})
+	g.Go(func() error {
+		dir, err := fetchSource(ctx, cfgSources.Showcase, showcaseRepo)
+		if err != nil {
+			return err
+		}
+		sources.Showcase = dir
+		return nil
+	})
+
+	if cfgSources.ProtobufSrc != nil {
+		g.Go(func() error {
+			dir, err := fetchSource(ctx, cfgSources.ProtobufSrc, protobufRepo)
+			if err != nil {
+				return err
+			}
+			sources.ProtobufSrc = filepath.Join(dir, cfgSources.ProtobufSrc.Subpath)
+			return nil
+		})
+	}
+	if err := g.Wait(); err != nil {
+		return nil, err
+	}
+	return sources, nil
+}
+
 func formatLibrary(ctx context.Context, language string, library *config.Library) error {
 	switch language {
 	case languageFake: