fix(internal/librarian/java): Improve error handling in generate.go and add back test case (#4707)

Improves error handling in generate.go, use sentinel error for checks in
tests. Add back temporary removed test case, and removes a redundant
test.

Fix #4517
Fix #4436

Author: zhumin8

internal/librarian/java/generate.go

@@ -17,6 +17,7 @@ package java
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"os/exec"
@@ -36,6 +37,13 @@ const (
 	commonProtos = "google/cloud/common_resources.proto"
 )
 
+var (
+	// errExtractVersion is returned when an API version cannot be extracted from its path.
+	errExtractVersion = errors.New("failed to extract version")
+	// errNoProtos is returned when no proto files are found in an API directory.
+	errNoProtos = errors.New("no protos found")
+)
+
 // Generate generates a Java client library.
 func Generate(ctx context.Context, cfg *config.Config, library *config.Library, srcs *sources.Sources) error {
 	outdir, err := filepath.Abs(library.Output)
@@ -49,7 +57,7 @@ func Generate(ctx context.Context, cfg *config.Config, library *config.Library,
 		return fmt.Errorf("failed to resolve googleapis directory path: %w", err)
 	}
 	if err := os.MkdirAll(outdir, 0755); err != nil {
-		return fmt.Errorf("failed to create output directory: %w", err)
+		return fmt.Errorf("failed to create output directory %q: %w", outdir, err)
 	}
 	for _, api := range library.APIs {
 		if err := generateAPI(ctx, cfg, api, library, googleapisDir, outdir); err != nil {
@@ -62,7 +70,7 @@ func Generate(ctx context.Context, cfg *config.Config, library *config.Library,
 func generateAPI(ctx context.Context, cfg *config.Config, api *config.API, library *config.Library, googleapisDir, outdir string) error {
 	version := serviceconfig.ExtractVersion(api.Path)
 	if version == "" {
-		return fmt.Errorf("failed to generate api: failed to extract version from api path %q", api.Path)
+		return fmt.Errorf("%s: %w", api.Path, errExtractVersion)
 	}
 	javaAPI := resolveJavaAPI(library, api)
 	p := postProcessParams{
@@ -77,7 +85,7 @@ func generateAPI(ctx context.Context, cfg *config.Config, api *config.API, libra
 	}
 	for _, dir := range []string{p.gapicDir, p.grpcDir, p.protoDir} {
 		if err := os.MkdirAll(dir, 0755); err != nil {
-			return fmt.Errorf("failed to create directory %s: %w", dir, err)
+			return fmt.Errorf("failed to create directory %q: %w", dir, err)
 		}
 	}
 
@@ -91,7 +99,7 @@ func generateAPI(ctx context.Context, cfg *config.Config, api *config.API, libra
 		return fmt.Errorf("failed to find protos: %w", err)
 	}
 	if len(apiProtos) == 0 {
-		return fmt.Errorf("failed to generate api: no protos found in api %q", api.Path)
+		return fmt.Errorf("%s: %w", api.Path, errNoProtos)
 	}
 	p.apiProtos = apiProtos
 
@@ -246,7 +254,7 @@ func Format(ctx context.Context, library *config.Library) error {
 
 	args := append([]string{"--replace"}, files...)
 	if err := command.Run(ctx, "google-java-format", args...); err != nil {
-		return fmt.Errorf("formatting failed: %w", err)
+		return fmt.Errorf("failed to format files: %w", err)
 	}
 	return nil
 }

internal/librarian/java/generate_test.go

@@ -16,9 +16,11 @@ package java
 
 import (
 	"context"
+	"errors"
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 
 	"sort"
 
@@ -409,7 +411,7 @@ func TestGenerateLibrary_Error(t *testing.T) {
 		name    string
 		library *config.Library
 		setup   func(t *testing.T, library *config.Library)
-		wantErr string
+		wantErr error
 	}{
 		{
 			name: "invalid version",
@@ -420,7 +422,18 @@ func TestGenerateLibrary_Error(t *testing.T) {
 					{Path: "google/cloud/secretmanager"}, // Missing version
 				},
 			},
-			wantErr: "failed to extract version from api path",
+			wantErr: errExtractVersion,
+		},
+		{
+			name: "no protos found",
+			library: &config.Library{
+				Name:   "test",
+				Output: t.TempDir(),
+				APIs: []*config.API{
+					{Path: "google/cloud/nonexistent/v1"},
+				},
+			},
+			wantErr: errNoProtos,
 		},
 		{
 			name: "mkdir failure for output dir",
@@ -437,7 +450,7 @@ func TestGenerateLibrary_Error(t *testing.T) {
 					t.Fatal(err)
 				}
 			},
-			wantErr: "failed to create output directory",
+			wantErr: syscall.ENOTDIR,
 		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
@@ -446,7 +459,7 @@ func TestGenerateLibrary_Error(t *testing.T) {
 			}
 			cfg := &config.Config{Language: "java"}
 			err := Generate(t.Context(), cfg, test.library, &sources.Sources{Googleapis: googleapisDir})
-			if err == nil || !strings.Contains(err.Error(), test.wantErr) {
+			if !errors.Is(err, test.wantErr) {
 				t.Errorf("generate() error = %v, wantErr %v", err, test.wantErr)
 			}
 		})