Address review feedback: move e2e tests to external test package

- Move e2e tests to token_file_call_creds_ext_test.go with package jwt_test
- Split into 4 focused tests instead of one complex table-driven test
- Copy helper functions (createTestJWT, writeTempTokenFile) to new file
- Rename expectedAuth to wantAuth per Go style guide

Author: rajiv-maersk

credentials/jwt/token_file_call_creds_ext_test.go

@@ -0,0 +1,271 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package jwt_test
+
+import (
+	"context"
+	"crypto/tls"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/credentials/jwt"
+	"google.golang.org/grpc/internal/stubserver"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/testdata"
+
+	testgrpc "google.golang.org/grpc/interop/grpc_testing"
+	testpb "google.golang.org/grpc/interop/grpc_testing"
+)
+
+const defaultTestTimeout = 5 * time.Second
+
+// TestJWTCallCredentials_InsecureTransport_AsCallOption verifies that when JWT
+// call credentials are passed as a per-RPC call option over an insecure
+// transport, the RPC fails with a meaningful error.
+func TestJWTCallCredentials_InsecureTransport_AsCallOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	ss := &stubserver.StubServer{}
+	if err := ss.StartServer(grpc.Creds(insecure.NewCredentials())); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	cc, err := grpc.NewClient(ss.Address, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		t.Fatalf("grpc.NewClient(%q) failed: %v", ss.Address, err)
+	}
+	defer cc.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	client := testgrpc.NewTestServiceClient(cc)
+	_, err = client.EmptyCall(ctx, &testpb.Empty{}, grpc.PerRPCCredentials(jwtCreds))
+
+	if err == nil || !strings.Contains(err.Error(), "cannot send secure credentials on an insecure connection") {
+		t.Fatalf("EmptyCall() error = %v; want error containing %q", err, "cannot send secure credentials on an insecure connection")
+	}
+}
+
+// TestJWTCallCredentials_InsecureTransport_AsDialOption verifies that when JWT
+// call credentials are passed as a dial option over an insecure transport, the
+// client creation fails with a meaningful error.
+func TestJWTCallCredentials_InsecureTransport_AsDialOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	ss := &stubserver.StubServer{}
+	if err := ss.StartServer(grpc.Creds(insecure.NewCredentials())); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	_, err = grpc.NewClient(ss.Address,
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithPerRPCCredentials(jwtCreds),
+	)
+
+	if err == nil || !strings.Contains(err.Error(), "the credentials require transport level security") {
+		t.Fatalf("grpc.NewClient() error = %v; want error containing %q", err, "the credentials require transport level security")
+	}
+}
+
+// TestJWTCallCredentials_SecureTransport_AsDialOption verifies that JWT call
+// credentials work correctly when passed as a dial option over a secure TLS
+// transport.
+func TestJWTCallCredentials_SecureTransport_AsDialOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	wantAuth := "Bearer " + token
+	ss := &stubserver.StubServer{
+		EmptyCallF: func(ctx context.Context, _ *testpb.Empty) (*testpb.Empty, error) {
+			md, ok := metadata.FromIncomingContext(ctx)
+			if !ok {
+				return nil, fmt.Errorf("no metadata received")
+			}
+			authHeaders := md.Get("authorization")
+			if len(authHeaders) != 1 || authHeaders[0] != wantAuth {
+				return nil, fmt.Errorf("authorization header mismatch: got %v, want %q", authHeaders, wantAuth)
+			}
+			return &testpb.Empty{}, nil
+		},
+	}
+
+	serverCert, err := tls.LoadX509KeyPair(testdata.Path("x509/server1_cert.pem"), testdata.Path("x509/server1_key.pem"))
+	if err != nil {
+		t.Fatalf("Failed to load server cert: %v", err)
+	}
+	if err := ss.StartServer(grpc.Creds(credentials.NewServerTLSFromCert(&serverCert))); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	clientCreds, err := credentials.NewClientTLSFromFile(testdata.Path("x509/server_ca_cert.pem"), "x.test.example.com")
+	if err != nil {
+		t.Fatalf("Failed to create client TLS credentials: %v", err)
+	}
+
+	cc, err := grpc.NewClient(ss.Address,
+		grpc.WithTransportCredentials(clientCreds),
+		grpc.WithPerRPCCredentials(jwtCreds),
+	)
+	if err != nil {
+		t.Fatalf("grpc.NewClient(%q) failed: %v", ss.Address, err)
+	}
+	defer cc.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	client := testgrpc.NewTestServiceClient(cc)
+	if _, err := client.EmptyCall(ctx, &testpb.Empty{}); err != nil {
+		t.Fatalf("EmptyCall() failed: %v", err)
+	}
+}
+
+// TestJWTCallCredentials_SecureTransport_AsCallOption verifies that JWT call
+// credentials work correctly when passed as a per-RPC call option over a secure
+// TLS transport.
+func TestJWTCallCredentials_SecureTransport_AsCallOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	wantAuth := "Bearer " + token
+	ss := &stubserver.StubServer{
+		EmptyCallF: func(ctx context.Context, _ *testpb.Empty) (*testpb.Empty, error) {
+			md, ok := metadata.FromIncomingContext(ctx)
+			if !ok {
+				return nil, fmt.Errorf("no metadata received")
+			}
+			authHeaders := md.Get("authorization")
+			if len(authHeaders) != 1 || authHeaders[0] != wantAuth {
+				return nil, fmt.Errorf("authorization header mismatch: got %v, want %q", authHeaders, wantAuth)
+			}
+			return &testpb.Empty{}, nil
+		},
+	}
+
+	serverCert, err := tls.LoadX509KeyPair(testdata.Path("x509/server1_cert.pem"), testdata.Path("x509/server1_key.pem"))
+	if err != nil {
+		t.Fatalf("Failed to load server cert: %v", err)
+	}
+	if err := ss.StartServer(grpc.Creds(credentials.NewServerTLSFromCert(&serverCert))); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	clientCreds, err := credentials.NewClientTLSFromFile(testdata.Path("x509/server_ca_cert.pem"), "x.test.example.com")
+	if err != nil {
+		t.Fatalf("Failed to create client TLS credentials: %v", err)
+	}
+
+	cc, err := grpc.NewClient(ss.Address, grpc.WithTransportCredentials(clientCreds))
+	if err != nil {
+		t.Fatalf("grpc.NewClient(%q) failed: %v", ss.Address, err)
+	}
+	defer cc.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	client := testgrpc.NewTestServiceClient(cc)
+	if _, err := client.EmptyCall(ctx, &testpb.Empty{}, grpc.PerRPCCredentials(jwtCreds)); err != nil {
+		t.Fatalf("EmptyCall() failed: %v", err)
+	}
+}
+
+// createTestJWT creates a test JWT token with the specified expiration.
+func createTestJWT(t *testing.T, expiration time.Time) string {
+	t.Helper()
+
+	claims := map[string]any{}
+	if !expiration.IsZero() {
+		claims["exp"] = expiration.Unix()
+	}
+
+	header := map[string]any{
+		"typ": "JWT",
+		"alg": "HS256",
+	}
+	headerBytes, err := json.Marshal(header)
+	if err != nil {
+		t.Fatalf("Failed to marshal header: %v", err)
+	}
+
+	claimsBytes, err := json.Marshal(claims)
+	if err != nil {
+		t.Fatalf("Failed to marshal claims: %v", err)
+	}
+
+	headerB64 := base64.URLEncoding.EncodeToString(headerBytes)
+	claimsB64 := base64.URLEncoding.EncodeToString(claimsBytes)
+
+	// Remove padding for URL-safe base64.
+	headerB64 = strings.TrimRight(headerB64, "=")
+	claimsB64 = strings.TrimRight(claimsB64, "=")
+
+	// For testing, we use a fake signature.
+	signature := base64.URLEncoding.EncodeToString([]byte("fake_signature"))
+	signature = strings.TrimRight(signature, "=")
+
+	return fmt.Sprintf("%s.%s.%s", headerB64, claimsB64, signature)
+}
+
+// writeTempTokenFile writes the token to a temporary file and returns the path.
+func writeTempTokenFile(t *testing.T, token string) string {
+	t.Helper()
+	tempDir := t.TempDir()
+	filePath := filepath.Join(tempDir, "token")
+	if err := os.WriteFile(filePath, []byte(token), 0600); err != nil {
+		t.Fatalf("Failed to write temp token file: %v", err)
+	}
+	return filePath
+}