fix(auth): restore transportFactory upon deserialization in InternalAwsSecurityCredentialsSupplier

Fixes #14050
Fixes #1538

In InternalAwsSecurityCredentialsSupplier, transportFactory is marked
transient but no transportFactoryClassName was retained and no readObject
was defined. Consequently, after Java deserialization (common in distributed
frameworks like Apache Flink and Apache Spark), transportFactory is null.
When deserialized AwsCredentials attempts to refresh its token, a
NullPointerException is thrown when retrieving AWS security credentials
or region from the metadata service.

This change:
1. Records transportFactoryClassName during construction.
2. Implements readObject to restore transportFactory using
   OAuth2Credentials.newInstance(transportFactoryClassName), with fallback
   to OAuth2Utils.HTTP_TRANSPORT_FACTORY for backwards compatibility with
   previously serialized streams.
3. Adds reproduction and serialization unit tests verifying credentials
   and region retrieval after deserialization, including token refresh in
   AwsCredentials.

Author: lqiu96

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/InternalAwsSecurityCredentialsSupplier.java

@@ -44,6 +44,7 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.ImmutableList;
 import java.io.IOException;
+import java.io.ObjectInputStream;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -73,6 +74,7 @@ class InternalAwsSecurityCredentialsSupplier implements AwsSecurityCredentialsSu
   private final AwsCredentialSource awsCredentialSource;
   private EnvironmentProvider environmentProvider;
   private transient HttpTransportFactory transportFactory;
+  private final @Nullable String transportFactoryClassName;
 
   /**
    * Constructor for InternalAwsSecurityCredentialsProvider
@@ -83,11 +85,35 @@ class InternalAwsSecurityCredentialsSupplier implements AwsSecurityCredentialsSu
    */
   InternalAwsSecurityCredentialsSupplier(
       AwsCredentialSource awsCredentialSource,
-      EnvironmentProvider environmentProvider,
-      HttpTransportFactory transportFactory) {
-    this.environmentProvider = environmentProvider;
+      @Nullable EnvironmentProvider environmentProvider,
+      @Nullable HttpTransportFactory transportFactory) {
+    this.environmentProvider =
+        environmentProvider == null ? SystemEnvironmentProvider.getInstance() : environmentProvider;
     this.awsCredentialSource = awsCredentialSource;
-    this.transportFactory = transportFactory;
+    this.transportFactory =
+        transportFactory != null ? transportFactory : OAuth2Utils.HTTP_TRANSPORT_FACTORY;
+    // If a custom transport factory is provided, record its class name to restore it upon
+    // deserialization. When using the default transport factory, keep this null to avoid
+    // serializing internal implementation class names across versions.
+    this.transportFactoryClassName =
+        transportFactory != null ? transportFactory.getClass().getName() : null;
+  }
+
+  @SuppressWarnings("unused")
+  private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
+    input.defaultReadObject();
+    if (transportFactoryClassName != null) {
+      transportFactory = OAuth2Credentials.newInstance(transportFactoryClassName);
+    } else {
+      // Fall back to default transport factory if null or previously serialized without a class
+      // name.
+      transportFactory = OAuth2Utils.HTTP_TRANSPORT_FACTORY;
+    }
+  }
+
+  @VisibleForTesting
+  HttpTransportFactory getTransportFactory() {
+    return transportFactory;
   }
 
   @Override

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/AwsCredentialsTest.java

@@ -1248,6 +1248,36 @@ void serialize() throws IOException, ClassNotFoundException {
     assertSame(Clock.SYSTEM, deserializedCredentials.clock);
   }
 
+  /**
+   * Verifies that {@link AwsCredentials} can successfully refresh access tokens after being
+   * serialized and deserialized.
+   */
+  @Test
+  void serialize_refreshAccessToken_success() throws IOException, ClassNotFoundException {
+    // Uses an in-memory MockHttpTransport (no network calls) that returns canned HTTP responses
+    // for both AWS IMDS metadata endpoints and the GCP STS token exchange endpoint.
+    MockExternalAccountCredentialsTransportFactory transportFactory =
+        new MockExternalAccountCredentialsTransportFactory();
+
+    // Use an IMDS credential source so that token refresh is forced to retrieve AWS credentials
+    // and region from the metadata server via HTTP, exercising the supplier's transportFactory.
+    AwsCredentials awsCredential =
+        AwsCredentials.newBuilder(AWS_CREDENTIAL)
+            .setTokenUrl(transportFactory.transport.getStsUrl())
+            .setHttpTransportFactory(transportFactory)
+            .setCredentialSource(buildAwsCredentialSource(transportFactory))
+            .build();
+
+    AwsCredentials deserialized = serializeAndDeserialize(awsCredential);
+
+    // refreshAccessToken() calls getCredentials(), getRegion(), and the STS token endpoint,
+    // verifying that the restored transportFactory is used for all HTTP requests.
+    AccessToken accessToken = deserialized.refreshAccessToken();
+
+    // Verifies the access token returned from the simulated STS exchange matches the mock.
+    assertEquals(transportFactory.transport.getAccessToken(), accessToken.getTokenValue());
+  }
+
   private static void ValidateRequest(
       MockLowLevelHttpRequest request, String expectedUrl, Map<String, String> expectedHeaders) {
     assertEquals(expectedUrl, request.getUrl());

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/InternalAwsSecurityCredentialsSupplierTest.java

@@ -32,7 +32,10 @@
 package com.google.auth.oauth2;
 
 import static com.google.auth.oauth2.AwsCredentialsTest.buildAwsImdsv2CredentialSource;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import com.google.auth.oauth2.ExternalAccountCredentialsTest.MockExternalAccountCredentialsTransportFactory;
@@ -41,7 +44,7 @@
 import org.junit.jupiter.api.Test;
 
 /** Tests for {@link InternalAwsSecurityCredentialsSupplier}. */
-class InternalAwsSecurityCredentialsSupplierTest {
+class InternalAwsSecurityCredentialsSupplierTest extends BaseSerializationTest {
   @Test
   void shouldUseMetadataServer_withRequiredEnvironmentVariables() {
     MockExternalAccountCredentialsTransportFactory transportFactory =
@@ -159,4 +162,79 @@ void shouldUseMetadataServer_noEnvironmentVars() {
             transportFactory);
     assertTrue(supplier.shouldUseMetadataServer());
   }
+
+  /**
+   * Verifies that {@link InternalAwsSecurityCredentialsSupplier} restores its {@code
+   * transportFactory} upon deserialization, enabling successful retrieval of AWS security
+   * credentials and region from the AWS EC2 metadata server.
+   */
+  @Test
+  void serializeAndDeserialize_retrievesCredentialsAndRegionSuccessfully() throws Exception {
+    MockExternalAccountCredentialsTransportFactory transportFactory =
+        new MockExternalAccountCredentialsTransportFactory();
+    InternalAwsSecurityCredentialsSupplier supplier =
+        new InternalAwsSecurityCredentialsSupplier(
+            buildAwsImdsv2CredentialSource(transportFactory),
+            // Pass null to use the default SystemEnvironmentProvider, which implements Serializable
+            // (unlike TestEnvironmentProvider).
+            /* environmentProvider= */ null,
+            transportFactory);
+
+    InternalAwsSecurityCredentialsSupplier deserialized = serializeAndDeserialize(supplier);
+    assertEquals(
+        MockExternalAccountCredentialsTransportFactory.class,
+        deserialized.getTransportFactory().getClass());
+
+    // Credentials and region are not serialized fields; they are retrieved on demand via HTTP.
+    // Calling getCredentials() and getRegion() verifies that the restored transportFactory
+    // successfully constructs and executes HTTP requests against the mock metadata server
+    // (rather than failing with a NullPointerException).
+    AwsSecurityCredentials credentials = deserialized.getCredentials(null);
+    assertNotNull(credentials);
+    assertEquals("accessKeyId", credentials.getAccessKeyId());
+    assertEquals("secretAccessKey", credentials.getSecretAccessKey());
+    assertEquals("token", credentials.getSessionToken());
+
+    String region = deserialized.getRegion(null);
+    assertEquals("us-east-1", region);
+  }
+
+  /**
+   * Verifies that {@link InternalAwsSecurityCredentialsSupplier} deserializes cleanly and falls
+   * back to the default {@link OAuth2Utils#HTTP_TRANSPORT_FACTORY} when no custom transport factory
+   * was provided.
+   */
+  @Test
+  void serializeAndDeserialize_defaultTransportFactory_success() throws Exception {
+    MockExternalAccountCredentialsTransportFactory transportFactory =
+        new MockExternalAccountCredentialsTransportFactory();
+    InternalAwsSecurityCredentialsSupplier supplier =
+        new InternalAwsSecurityCredentialsSupplier(
+            buildAwsImdsv2CredentialSource(transportFactory),
+            /* environmentProvider= */ null,
+            /* transportFactory= */ null);
+
+    InternalAwsSecurityCredentialsSupplier deserialized = serializeAndDeserialize(supplier);
+    assertNotNull(deserialized);
+    assertSame(OAuth2Utils.HTTP_TRANSPORT_FACTORY, deserialized.getTransportFactory());
+  }
+
+  /**
+   * Verifies that {@link InternalAwsSecurityCredentialsSupplier} can be serialized and deserialized
+   * when an explicit {@link EnvironmentProvider} is provided.
+   */
+  @Test
+  void serializeAndDeserialize_withEnvironmentVariables_success() throws Exception {
+    MockExternalAccountCredentialsTransportFactory transportFactory =
+        new MockExternalAccountCredentialsTransportFactory();
+    SystemEnvironmentProvider environmentProvider = SystemEnvironmentProvider.getInstance();
+    InternalAwsSecurityCredentialsSupplier supplier =
+        new InternalAwsSecurityCredentialsSupplier(
+            buildAwsImdsv2CredentialSource(transportFactory),
+            environmentProvider,
+            transportFactory);
+
+    InternalAwsSecurityCredentialsSupplier deserialized = serializeAndDeserialize(supplier);
+    assertNotNull(deserialized);
+  }
 }