fix: Create a single S2AChannelCredentials per application

gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

@@ -161,6 +161,23 @@ public final class InstantiatingGrpcChannelProvider implements TransportChannelP
   @Nullable
   private final ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> channelConfigurator;
 
+  // This is initialized once for the lifetime of the application. This enables re-using
+  // channels to S2A.
+  private static volatile ChannelCredentials s2aChannelCredentials;
+
+  /**
+   * Resets the s2aChannelCredentials of the {@link InstantiatingGrpcChannelProvider} class for
+   * testing purposes.
+   *
+   * <p>This should only be called from tests.
+   */
+  @VisibleForTesting
+  static void resetS2AChannelCredentials() {
+    synchronized (InstantiatingGrpcChannelProvider.class) {
+      s2aChannelCredentials = null;
+    }
+  }
+
   /*
    * Experimental feature
    *
@@ -595,43 +612,60 @@ ChannelCredentials createPlaintextToS2AChannelCredentials(String plaintextAddres
    * @return {@link ChannelCredentials} configured to use S2A to create mTLS connection.
    */
   ChannelCredentials createS2ASecuredChannelCredentials() {
-    SecureSessionAgentConfig config = s2aConfigProvider.getConfig();
-    String plaintextAddress = config.getPlaintextAddress();
-    String mtlsAddress = config.getMtlsAddress();
-    if (Strings.isNullOrEmpty(mtlsAddress)) {
-      // Fallback to plaintext connection to S2A.
-      LOG.log(
-          Level.INFO,
-          "Cannot establish an mTLS connection to S2A because autoconfig endpoint did not return a mtls address to reach S2A.");
-      return createPlaintextToS2AChannelCredentials(plaintextAddress);
-    }
-    // Currently, MTLS to MDS is only available on GCE. See:
-    // https://cloud.google.com/compute/docs/metadata/overview#https-mds
-    // Try to load MTLS-MDS creds.
-    File rootFile = new File(MTLS_MDS_ROOT_PATH);
-    File certKeyFile = new File(MTLS_MDS_CERT_CHAIN_AND_KEY_PATH);
-    if (rootFile.isFile() && certKeyFile.isFile()) {
-      // Try to connect to S2A using mTLS.
-      ChannelCredentials mtlsToS2AChannelCredentials = null;
-      try {
-        mtlsToS2AChannelCredentials =
-            createMtlsToS2AChannelCredentials(rootFile, certKeyFile, certKeyFile);
-      } catch (IOException ignore) {
-        // Fallback to plaintext-to-S2A connection on error.
-        LOG.log(
-            Level.WARNING,
-            "Cannot establish an mTLS connection to S2A due to error creating MTLS to MDS TlsChannelCredentials credentials, falling back to plaintext connection to S2A: "
-                + ignore.getMessage());
-        return createPlaintextToS2AChannelCredentials(plaintextAddress);
+    if (s2aChannelCredentials == null) {
+      // s2aChannelCredentials is initialized once and shared by all instances of the class.
+      // To prevent a race on initialization, the object initialization is synchronized on the class
+      // object.
+      synchronized (InstantiatingGrpcChannelProvider.class) {
+        if (s2aChannelCredentials != null) {
+          return s2aChannelCredentials;
+        }
+        SecureSessionAgentConfig config = s2aConfigProvider.getConfig();
+        String plaintextAddress = config.getPlaintextAddress();
+        String mtlsAddress = config.getMtlsAddress();
+        if (Strings.isNullOrEmpty(mtlsAddress)) {
+          // Fallback to plaintext connection to S2A.
+          LOG.log(
+              Level.INFO,
+              "Cannot establish an mTLS connection to S2A because autoconfig endpoint did not return a mtls address to reach S2A.");
+          s2aChannelCredentials = createPlaintextToS2AChannelCredentials(plaintextAddress);
+          return s2aChannelCredentials;
+        }
+        // Currently, MTLS to MDS is only available on GCE. See:
+        // https://cloud.google.com/compute/docs/metadata/overview#https-mds
+        // Try to load MTLS-MDS creds.
+        File rootFile = new File(MTLS_MDS_ROOT_PATH);
+        File certKeyFile = new File(MTLS_MDS_CERT_CHAIN_AND_KEY_PATH);
+        if (rootFile.isFile() && certKeyFile.isFile()) {
+          // Try to connect to S2A using mTLS.
+          ChannelCredentials mtlsToS2AChannelCredentials = null;
+          try {
+            mtlsToS2AChannelCredentials =
+                createMtlsToS2AChannelCredentials(rootFile, certKeyFile, certKeyFile);
+          } catch (IOException ignore) {
+            // Fallback to plaintext-to-S2A connection on error.
+            LOG.log(
+                Level.WARNING,
+                "Cannot establish an mTLS connection to S2A due to error creating MTLS to MDS TlsChannelCredentials credentials, falling back to plaintext connection to S2A: "
+                    + ignore.getMessage());
+            s2aChannelCredentials = createPlaintextToS2AChannelCredentials(plaintextAddress);
+            return s2aChannelCredentials;
+          }
+          s2aChannelCredentials =
+              buildS2AChannelCredentials(mtlsAddress, mtlsToS2AChannelCredentials);
+          return s2aChannelCredentials;
+        } else {
+          // Fallback to plaintext-to-S2A connection if MTLS-MDS creds do not exist.
+          LOG.log(
+              Level.INFO,
+              "Cannot establish an mTLS connection to S2A because MTLS to MDS credentials do not"
+                  + " exist on filesystem, falling back to plaintext connection to S2A");
+          s2aChannelCredentials = createPlaintextToS2AChannelCredentials(plaintextAddress);
+          return s2aChannelCredentials;
+        }
       }
-      return buildS2AChannelCredentials(mtlsAddress, mtlsToS2AChannelCredentials);
-    } else {
-      // Fallback to plaintext-to-S2A connection if MTLS-MDS creds do not exist.
-      LOG.log(
-          Level.INFO,
-          "Cannot establish an mTLS connection to S2A because MTLS to MDS credentials do not exist on filesystem, falling back to plaintext connection to S2A");
-      return createPlaintextToS2AChannelCredentials(plaintextAddress);
     }
+    return s2aChannelCredentials;
   }
 
   private ManagedChannel createSingleChannel() throws IOException {

gax-java/gax-grpc/src/test/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProviderTest.java

@@ -1155,6 +1155,7 @@ void createS2ASecuredChannelCredentials_bothS2AAddressesNull_returnsNull() {
             .setS2AConfigProvider(s2aConfigProvider)
             .build();
     assertThat(provider.createS2ASecuredChannelCredentials()).isNull();
+    InstantiatingGrpcChannelProvider.resetS2AChannelCredentials();
   }
 
   @Test
@@ -1175,6 +1176,28 @@ void createS2ASecuredChannelCredentials_bothS2AAddressesNull_returnsNull() {
         .contains(
             "Cannot establish an mTLS connection to S2A because autoconfig endpoint did not return a mtls address to reach S2A.");
     InstantiatingGrpcChannelProvider.LOG.removeHandler(logHandler);
+    InstantiatingGrpcChannelProvider.resetS2AChannelCredentials();
+  }
+
+  @Test
+  void
+      createTwoS2ASecuredChannelCredentials_mtlsS2AAddressNull_returnsSamePlaintextToS2AS2AChannelCredentials() {
+    SecureSessionAgent s2aConfigProvider = Mockito.mock(SecureSessionAgent.class);
+    SecureSessionAgentConfig config =
+        SecureSessionAgentConfig.createBuilder().setPlaintextAddress("localhost:8080").build();
+    Mockito.when(s2aConfigProvider.getConfig()).thenReturn(config);
+    InstantiatingGrpcChannelProvider provider =
+        InstantiatingGrpcChannelProvider.newBuilder()
+            .setS2AConfigProvider(s2aConfigProvider)
+            .build();
+    assertThat(provider.createS2ASecuredChannelCredentials()).isNotNull();
+    InstantiatingGrpcChannelProvider provider2 =
+        InstantiatingGrpcChannelProvider.newBuilder()
+            .setS2AConfigProvider(s2aConfigProvider)
+            .build();
+    assertThat(provider2.createS2ASecuredChannelCredentials()).isNotNull();
+    assertEquals(provider, provider2);
+    InstantiatingGrpcChannelProvider.resetS2AChannelCredentials();
   }
 
   @Test
@@ -1197,6 +1220,7 @@ void createS2ASecuredChannelCredentials_returnsPlaintextToS2AS2AChannelCredentia
         .contains(
             "Cannot establish an mTLS connection to S2A because MTLS to MDS credentials do not exist on filesystem, falling back to plaintext connection to S2A");
     InstantiatingGrpcChannelProvider.LOG.removeHandler(logHandler);
+    InstantiatingGrpcChannelProvider.resetS2AChannelCredentials();
   }
 
   @Test