xds: add env var protection for google-c2p resolver (#7918)

xds/src/main/java/io/grpc/xds/GoogleCloudToProdNameResolverProvider.java

@@ -29,6 +29,8 @@ import java.net.URI;
 @Internal
 public final class GoogleCloudToProdNameResolverProvider extends NameResolverProvider {
 
+  private static final boolean enableC2PResolver =
+      Boolean.parseBoolean(System.getenv("GRPC_EXPERIMENTAL_GOOGLE_C2P_RESOLVER"));
   private static final String SCHEME = "google-c2p";
 
   @Override
@@ -48,7 +50,10 @@ public final class GoogleCloudToProdNameResolverProvider extends NameResolverPro
 
   @Override
   protected boolean isAvailable() {
-    return true;
+    if (enableC2PResolver) {
+      return true;
+    }
+    return false;
   }
 
   @Override

xds/src/test/java/io/grpc/xds/GoogleCloudToProdNameResolverProviderTest.java

@@ -17,12 +17,14 @@
 package io.grpc.xds;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.fail;
 import static org.mockito.Mockito.mock;
 
 import io.grpc.ChannelLogger;
+import io.grpc.InternalServiceProviders;
 import io.grpc.NameResolver;
 import io.grpc.NameResolver.ServiceConfigParser;
-import io.grpc.NameResolverRegistry;
+import io.grpc.NameResolverProvider;
 import io.grpc.SynchronizationContext;
 import io.grpc.internal.FakeClock;
 import io.grpc.internal.GrpcUtil;
@@ -54,12 +56,24 @@ public class GoogleCloudToProdNameResolverProviderTest {
       .setChannelLogger(mock(ChannelLogger.class))
       .build();
 
-  private final NameResolverRegistry nsRegistry = NameResolverRegistry.getDefaultRegistry();
+  private GoogleCloudToProdNameResolverProvider provider =
+      new GoogleCloudToProdNameResolverProvider();
 
   @Test
   public void provided() {
-    NameResolver resolver = nsRegistry.asFactory().newNameResolver(
-        URI.create("google-c2p:///foo.googleapis.com"), args);
-    assertThat(resolver).isInstanceOf(GoogleCloudToProdNameResolver.class);
+    for (NameResolverProvider current
+        : InternalServiceProviders.getCandidatesViaServiceLoader(
+        NameResolverProvider.class, getClass().getClassLoader())) {
+      if (current instanceof GoogleCloudToProdNameResolverProvider) {
+        return;
+      }
+    }
+    fail("GoogleCloudToProdNameResolverProvider not registered");
   }
-}
+
+  @Test
+  public void newNameResolver() {
+    assertThat(provider.newNameResolver(URI.create("google-c2p:///foo.googleapis.com"), args))
+        .isInstanceOf(GoogleCloudToProdNameResolver.class);
+  }
+}