xds: Handle empty target URI authorities the same as null.

Fixes an oversight in grpc#12660. io.grpc.Uri#getAuthority makes a
distinction between xds:///service and xds:/service but java.net.URI
does not. XdsNameResolver wants to treat those two cases the same.

Author: jdcormie

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

@@ -138,6 +138,13 @@ final class XdsNameResolver extends NameResolver {
   private CallCounterProvider callCounterProvider;
   private ResolveState resolveState;
 
+  /**
+   * Constructs a new instance.
+   *
+   * @param target the target URI to resolve
+   * @param targetAuthority the authority component of `target`, possibly the empty string, or null
+   *     if 'target' has no such component
+   */
   XdsNameResolver(
       String target, @Nullable String targetAuthority, String name,
       @Nullable String overrideAuthority, ServiceConfigParser serviceConfigParser,
@@ -208,7 +215,9 @@ public void start(Listener2 listener) {
     }
     BootstrapInfo bootstrapInfo = xdsClient.getBootstrapInfo();
     String listenerNameTemplate;
-    if (targetAuthority == null) {
+    if (targetAuthority == null || targetAuthority.isEmpty()) {
+      // Both https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md and
+      // A47-xds-federation.md seem to treat an empty authority the same as an undefined one.
       listenerNameTemplate = bootstrapInfo.clientDefaultListenerResourceNameTemplate();
     } else {
       AuthorityInfo authorityInfo = bootstrapInfo.authorities().get(targetAuthority);

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

@@ -87,7 +87,11 @@ public XdsNameResolver newNameResolver(URI targetUri, Args args) {
           targetPath,
           targetUri);
       String name = targetPath.substring(1);
-      return newNameResolver(targetUri.toString(), targetUri.getAuthority(), name, args);
+      // TODO(jdcormie): java.net.URI#getAuthority incorrectly returns null for both xds:///service
+      //  and xds:/service. This doesn't matter for now since XdsNameResolver treats them the same
+      //  anyway and all this code will go away once newNameResolver(io.grpc.Uri) launches.
+      String targetAuthority = targetUri.getAuthority();
+      return newNameResolver(targetUri.toString(), targetAuthority, name, args);
     }
     return null;
   }

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

@@ -297,6 +297,40 @@ public void resolving_noTargetAuthority_templateWithoutXdstp() {
     verify(mockListener, never()).onError(any(Status.class));
   }
 
+  @Test
+  public void resolving_emptyTargetAuthority_templateWithXdstp() {
+    bootstrapInfo =
+        BootstrapInfo.builder()
+            .servers(
+                ImmutableList.of(
+                    ServerInfo.create("td.googleapis.com", InsecureChannelCredentials.create())))
+            .node(Node.newBuilder().build())
+            .clientDefaultListenerResourceNameTemplate(
+                "xdstp://xds.authority.com/envoy.config.listener.v3.Listener/%s?id=1")
+            .build();
+    String serviceAuthority = "[::FFFF:129.144.52.38]:80";
+    expectedLdsResourceName =
+        "xdstp://xds.authority.com/envoy.config.listener.v3.Listener/"
+            + "%5B::FFFF:129.144.52.38%5D:80?id=1";
+    resolver =
+        new XdsNameResolver(
+            "xds:///foo.googleapis.com",
+            "",
+            serviceAuthority,
+            null,
+            serviceConfigParser,
+            syncContext,
+            scheduler,
+            xdsClientPoolFactory,
+            mockRandom,
+            FilterRegistry.getDefaultRegistry(),
+            rawBootstrap,
+            metricRecorder,
+            nameResolverArgs);
+    resolver.start(mockListener);
+    verify(mockListener, never()).onError(any(Status.class));
+  }
+
   @Test
   public void resolving_noTargetAuthority_templateWithXdstp() {
     bootstrapInfo = BootstrapInfo.builder()