Fix NameResolvers calling Listener2.onResult2 outside of the synchronization context to call it from inside of the synchronization context.

Fixes grpc#11662.

Author: kannanjgithub

api/src/main/java/io/grpc/NameResolver.java

@@ -219,13 +219,15 @@ public abstract static class Listener2 implements Listener {
     @Override
     @Deprecated
     @InlineMe(
-        replacement = "this.onResult2(ResolutionResult.newBuilder().setAddressesOrError("
+        replacement = "this.onResult(ResolutionResult.newBuilder().setAddressesOrError("
             + "StatusOr.fromValue(servers)).setAttributes(attributes).build())",
         imports = {"io.grpc.NameResolver.ResolutionResult", "io.grpc.StatusOr"})
     public final void onAddresses(
         List<EquivalentAddressGroup> servers, @ResolutionResultAttr Attributes attributes) {
       // TODO(jihuncho) need to promote Listener2 if we want to use ConfigOrError
-      onResult2(
+      // Calling onResult and not onResult2 because onResult2 can only be called from a
+      // synchronization context.
+      onResult(
           ResolutionResult.newBuilder().setAddressesOrError(
               StatusOr.fromValue(servers)).setAttributes(attributes).build());
     }

core/src/main/java/io/grpc/internal/ManagedChannelImplBuilder.java

@@ -878,13 +878,14 @@ public String getServiceAuthority() {
 
         @Override
         public void start(Listener2 listener) {
-          listener.onResult2(
-              ResolutionResult.newBuilder()
-                  .setAddressesOrError(
-                      StatusOr.fromValue(
-                          Collections.singletonList(new EquivalentAddressGroup(address))))
-                  .setAttributes(Attributes.EMPTY)
-                  .build());
+          args.getSynchronizationContext().execute(() ->
+              listener.onResult2(
+                  ResolutionResult.newBuilder()
+                      .setAddressesOrError(
+                          StatusOr.fromValue(
+                              Collections.singletonList(new EquivalentAddressGroup(address))))
+                      .setAttributes(Attributes.EMPTY)
+                      .build()));
         }
 
         @Override

netty/src/main/java/io/grpc/netty/UdsNameResolver.java

@@ -30,10 +30,12 @@
 final class UdsNameResolver extends NameResolver {
   private NameResolver.Listener2 listener;
   private final String authority;
+  private final Args args;
 
   UdsNameResolver(String authority, String targetPath, Args args) {
     checkArgument(authority == null, "non-null authority not supported");
     this.authority = targetPath;
+    this.args = args;
   }
 
   @Override
@@ -58,7 +60,8 @@ private void resolve() {
     List<EquivalentAddressGroup> servers = new ArrayList<>(1);
     servers.add(new EquivalentAddressGroup(new DomainSocketAddress(authority)));
     resolutionResultBuilder.setAddressesOrError(StatusOr.fromValue(servers));
-    listener.onResult2(resolutionResultBuilder.build());
+    args.getSynchronizationContext().execute(() ->
+        listener.onResult2(resolutionResultBuilder.build()));
   }
 
   @Override