core: Clear ConfigSelector in panic mode

If the failure is before the NameResolver has returned the first time,
RPCs would be queued waiting for service config. We don't want to use
the ConfigSelector, as we are trying to circumvent the NameResolver and
LoadBalancer.

Fixes #9257

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

@@ -887,6 +887,7 @@ public String toString() {
     }
 
     updateSubchannelPicker(new PanicSubchannelPicker());
+    realChannel.updateConfigSelector(null);
     channelLogger.log(ChannelLogLevel.ERROR, "PANIC! Entering TRANSIENT_FAILURE");
     channelStateManager.gotoState(TRANSIENT_FAILURE);
   }
@@ -1755,6 +1756,9 @@ final class NamesResolved implements Runnable {
         @SuppressWarnings("ReferenceEquality")
         @Override
         public void run() {
+          if (ManagedChannelImpl.this.nameResolver != resolver) {
+            return;
+          }
 
           List<EquivalentAddressGroup> servers = resolutionResult.getAddresses();
           channelLogger.log(

core/src/test/java/io/grpc/internal/ManagedChannelImplTest.java

@@ -2799,6 +2799,40 @@ public void run() {
     panicExpected = true;
   }
 
+  @Test
+  public void panic_atStart() {
+    final RuntimeException panicReason = new RuntimeException("Simulated NR exception");
+    final NameResolver failingResolver = new NameResolver() {
+      @Override public String getServiceAuthority() {
+        return "fake-authority";
+      }
+
+      @Override public void start(Listener2 listener) {
+        throw panicReason;
+      }
+
+      @Override public void shutdown() {}
+    };
+    channelBuilder.nameResolverFactory(new NameResolver.Factory() {
+      @Override public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
+        return failingResolver;
+      }
+
+      @Override public String getDefaultScheme() {
+        return "fakescheme";
+      }
+    });
+    createChannel();
+
+    // RPCs fail immediately
+    ClientCall<String, Integer> call =
+        channel.newCall(method, CallOptions.DEFAULT.withoutWaitForReady());
+    call.start(mockCallListener, new Metadata());
+    executor.runDueTasks();
+    verifyCallListenerClosed(mockCallListener, Status.Code.INTERNAL, panicReason);
+    panicExpected = true;
+  }
+
   private void verifyPanicMode(Throwable cause) {
     panicExpected = true;
     @SuppressWarnings("unchecked")