core: Handle NR/LB exceptions when panicking

If a panic is followed a panic, we'd ignore the second. But if an exception happens while entering panic mode we may fail to update the picker with the first error. This is "fine" from a correctness standpoint; all bets are off when panicking and we've already logged the first error. But failing RPCs can often be more easily seen than just the log. Noticed because of http://yaqs/8493785598685872128

Author: ejona86

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

@@ -779,30 +779,16 @@ void panic(final Throwable t) {
  return;
  }
  panicMode = true;
- cancelIdleTimer(/* permanent= */ true);
- shutdownNameResolverAndLoadBalancer(false);
- final class PanicSubchannelPicker extends SubchannelPicker {
- private final PickResult panicPickResult =
- PickResult.withDrop(
- Status.INTERNAL.withDescription("Panic! This is a bug!").withCause(t));
-
- @Override
- public PickResult pickSubchannel(PickSubchannelArgs args) {
- return panicPickResult;
- }
-
- @Override
- public String toString() {
- return MoreObjects.toStringHelper(PanicSubchannelPicker.class)
- .add("panicPickResult", panicPickResult)
- .toString();
- }
+ try {
+ cancelIdleTimer(/* permanent= */ true);
+ shutdownNameResolverAndLoadBalancer(false);
+ } finally {
+ updateSubchannelPicker(new LoadBalancer.FixedResultPicker(PickResult.withDrop(
+ Status.INTERNAL.withDescription("Panic! This is a bug!").withCause(t))));
+ realChannel.updateConfigSelector(null);
+ channelLogger.log(ChannelLogLevel.ERROR, "PANIC! Entering TRANSIENT_FAILURE");
+ channelStateManager.gotoState(TRANSIENT_FAILURE);
  }
-
- updateSubchannelPicker(new PanicSubchannelPicker());
- realChannel.updateConfigSelector(null);
- channelLogger.log(ChannelLogLevel.ERROR, "PANIC! Entering TRANSIENT_FAILURE");
- channelStateManager.gotoState(TRANSIENT_FAILURE);
  }
 
  @VisibleForTesting
@@ -1404,7 +1390,7 @@ public void updateBalancingState(
  final class UpdateBalancingState implements Runnable {
  @Override
  public void run() {
- if (LbHelperImpl.this != lbHelper) {
+ if (LbHelperImpl.this != lbHelper || panicMode) {
  return;
  }
  updateSubchannelPicker(newPicker);