#19390

Committed 02 Aug 2024 03:10PM UTC coverage: 84.454% (-0.02%) from 84.476%

Build # #19390

Build Type

push

github

Committed by

web-flow

Commit Message

Introduce onResult2 in NameResolver Listener2 that returns Status

Lets the Name Resolver receive the status of the acceptance of the name resolution by the load balancer.

Run Details

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93.67

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

/*
 * Copyright 2016 The gRPC Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.grpc.internal;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static io.grpc.ClientStreamTracer.NAME_RESOLUTION_DELAYED;
import static io.grpc.ConnectivityState.CONNECTING;
import static io.grpc.ConnectivityState.IDLE;
import static io.grpc.ConnectivityState.SHUTDOWN;
import static io.grpc.ConnectivityState.TRANSIENT_FAILURE;
import static io.grpc.EquivalentAddressGroup.ATTR_AUTHORITY_OVERRIDE;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.MoreObjects;
import com.google.common.base.Stopwatch;
import com.google.common.base.Supplier;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.common.util.concurrent.SettableFuture;
import io.grpc.Attributes;
import io.grpc.CallCredentials;
import io.grpc.CallOptions;
import io.grpc.Channel;
import io.grpc.ChannelCredentials;
import io.grpc.ChannelLogger;
import io.grpc.ChannelLogger.ChannelLogLevel;
import io.grpc.ClientCall;
import io.grpc.ClientInterceptor;
import io.grpc.ClientInterceptors;
import io.grpc.ClientStreamTracer;
import io.grpc.ClientTransportFilter;
import io.grpc.CompressorRegistry;
import io.grpc.ConnectivityState;
import io.grpc.ConnectivityStateInfo;
import io.grpc.Context;
import io.grpc.Deadline;
import io.grpc.DecompressorRegistry;
import io.grpc.EquivalentAddressGroup;
import io.grpc.ForwardingChannelBuilder2;
import io.grpc.ForwardingClientCall;
import io.grpc.Grpc;
import io.grpc.InternalChannelz;
import io.grpc.InternalChannelz.ChannelStats;
import io.grpc.InternalChannelz.ChannelTrace;
import io.grpc.InternalConfigSelector;
import io.grpc.InternalInstrumented;
import io.grpc.InternalLogId;
import io.grpc.InternalWithLogId;
import io.grpc.LoadBalancer;
import io.grpc.LoadBalancer.CreateSubchannelArgs;
import io.grpc.LoadBalancer.PickResult;
import io.grpc.LoadBalancer.PickSubchannelArgs;
import io.grpc.LoadBalancer.ResolvedAddresses;
import io.grpc.LoadBalancer.SubchannelPicker;
import io.grpc.LoadBalancer.SubchannelStateListener;
import io.grpc.ManagedChannel;
import io.grpc.ManagedChannelBuilder;
import io.grpc.Metadata;
import io.grpc.MethodDescriptor;
import io.grpc.MetricInstrumentRegistry;
import io.grpc.MetricRecorder;
import io.grpc.NameResolver;
import io.grpc.NameResolver.ConfigOrError;
import io.grpc.NameResolver.ResolutionResult;
import io.grpc.NameResolverProvider;
import io.grpc.NameResolverRegistry;
import io.grpc.ProxyDetector;
import io.grpc.Status;
import io.grpc.SynchronizationContext;
import io.grpc.SynchronizationContext.ScheduledHandle;
import io.grpc.internal.AutoConfiguredLoadBalancerFactory.AutoConfiguredLoadBalancer;
import io.grpc.internal.ClientCallImpl.ClientStreamProvider;
import io.grpc.internal.ClientTransportFactory.SwapChannelCredentialsResult;
import io.grpc.internal.ManagedChannelImplBuilder.ClientTransportFactoryBuilder;
import io.grpc.internal.ManagedChannelImplBuilder.FixedPortProvider;
import io.grpc.internal.ManagedChannelServiceConfig.MethodInfo;
import io.grpc.internal.ManagedChannelServiceConfig.ServiceConfigConvertedSelector;
import io.grpc.internal.RetriableStream.ChannelBufferMeter;
import io.grpc.internal.RetriableStream.Throttle;
import io.grpc.internal.RetryingNameResolver.ResolutionResultListener;
import java.net.URI;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;
import javax.annotation.concurrent.ThreadSafe;

/** A communication channel for making outgoing RPCs. */
@ThreadSafe
final class ManagedChannelImpl extends ManagedChannel implements
    InternalInstrumented<ChannelStats> {
  @VisibleForTesting
  static final Logger logger = Logger.getLogger(ManagedChannelImpl.class.getName());

  static final long IDLE_TIMEOUT_MILLIS_DISABLE = -1;

  static final long SUBCHANNEL_SHUTDOWN_DELAY_SECONDS = 5;

  @VisibleForTesting
  static final Status SHUTDOWN_NOW_STATUS =
      Status.UNAVAILABLE.withDescription("Channel shutdownNow invoked");

  @VisibleForTesting
  static final Status SHUTDOWN_STATUS =
      Status.UNAVAILABLE.withDescription("Channel shutdown invoked");

  @VisibleForTesting
  static final Status SUBCHANNEL_SHUTDOWN_STATUS =
      Status.UNAVAILABLE.withDescription("Subchannel shutdown invoked");

  private static final ManagedChannelServiceConfig EMPTY_SERVICE_CONFIG =
      ManagedChannelServiceConfig.empty();
  private static final InternalConfigSelector INITIAL_PENDING_SELECTOR =
      new InternalConfigSelector() {
        @Override
        public Result selectConfig(PickSubchannelArgs args) {
          throw new IllegalStateException("Resolution is pending");
        }
      };
  private static final LoadBalancer.PickDetailsConsumer NOOP_PICK_DETAILS_CONSUMER =
      new LoadBalancer.PickDetailsConsumer() {};

  private final InternalLogId logId;
  private final String target;
  @Nullable
  private final String authorityOverride;
  private final NameResolverRegistry nameResolverRegistry;
  private final URI targetUri;
  private final NameResolverProvider nameResolverProvider;
  private final NameResolver.Args nameResolverArgs;
  private final AutoConfiguredLoadBalancerFactory loadBalancerFactory;
  private final ClientTransportFactory originalTransportFactory;
  @Nullable
  private final ChannelCredentials originalChannelCreds;
  private final ClientTransportFactory transportFactory;
  private final ClientTransportFactory oobTransportFactory;
  private final RestrictedScheduledExecutor scheduledExecutor;
  private final Executor executor;
  private final ObjectPool<? extends Executor> executorPool;
  private final ObjectPool<? extends Executor> balancerRpcExecutorPool;
  private final ExecutorHolder balancerRpcExecutorHolder;
  private final ExecutorHolder offloadExecutorHolder;
  private final TimeProvider timeProvider;
  private final int maxTraceEvents;

  @VisibleForTesting
  final SynchronizationContext syncContext = new SynchronizationContext(
      new Thread.UncaughtExceptionHandler() {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
          logger.log(
              Level.SEVERE,
              "[" + getLogId() + "] Uncaught exception in the SynchronizationContext. Panic!",
              e);
          panic(e);
        }
      });

  private boolean fullStreamDecompression;

  private final DecompressorRegistry decompressorRegistry;
  private final CompressorRegistry compressorRegistry;

  private final Supplier<Stopwatch> stopwatchSupplier;
  /** The timeout before entering idle mode. */
  private final long idleTimeoutMillis;

  private final ConnectivityStateManager channelStateManager = new ConnectivityStateManager();
  private final BackoffPolicy.Provider backoffPolicyProvider;

  /**
   * We delegate to this channel, so that we can have interceptors as necessary. If there aren't
   * any interceptors and the {@link io.grpc.BinaryLog} is {@code null} then this will just be a
   * {@link RealChannel}.
   */
  private final Channel interceptorChannel;

  private final List<ClientTransportFilter> transportFilters;
  @Nullable private final String userAgent;

  // Only null after channel is terminated. Must be assigned from the syncContext.
  private NameResolver nameResolver;

  // Must be accessed from the syncContext.
  private boolean nameResolverStarted;

  // null when channel is in idle mode.  Must be assigned from syncContext.
  @Nullable
  private LbHelperImpl lbHelper;

  // Must ONLY be assigned from updateSubchannelPicker(), which is called from syncContext.
  // null if channel is in idle mode.
  @Nullable
  private volatile SubchannelPicker subchannelPicker;

  // Must be accessed from the syncContext
  private boolean panicMode;

  // Must be mutated from syncContext
  // If any monitoring hook to be added later needs to get a snapshot of this Set, we could
  // switch to a ConcurrentHashMap.
  private final Set<InternalSubchannel> subchannels = new HashSet<>(16, .75f);

  // Must be accessed from syncContext
  @Nullable
  private Collection<RealChannel.PendingCall<?, ?>> pendingCalls;
  private final Object pendingCallsInUseObject = new Object();

  // Must be mutated from syncContext
  private final Set<OobChannel> oobChannels = new HashSet<>(1, .75f);

  // reprocess() must be run from syncContext
  private final DelayedClientTransport delayedTransport;
  private final UncommittedRetriableStreamsRegistry uncommittedRetriableStreamsRegistry
      = new UncommittedRetriableStreamsRegistry();

  // Shutdown states.
  //
  // Channel's shutdown process:
  // 1. shutdown(): stop accepting new calls from applications
  //   1a shutdown <- true
  //   1b subchannelPicker <- null
  //   1c delayedTransport.shutdown()
  // 2. delayedTransport terminated: stop stream-creation functionality
  //   2a terminating <- true
  //   2b loadBalancer.shutdown()
  //     * LoadBalancer will shutdown subchannels and OOB channels
  //   2c loadBalancer <- null
  //   2d nameResolver.shutdown()
  //   2e nameResolver <- null
  // 3. All subchannels and OOB channels terminated: Channel considered terminated

  private final AtomicBoolean shutdown = new AtomicBoolean(false);
  // Must only be mutated and read from syncContext
  private boolean shutdownNowed;
  // Must only be mutated from syncContext
  private boolean terminating;
  // Must be mutated from syncContext
  private volatile boolean terminated;
  private final CountDownLatch terminatedLatch = new CountDownLatch(1);

  private final CallTracer.Factory callTracerFactory;
  private final CallTracer channelCallTracer;
  private final ChannelTracer channelTracer;
  private final ChannelLogger channelLogger;
  private final InternalChannelz channelz;
  private final RealChannel realChannel;
  // Must be mutated and read from syncContext
  // a flag for doing channel tracing when flipped
  private ResolutionState lastResolutionState = ResolutionState.NO_RESOLUTION;
  // Must be mutated and read from constructor or syncContext
  // used for channel tracing when value changed
  private ManagedChannelServiceConfig lastServiceConfig = EMPTY_SERVICE_CONFIG;

  @Nullable
  private final ManagedChannelServiceConfig defaultServiceConfig;
  // Must be mutated and read from constructor or syncContext
  private boolean serviceConfigUpdated = false;
  private final boolean lookUpServiceConfig;

  // One instance per channel.
  private final ChannelBufferMeter channelBufferUsed = new ChannelBufferMeter();

  private final long perRpcBufferLimit;
  private final long channelBufferLimit;

  // Temporary false flag that can skip the retry code path.
  private final boolean retryEnabled;

  private final Deadline.Ticker ticker = Deadline.getSystemTicker();

  // Called from syncContext
  private final ManagedClientTransport.Listener delayedTransportListener =
      new DelayedTransportListener();

  // Must be called from syncContext
  private void maybeShutdownNowSubchannels() {
    if (shutdownNowed) {
      for (InternalSubchannel subchannel : subchannels) {
        subchannel.shutdownNow(SHUTDOWN_NOW_STATUS);
      }
      for (OobChannel oobChannel : oobChannels) {
        oobChannel.getInternalSubchannel().shutdownNow(SHUTDOWN_NOW_STATUS);
      }
    }
  }

  // Must be accessed from syncContext
  @VisibleForTesting
  final InUseStateAggregator<Object> inUseStateAggregator = new IdleModeStateAggregator();

  @Override
  public ListenableFuture<ChannelStats> getStats() {
    final SettableFuture<ChannelStats> ret = SettableFuture.create();
    final class StatsFetcher implements Runnable {
      @Override
      public void run() {
        ChannelStats.Builder builder = new InternalChannelz.ChannelStats.Builder();
        channelCallTracer.updateBuilder(builder);
        channelTracer.updateBuilder(builder);
        builder.setTarget(target).setState(channelStateManager.getState());
        List<InternalWithLogId> children = new ArrayList<>();
        children.addAll(subchannels);
        children.addAll(oobChannels);
        builder.setSubchannels(children);
        ret.set(builder.build());
      }
    }

    // subchannels and oobchannels can only be accessed from syncContext
    syncContext.execute(new StatsFetcher());
    return ret;
  }

  @Override
  public InternalLogId getLogId() {
    return logId;
  }

  // Run from syncContext
  private class IdleModeTimer implements Runnable {

    @Override
    public void run() {
      // Workaround timer scheduled while in idle mode. This can happen from handleNotInUse() after
      // an explicit enterIdleMode() by the user. Protecting here as other locations are a bit too
      // subtle to change rapidly to resolve the channel panic. See #8714
      if (lbHelper == null) {
        return;
      }
      enterIdleMode();
    }
  }

  // Must be called from syncContext
  private void shutdownNameResolverAndLoadBalancer(boolean channelIsActive) {
    syncContext.throwIfNotInThisSynchronizationContext();
    if (channelIsActive) {
      checkState(nameResolverStarted, "nameResolver is not started");
      checkState(lbHelper != null, "lbHelper is null");
    }
    if (nameResolver != null) {
      nameResolver.shutdown();
      nameResolverStarted = false;
      if (channelIsActive) {
        nameResolver = getNameResolver(
            targetUri, authorityOverride, nameResolverProvider, nameResolverArgs);
      } else {
        nameResolver = null;
      }
    }
    if (lbHelper != null) {
      lbHelper.lb.shutdown();
      lbHelper = null;
    }
    subchannelPicker = null;
  }

  /**
   * Make the channel exit idle mode, if it's in it.
   *
   * <p>Must be called from syncContext
   */
  @VisibleForTesting
  void exitIdleMode() {
    syncContext.throwIfNotInThisSynchronizationContext();
    if (shutdown.get() || panicMode) {
      return;
    }
    if (inUseStateAggregator.isInUse()) {
      // Cancel the timer now, so that a racing due timer will not put Channel on idleness
      // when the caller of exitIdleMode() is about to use the returned loadBalancer.
      cancelIdleTimer(false);
    } else {
      // exitIdleMode() may be called outside of inUseStateAggregator.handleNotInUse() while
      // isInUse() == false, in which case we still need to schedule the timer.
      rescheduleIdleTimer();
    }
    if (lbHelper != null) {
      return;
    }
    channelLogger.log(ChannelLogLevel.INFO, "Exiting idle mode");
    LbHelperImpl lbHelper = new LbHelperImpl();
    lbHelper.lb = loadBalancerFactory.newLoadBalancer(lbHelper);
    // Delay setting lbHelper until fully initialized, since loadBalancerFactory is user code and
    // may throw. We don't want to confuse our state, even if we will enter panic mode.
    this.lbHelper = lbHelper;

    channelStateManager.gotoState(CONNECTING);
    NameResolverListener listener = new NameResolverListener(lbHelper, nameResolver);
    nameResolver.start(listener);
    nameResolverStarted = true;
  }

  // Must be run from syncContext
  private void enterIdleMode() {
    // nameResolver and loadBalancer are guaranteed to be non-null.  If any of them were null,
    // either the idleModeTimer ran twice without exiting the idle mode, or the task in shutdown()
    // did not cancel idleModeTimer, or enterIdle() ran while shutdown or in idle, all of
    // which are bugs.
    shutdownNameResolverAndLoadBalancer(true);
    delayedTransport.reprocess(null);
    channelLogger.log(ChannelLogLevel.INFO, "Entering IDLE state");
    channelStateManager.gotoState(IDLE);
    // If the inUseStateAggregator still considers pending calls to be queued up or the delayed
    // transport to be holding some we need to exit idle mode to give these calls a chance to
    // be processed.
    if (inUseStateAggregator.anyObjectInUse(pendingCallsInUseObject, delayedTransport)) {
      exitIdleMode();
    }
  }

  // Must be run from syncContext
  private void cancelIdleTimer(boolean permanent) {
    idleTimer.cancel(permanent);
  }

  // Always run from syncContext
  private void rescheduleIdleTimer() {
    if (idleTimeoutMillis == IDLE_TIMEOUT_MILLIS_DISABLE) {
      return;
    }
    idleTimer.reschedule(idleTimeoutMillis, TimeUnit.MILLISECONDS);
  }

  /**
   * Force name resolution refresh to happen immediately. Must be run
   * from syncContext.
   */
  private void refreshNameResolution() {
    syncContext.throwIfNotInThisSynchronizationContext();
    if (nameResolverStarted) {
      nameResolver.refresh();
    }
  }

  private final class ChannelStreamProvider implements ClientStreamProvider {
    volatile Throttle throttle;

    @Override
    public ClientStream newStream(
        final MethodDescriptor<?, ?> method,
        final CallOptions callOptions,
        final Metadata headers,
        final Context context) {
      // There is no need to reschedule the idle timer here. If the channel isn't shut down, either
      // the delayed transport or a real transport will go in-use and cancel the idle timer.
      if (!retryEnabled) {
        ClientStreamTracer[] tracers = GrpcUtil.getClientStreamTracers(
            callOptions, headers, 0, /* isTransparentRetry= */ false);
        Context origContext = context.attach();
        try {
          return delayedTransport.newStream(method, headers, callOptions, tracers);
        } finally {
          context.detach(origContext);
        }
      } else {
        MethodInfo methodInfo = callOptions.getOption(MethodInfo.KEY);
        final RetryPolicy retryPolicy = methodInfo == null ? null : methodInfo.retryPolicy;
        final HedgingPolicy hedgingPolicy = methodInfo == null ? null : methodInfo.hedgingPolicy;
        final class RetryStream<ReqT> extends RetriableStream<ReqT> {
          @SuppressWarnings("unchecked")
          RetryStream() {
            super(
                (MethodDescriptor<ReqT, ?>) method,
                headers,
                channelBufferUsed,
                perRpcBufferLimit,
                channelBufferLimit,
                getCallExecutor(callOptions),
                transportFactory.getScheduledExecutorService(),
                retryPolicy,
                hedgingPolicy,
                throttle);
          }

          @Override
          Status prestart() {
            return uncommittedRetriableStreamsRegistry.add(this);
          }

          @Override
          void postCommit() {
            uncommittedRetriableStreamsRegistry.remove(this);
          }

          @Override
          ClientStream newSubstream(
              Metadata newHeaders, ClientStreamTracer.Factory factory, int previousAttempts,
              boolean isTransparentRetry) {
            CallOptions newOptions = callOptions.withStreamTracerFactory(factory);
            ClientStreamTracer[] tracers = GrpcUtil.getClientStreamTracers(
                newOptions, newHeaders, previousAttempts, isTransparentRetry);
            Context origContext = context.attach();
            try {
              return delayedTransport.newStream(method, newHeaders, newOptions, tracers);
            } finally {
              context.detach(origContext);
            }
          }
        }

        return new RetryStream<>();
      }
    }
  }

  private final ChannelStreamProvider transportProvider = new ChannelStreamProvider();

  private final Rescheduler idleTimer;
  private final MetricRecorder metricRecorder;

  ManagedChannelImpl(
      ManagedChannelImplBuilder builder,
      ClientTransportFactory clientTransportFactory,
      URI targetUri,
      NameResolverProvider nameResolverProvider,
      BackoffPolicy.Provider backoffPolicyProvider,
      ObjectPool<? extends Executor> balancerRpcExecutorPool,
      Supplier<Stopwatch> stopwatchSupplier,
      List<ClientInterceptor> interceptors,
      final TimeProvider timeProvider) {
    this.target = checkNotNull(builder.target, "target");
    this.logId = InternalLogId.allocate("Channel", target);
    this.timeProvider = checkNotNull(timeProvider, "timeProvider");
    this.executorPool = checkNotNull(builder.executorPool, "executorPool");
    this.executor = checkNotNull(executorPool.getObject(), "executor");
    this.originalChannelCreds = builder.channelCredentials;
    this.originalTransportFactory = clientTransportFactory;
    this.offloadExecutorHolder =
        new ExecutorHolder(checkNotNull(builder.offloadExecutorPool, "offloadExecutorPool"));
    this.transportFactory = new CallCredentialsApplyingTransportFactory(
        clientTransportFactory, builder.callCredentials, this.offloadExecutorHolder);
    this.oobTransportFactory = new CallCredentialsApplyingTransportFactory(
        clientTransportFactory, null, this.offloadExecutorHolder);
    this.scheduledExecutor =
        new RestrictedScheduledExecutor(transportFactory.getScheduledExecutorService());
    maxTraceEvents = builder.maxTraceEvents;
    channelTracer = new ChannelTracer(
        logId, builder.maxTraceEvents, timeProvider.currentTimeNanos(),
        "Channel for '" + target + "'");
    channelLogger = new ChannelLoggerImpl(channelTracer, timeProvider);
    ProxyDetector proxyDetector =
        builder.proxyDetector != null ? builder.proxyDetector : GrpcUtil.DEFAULT_PROXY_DETECTOR;
    this.retryEnabled = builder.retryEnabled;
    this.loadBalancerFactory = new AutoConfiguredLoadBalancerFactory(builder.defaultLbPolicy);
    this.nameResolverRegistry = builder.nameResolverRegistry;
    this.targetUri = checkNotNull(targetUri, "targetUri");
    this.nameResolverProvider = checkNotNull(nameResolverProvider, "nameResolverProvider");
    ScParser serviceConfigParser =
        new ScParser(
            retryEnabled,
            builder.maxRetryAttempts,
            builder.maxHedgedAttempts,
            loadBalancerFactory);
    this.authorityOverride = builder.authorityOverride;
    this.nameResolverArgs =
        NameResolver.Args.newBuilder()
            .setDefaultPort(builder.getDefaultPort())
            .setProxyDetector(proxyDetector)
            .setSynchronizationContext(syncContext)
            .setScheduledExecutorService(scheduledExecutor)
            .setServiceConfigParser(serviceConfigParser)
            .setChannelLogger(channelLogger)
            .setOffloadExecutor(this.offloadExecutorHolder)
            .setOverrideAuthority(this.authorityOverride)
            .build();
    this.nameResolver = getNameResolver(
        targetUri, authorityOverride, nameResolverProvider, nameResolverArgs);
    this.balancerRpcExecutorPool = checkNotNull(balancerRpcExecutorPool, "balancerRpcExecutorPool");
    this.balancerRpcExecutorHolder = new ExecutorHolder(balancerRpcExecutorPool);
    this.delayedTransport = new DelayedClientTransport(this.executor, this.syncContext);
    this.delayedTransport.start(delayedTransportListener);
    this.backoffPolicyProvider = backoffPolicyProvider;

    if (builder.defaultServiceConfig != null) {
      ConfigOrError parsedDefaultServiceConfig =
          serviceConfigParser.parseServiceConfig(builder.defaultServiceConfig);
      checkState(
          parsedDefaultServiceConfig.getError() == null,
          "Default config is invalid: %s",
          parsedDefaultServiceConfig.getError());
      this.defaultServiceConfig =
          (ManagedChannelServiceConfig) parsedDefaultServiceConfig.getConfig();
      this.transportProvider.throttle = this.defaultServiceConfig.getRetryThrottling();
    } else {
      this.defaultServiceConfig = null;
    }
    this.lookUpServiceConfig = builder.lookUpServiceConfig;
    realChannel = new RealChannel(nameResolver.getServiceAuthority());
    Channel channel = realChannel;
    if (builder.binlog != null) {
      channel = builder.binlog.wrapChannel(channel);
    }
    this.interceptorChannel = ClientInterceptors.intercept(channel, interceptors);
    this.transportFilters = new ArrayList<>(builder.transportFilters);
    this.stopwatchSupplier = checkNotNull(stopwatchSupplier, "stopwatchSupplier");
    if (builder.idleTimeoutMillis == IDLE_TIMEOUT_MILLIS_DISABLE) {
      this.idleTimeoutMillis = builder.idleTimeoutMillis;
    } else {
      checkArgument(
          builder.idleTimeoutMillis
              >= ManagedChannelImplBuilder.IDLE_MODE_MIN_TIMEOUT_MILLIS,
          "invalid idleTimeoutMillis %s", builder.idleTimeoutMillis);
      this.idleTimeoutMillis = builder.idleTimeoutMillis;
    }

    idleTimer = new Rescheduler(
        new IdleModeTimer(),
        syncContext,
        transportFactory.getScheduledExecutorService(),
        stopwatchSupplier.get());
    this.fullStreamDecompression = builder.fullStreamDecompression;
    this.decompressorRegistry = checkNotNull(builder.decompressorRegistry, "decompressorRegistry");
    this.compressorRegistry = checkNotNull(builder.compressorRegistry, "compressorRegistry");
    this.userAgent = builder.userAgent;

    this.channelBufferLimit = builder.retryBufferSize;
    this.perRpcBufferLimit = builder.perRpcBufferLimit;
    final class ChannelCallTracerFactory implements CallTracer.Factory {
      @Override
      public CallTracer create() {
        return new CallTracer(timeProvider);
      }
    }

    this.callTracerFactory = new ChannelCallTracerFactory();
    channelCallTracer = callTracerFactory.create();
    this.channelz = checkNotNull(builder.channelz);
    channelz.addRootChannel(this);

    if (!lookUpServiceConfig) {
      if (defaultServiceConfig != null) {
        channelLogger.log(
            ChannelLogLevel.INFO, "Service config look-up disabled, using default service config");
      }
      serviceConfigUpdated = true;
    }
    this.metricRecorder = new MetricRecorderImpl(builder.metricSinks,
        MetricInstrumentRegistry.getDefaultRegistry());
  }

  @VisibleForTesting
  static NameResolver getNameResolver(
      URI targetUri, @Nullable final String overrideAuthority,
      NameResolverProvider provider, NameResolver.Args nameResolverArgs) {
    NameResolver resolver = provider.newNameResolver(targetUri, nameResolverArgs);
    if (resolver == null) {
      throw new IllegalArgumentException("cannot create a NameResolver for " + targetUri);
    }

    // We wrap the name resolver in a RetryingNameResolver to give it the ability to retry failures.
    // TODO: After a transition period, all NameResolver implementations that need retry should use
    //       RetryingNameResolver directly and this step can be removed.
    NameResolver usedNameResolver = new RetryingNameResolver(resolver,
          new BackoffPolicyRetryScheduler(new ExponentialBackoffPolicy.Provider(),
              nameResolverArgs.getScheduledExecutorService(),
              nameResolverArgs.getSynchronizationContext()),
          nameResolverArgs.getSynchronizationContext());

    if (overrideAuthority == null) {
      return usedNameResolver;
    }

    return new ForwardingNameResolver(usedNameResolver) {
      @Override
      public String getServiceAuthority() {
        return overrideAuthority;
      }
    };
  }

  @VisibleForTesting
  InternalConfigSelector getConfigSelector() {
    return realChannel.configSelector.get();
  }
  
  @VisibleForTesting
  boolean hasThrottle() {
    return this.transportProvider.throttle != null;
  }

  /**
   * Initiates an orderly shutdown in which preexisting calls continue but new calls are immediately
   * cancelled.
   */
  @Override
  public ManagedChannelImpl shutdown() {
    channelLogger.log(ChannelLogLevel.DEBUG, "shutdown() called");
    if (!shutdown.compareAndSet(false, true)) {
      return this;
    }
    final class Shutdown implements Runnable {
      @Override
      public void run() {
        channelLogger.log(ChannelLogLevel.INFO, "Entering SHUTDOWN state");
        channelStateManager.gotoState(SHUTDOWN);
      }
    }

    syncContext.execute(new Shutdown());
    realChannel.shutdown();
    final class CancelIdleTimer implements Runnable {
      @Override
      public void run() {
        cancelIdleTimer(/* permanent= */ true);
      }
    }

    syncContext.execute(new CancelIdleTimer());
    return this;
  }

  /**
   * Initiates a forceful shutdown in which preexisting and new calls are cancelled. Although
   * forceful, the shutdown process is still not instantaneous; {@link #isTerminated()} will likely
   * return {@code false} immediately after this method returns.
   */
  @Override
  public ManagedChannelImpl shutdownNow() {
    channelLogger.log(ChannelLogLevel.DEBUG, "shutdownNow() called");
    shutdown();
    realChannel.shutdownNow();
    final class ShutdownNow implements Runnable {
      @Override
      public void run() {
        if (shutdownNowed) {
          return;
        }
        shutdownNowed = true;
        maybeShutdownNowSubchannels();
      }
    }

    syncContext.execute(new ShutdownNow());
    return this;
  }

  // Called from syncContext
  @VisibleForTesting
  void panic(final Throwable t) {
    if (panicMode) {
      // Preserve the first panic information
      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();
      }
    }

    updateSubchannelPicker(new PanicSubchannelPicker());
    realChannel.updateConfigSelector(null);
    channelLogger.log(ChannelLogLevel.ERROR, "PANIC! Entering TRANSIENT_FAILURE");
    channelStateManager.gotoState(TRANSIENT_FAILURE);
  }

  @VisibleForTesting
  boolean isInPanicMode() {
    return panicMode;
  }

  // Called from syncContext
  private void updateSubchannelPicker(SubchannelPicker newPicker) {
    subchannelPicker = newPicker;
    delayedTransport.reprocess(newPicker);
  }

  @Override
  public boolean isShutdown() {
    return shutdown.get();
  }

  @Override
  public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
    return terminatedLatch.await(timeout, unit);
  }

  @Override
  public boolean isTerminated() {
    return terminated;
  }

  /*
   * Creates a new outgoing call on the channel.
   */
  @Override
  public <ReqT, RespT> ClientCall<ReqT, RespT> newCall(MethodDescriptor<ReqT, RespT> method,
      CallOptions callOptions) {
    return interceptorChannel.newCall(method, callOptions);
  }

  @Override
  public String authority() {
    return interceptorChannel.authority();
  }

  private Executor getCallExecutor(CallOptions callOptions) {
    Executor executor = callOptions.getExecutor();
    if (executor == null) {
      executor = this.executor;
    }
    return executor;
  }

  private class RealChannel extends Channel {
    // Reference to null if no config selector is available from resolution result
    // Reference must be set() from syncContext
    private final AtomicReference<InternalConfigSelector> configSelector =
        new AtomicReference<>(INITIAL_PENDING_SELECTOR);
    // Set when the NameResolver is initially created. When we create a new NameResolver for the
    // same target, the new instance must have the same value.
    private final String authority;

    private final Channel clientCallImplChannel = new Channel() {
      @Override
      public <RequestT, ResponseT> ClientCall<RequestT, ResponseT> newCall(
          MethodDescriptor<RequestT, ResponseT> method, CallOptions callOptions) {
        return new ClientCallImpl<>(
            method,
            getCallExecutor(callOptions),
            callOptions,
            transportProvider,
            terminated ? null : transportFactory.getScheduledExecutorService(),
            channelCallTracer,
            null)
            .setFullStreamDecompression(fullStreamDecompression)
            .setDecompressorRegistry(decompressorRegistry)
            .setCompressorRegistry(compressorRegistry);
      }

      @Override
      public String authority() {
        return authority;
      }
    };

    private RealChannel(String authority) {
      this.authority =  checkNotNull(authority, "authority");
    }

    @Override
    public <ReqT, RespT> ClientCall<ReqT, RespT> newCall(
        MethodDescriptor<ReqT, RespT> method, CallOptions callOptions) {
      if (configSelector.get() != INITIAL_PENDING_SELECTOR) {
        return newClientCall(method, callOptions);
      }
      syncContext.execute(new Runnable() {
        @Override
        public void run() {
          exitIdleMode();
        }
      });
      if (configSelector.get() != INITIAL_PENDING_SELECTOR) {
        // This is an optimization for the case (typically with InProcessTransport) when name
        // resolution result is immediately available at this point. Otherwise, some users'
        // tests might observe slight behavior difference from earlier grpc versions.
        return newClientCall(method, callOptions);
      }
      if (shutdown.get()) {
        // Return a failing ClientCall.
        return new ClientCall<ReqT, RespT>() {
          @Override
          public void start(Listener<RespT> responseListener, Metadata headers) {
            responseListener.onClose(SHUTDOWN_STATUS, new Metadata());
          }

          @Override public void request(int numMessages) {}

          @Override public void cancel(@Nullable String message, @Nullable Throwable cause) {}

          @Override public void halfClose() {}

          @Override public void sendMessage(ReqT message) {}
        };
      }
      Context context = Context.current();
      final PendingCall<ReqT, RespT> pendingCall = new PendingCall<>(context, method, callOptions);
      syncContext.execute(new Runnable() {
        @Override
        public void run() {
          if (configSelector.get() == INITIAL_PENDING_SELECTOR) {
            if (pendingCalls == null) {
              pendingCalls = new LinkedHashSet<>();
              inUseStateAggregator.updateObjectInUse(pendingCallsInUseObject, true);
            }
            pendingCalls.add(pendingCall);
          } else {
            pendingCall.reprocess();
          }
        }
      });
      return pendingCall;
    }

    // Must run in SynchronizationContext.
    void updateConfigSelector(@Nullable InternalConfigSelector config) {
      InternalConfigSelector prevConfig = configSelector.get();
      configSelector.set(config);
      if (prevConfig == INITIAL_PENDING_SELECTOR && pendingCalls != null) {
        for (RealChannel.PendingCall<?, ?> pendingCall : pendingCalls) {
          pendingCall.reprocess();
        }
      }
    }

    // Must run in SynchronizationContext.
    void onConfigError() {
      if (configSelector.get() == INITIAL_PENDING_SELECTOR) {
        updateConfigSelector(null);
      }
    }

    void shutdown() {
      final class RealChannelShutdown implements Runnable {
        @Override
        public void run() {
          if (pendingCalls == null) {
            if (configSelector.get() == INITIAL_PENDING_SELECTOR) {
              configSelector.set(null);
            }
            uncommittedRetriableStreamsRegistry.onShutdown(SHUTDOWN_STATUS);
          }
        }
      }

      syncContext.execute(new RealChannelShutdown());
    }

    void shutdownNow() {
      final class RealChannelShutdownNow implements Runnable {
        @Override
        public void run() {
          if (configSelector.get() == INITIAL_PENDING_SELECTOR) {
            configSelector.set(null);
          }
          if (pendingCalls != null) {
            for (RealChannel.PendingCall<?, ?> pendingCall : pendingCalls) {
              pendingCall.cancel("Channel is forcefully shutdown", null);
            }
          }
          uncommittedRetriableStreamsRegistry.onShutdownNow(SHUTDOWN_NOW_STATUS);
        }
      }

      syncContext.execute(new RealChannelShutdownNow());
    }

    @Override
    public String authority() {
      return authority;
    }

    private final class PendingCall<ReqT, RespT> extends DelayedClientCall<ReqT, RespT> {
      final Context context;
      final MethodDescriptor<ReqT, RespT> method;
      final CallOptions callOptions;
      private final long callCreationTime;

      PendingCall(
          Context context, MethodDescriptor<ReqT, RespT> method, CallOptions callOptions) {
        super(getCallExecutor(callOptions), scheduledExecutor, callOptions.getDeadline());
        this.context = context;
        this.method = method;
        this.callOptions = callOptions;
        this.callCreationTime = ticker.nanoTime();
      }

      /** Called when it's ready to create a real call and reprocess the pending call. */
      void reprocess() {
        ClientCall<ReqT, RespT> realCall;
        Context previous = context.attach();
        try {
          CallOptions delayResolutionOption = callOptions.withOption(NAME_RESOLUTION_DELAYED,
              ticker.nanoTime() - callCreationTime);
          realCall = newClientCall(method, delayResolutionOption);
        } finally {
          context.detach(previous);
        }
        Runnable toRun = setCall(realCall);
        if (toRun == null) {
          syncContext.execute(new PendingCallRemoval());
        } else {
          getCallExecutor(callOptions).execute(new Runnable() {
            @Override
            public void run() {
              toRun.run();
              syncContext.execute(new PendingCallRemoval());
            }
          });
        }
      }

      @Override
      protected void callCancelled() {
        super.callCancelled();
        syncContext.execute(new PendingCallRemoval());
      }

      final class PendingCallRemoval implements Runnable {
        @Override
        public void run() {
          if (pendingCalls != null) {
            pendingCalls.remove(PendingCall.this);
            if (pendingCalls.isEmpty()) {
              inUseStateAggregator.updateObjectInUse(pendingCallsInUseObject, false);
              pendingCalls = null;
              if (shutdown.get()) {
                uncommittedRetriableStreamsRegistry.onShutdown(SHUTDOWN_STATUS);
              }
            }
          }
        }
      }
    }

    private <ReqT, RespT> ClientCall<ReqT, RespT> newClientCall(
        MethodDescriptor<ReqT, RespT> method, CallOptions callOptions) {
      InternalConfigSelector selector = configSelector.get();
      if (selector == null) {
        return clientCallImplChannel.newCall(method, callOptions);
      }
      if (selector instanceof ServiceConfigConvertedSelector) {
        MethodInfo methodInfo =
            ((ServiceConfigConvertedSelector) selector).config.getMethodConfig(method);
        if (methodInfo != null) {
          callOptions = callOptions.withOption(MethodInfo.KEY, methodInfo);
        }
        return clientCallImplChannel.newCall(method, callOptions);
      }
      return new ConfigSelectingClientCall<>(
          selector, clientCallImplChannel, executor, method, callOptions);
    }
  }

  /**
   * A client call for a given channel that applies a given config selector when it starts.
   */
  static final class ConfigSelectingClientCall<ReqT, RespT>
      extends ForwardingClientCall<ReqT, RespT> {

    private final InternalConfigSelector configSelector;
    private final Channel channel;
    private final Executor callExecutor;
    private final MethodDescriptor<ReqT, RespT> method;
    private final Context context;
    private CallOptions callOptions;

    private ClientCall<ReqT, RespT> delegate;

    ConfigSelectingClientCall(
        InternalConfigSelector configSelector, Channel channel, Executor channelExecutor,
        MethodDescriptor<ReqT, RespT> method,
        CallOptions callOptions) {
      this.configSelector = configSelector;
      this.channel = channel;
      this.method = method;
      this.callExecutor =
          callOptions.getExecutor() == null ? channelExecutor : callOptions.getExecutor();
      this.callOptions = callOptions.withExecutor(callExecutor);
      this.context = Context.current();
    }

    @Override
    protected ClientCall<ReqT, RespT> delegate() {
      return delegate;
    }

    @SuppressWarnings("unchecked")
    @Override
    public void start(Listener<RespT> observer, Metadata headers) {
      PickSubchannelArgs args =
          new PickSubchannelArgsImpl(method, headers, callOptions, NOOP_PICK_DETAILS_CONSUMER);
      InternalConfigSelector.Result result = configSelector.selectConfig(args);
      Status status = result.getStatus();
      if (!status.isOk()) {
        executeCloseObserverInContext(observer,
            GrpcUtil.replaceInappropriateControlPlaneStatus(status));
        delegate = (ClientCall<ReqT, RespT>) NOOP_CALL;
        return;
      }
      ClientInterceptor interceptor = result.getInterceptor();
      ManagedChannelServiceConfig config = (ManagedChannelServiceConfig) result.getConfig();
      MethodInfo methodInfo = config.getMethodConfig(method);
      if (methodInfo != null) {
        callOptions = callOptions.withOption(MethodInfo.KEY, methodInfo);
      }
      if (interceptor != null) {
        delegate = interceptor.interceptCall(method, callOptions, channel);
      } else {
        delegate = channel.newCall(method, callOptions);
      }
      delegate.start(observer, headers);
    }

    private void executeCloseObserverInContext(
        final Listener<RespT> observer, final Status status) {
      class CloseInContext extends ContextRunnable {
        CloseInContext() {
          super(context);
        }

        @Override
        public void runInContext() {
          observer.onClose(status, new Metadata());
        }
      }

      callExecutor.execute(new CloseInContext());
    }

    @Override
    public void cancel(@Nullable String message, @Nullable Throwable cause) {
      if (delegate != null) {
        delegate.cancel(message, cause);
      }
    }
  }

  private static final ClientCall<Object, Object> NOOP_CALL = new ClientCall<Object, Object>() {
    @Override
    public void start(Listener<Object> responseListener, Metadata headers) {}

    @Override
    public void request(int numMessages) {}

    @Override
    public void cancel(String message, Throwable cause) {}

    @Override
    public void halfClose() {}

    @Override
    public void sendMessage(Object message) {}

    // Always returns {@code false}, since this is only used when the startup of the call fails.
    @Override
    public boolean isReady() {
      return false;
    }
  };

  /**
   * Terminate the channel if termination conditions are met.
   */
  // Must be run from syncContext
  private void maybeTerminateChannel() {
    if (terminated) {
      return;
    }
    if (shutdown.get() && subchannels.isEmpty() && oobChannels.isEmpty()) {
      channelLogger.log(ChannelLogLevel.INFO, "Terminated");
      channelz.removeRootChannel(this);
      executorPool.returnObject(executor);
      balancerRpcExecutorHolder.release();
      offloadExecutorHolder.release();
      // Release the transport factory so that it can deallocate any resources.
      transportFactory.close();

      terminated = true;
      terminatedLatch.countDown();
    }
  }

  // Must be called from syncContext
  private void handleInternalSubchannelState(ConnectivityStateInfo newState) {
    if (newState.getState() == TRANSIENT_FAILURE || newState.getState() == IDLE) {
      refreshNameResolution();
    }
  }

  @Override
  @SuppressWarnings("deprecation")
  public ConnectivityState getState(boolean requestConnection) {
    ConnectivityState savedChannelState = channelStateManager.getState();
    if (requestConnection && savedChannelState == IDLE) {
      final class RequestConnection implements Runnable {
        @Override
        public void run() {
          exitIdleMode();
          if (subchannelPicker != null) {
            subchannelPicker.requestConnection();
          }
          if (lbHelper != null) {
            lbHelper.lb.requestConnection();
          }
        }
      }

      syncContext.execute(new RequestConnection());
    }
    return savedChannelState;
  }

  @Override
  public void notifyWhenStateChanged(final ConnectivityState source, final Runnable callback) {
    final class NotifyStateChanged implements Runnable {
      @Override
      public void run() {
        channelStateManager.notifyWhenStateChanged(callback, executor, source);
      }
    }

    syncContext.execute(new NotifyStateChanged());
  }

  @Override
  public void resetConnectBackoff() {
    final class ResetConnectBackoff implements Runnable {
      @Override
      public void run() {
        if (shutdown.get()) {
          return;
        }
        if (nameResolverStarted) {
          refreshNameResolution();
        }
        for (InternalSubchannel subchannel : subchannels) {
          subchannel.resetConnectBackoff();
        }
        for (OobChannel oobChannel : oobChannels) {
          oobChannel.resetConnectBackoff();
        }
      }
    }

    syncContext.execute(new ResetConnectBackoff());
  }

  @Override
  public void enterIdle() {
    final class PrepareToLoseNetworkRunnable implements Runnable {
      @Override
      public void run() {
        if (shutdown.get() || lbHelper == null) {
          return;
        }
        cancelIdleTimer(/* permanent= */ false);
        enterIdleMode();
      }
    }

    syncContext.execute(new PrepareToLoseNetworkRunnable());
  }

  /**
   * A registry that prevents channel shutdown from killing existing retry attempts that are in
   * backoff.
   */
  private final class UncommittedRetriableStreamsRegistry {
    // TODO(zdapeng): This means we would acquire a lock for each new retry-able stream,
    // it's worthwhile to look for a lock-free approach.
    final Object lock = new Object();

    @GuardedBy("lock")
    Collection<ClientStream> uncommittedRetriableStreams = new HashSet<>();

    @GuardedBy("lock")
    Status shutdownStatus;

    void onShutdown(Status reason) {
      boolean shouldShutdownDelayedTransport = false;
      synchronized (lock) {
        if (shutdownStatus != null) {
          return;
        }
        shutdownStatus = reason;
        // Keep the delayedTransport open until there is no more uncommitted streams, b/c those
        // retriable streams, which may be in backoff and not using any transport, are already
        // started RPCs.
        if (uncommittedRetriableStreams.isEmpty()) {
          shouldShutdownDelayedTransport = true;
        }
      }

      if (shouldShutdownDelayedTransport) {
        delayedTransport.shutdown(reason);
      }
    }

    void onShutdownNow(Status reason) {
      onShutdown(reason);
      Collection<ClientStream> streams;

      synchronized (lock) {
        streams = new ArrayList<>(uncommittedRetriableStreams);
      }

      for (ClientStream stream : streams) {
        stream.cancel(reason);
      }
      delayedTransport.shutdownNow(reason);
    }

    /**
     * Registers a RetriableStream and return null if not shutdown, otherwise just returns the
     * shutdown Status.
     */
    @Nullable
    Status add(RetriableStream<?> retriableStream) {
      synchronized (lock) {
        if (shutdownStatus != null) {
          return shutdownStatus;
        }
        uncommittedRetriableStreams.add(retriableStream);
        return null;
      }
    }

    void remove(RetriableStream<?> retriableStream) {
      Status shutdownStatusCopy = null;

      synchronized (lock) {
        uncommittedRetriableStreams.remove(retriableStream);
        if (uncommittedRetriableStreams.isEmpty()) {
          shutdownStatusCopy = shutdownStatus;
          // Because retriable transport is long-lived, we take this opportunity to down-size the
          // hashmap.
          uncommittedRetriableStreams = new HashSet<>();
        }
      }

      if (shutdownStatusCopy != null) {
        delayedTransport.shutdown(shutdownStatusCopy);
      }
    }
  }

  private final class LbHelperImpl extends LoadBalancer.Helper {
    AutoConfiguredLoadBalancer lb;

    @Override
    public AbstractSubchannel createSubchannel(CreateSubchannelArgs args) {
      syncContext.throwIfNotInThisSynchronizationContext();
      // No new subchannel should be created after load balancer has been shutdown.
      checkState(!terminating, "Channel is being terminated");
      return new SubchannelImpl(args);
    }

    @Override
    public void updateBalancingState(
        final ConnectivityState newState, final SubchannelPicker newPicker) {
      syncContext.throwIfNotInThisSynchronizationContext();
      checkNotNull(newState, "newState");
      checkNotNull(newPicker, "newPicker");
      final class UpdateBalancingState implements Runnable {
        @Override
        public void run() {
          if (LbHelperImpl.this != lbHelper) {
            return;
          }
          updateSubchannelPicker(newPicker);
          // It's not appropriate to report SHUTDOWN state from lb.
          // Ignore the case of newState == SHUTDOWN for now.
          if (newState != SHUTDOWN) {
            channelLogger.log(
                ChannelLogLevel.INFO, "Entering {0} state with picker: {1}", newState, newPicker);
            channelStateManager.gotoState(newState);
          }
        }
      }

      syncContext.execute(new UpdateBalancingState());
    }

    @Override
    public void refreshNameResolution() {
      syncContext.throwIfNotInThisSynchronizationContext();
      final class LoadBalancerRefreshNameResolution implements Runnable {
        @Override
        public void run() {
          ManagedChannelImpl.this.refreshNameResolution();
        }
      }

      syncContext.execute(new LoadBalancerRefreshNameResolution());
    }

    @Override
    public ManagedChannel createOobChannel(EquivalentAddressGroup addressGroup, String authority) {
      return createOobChannel(Collections.singletonList(addressGroup), authority);
    }

    @Override
    public ManagedChannel createOobChannel(List<EquivalentAddressGroup> addressGroup,
        String authority) {
      // TODO(ejona): can we be even stricter? Like terminating?
      checkState(!terminated, "Channel is terminated");
      long oobChannelCreationTime = timeProvider.currentTimeNanos();
      InternalLogId oobLogId = InternalLogId.allocate("OobChannel", /*details=*/ null);
      InternalLogId subchannelLogId =
          InternalLogId.allocate("Subchannel-OOB", /*details=*/ authority);
      ChannelTracer oobChannelTracer =
          new ChannelTracer(
              oobLogId, maxTraceEvents, oobChannelCreationTime,
              "OobChannel for " + addressGroup);
      final OobChannel oobChannel = new OobChannel(
          authority, balancerRpcExecutorPool, oobTransportFactory.getScheduledExecutorService(),
          syncContext, callTracerFactory.create(), oobChannelTracer, channelz, timeProvider);
      channelTracer.reportEvent(new ChannelTrace.Event.Builder()
          .setDescription("Child OobChannel created")
          .setSeverity(ChannelTrace.Event.Severity.CT_INFO)
          .setTimestampNanos(oobChannelCreationTime)
          .setChannelRef(oobChannel)
          .build());
      ChannelTracer subchannelTracer =
          new ChannelTracer(subchannelLogId, maxTraceEvents, oobChannelCreationTime,
              "Subchannel for " + addressGroup);
      ChannelLogger subchannelLogger = new ChannelLoggerImpl(subchannelTracer, timeProvider);
      final class ManagedOobChannelCallback extends InternalSubchannel.Callback {
        @Override
        void onTerminated(InternalSubchannel is) {
          oobChannels.remove(oobChannel);
          channelz.removeSubchannel(is);
          oobChannel.handleSubchannelTerminated();
          maybeTerminateChannel();
        }

        @Override
        void onStateChange(InternalSubchannel is, ConnectivityStateInfo newState) {
          // TODO(chengyuanzhang): change to let LB policies explicitly manage OOB channel's
          //  state and refresh name resolution if necessary.
          handleInternalSubchannelState(newState);
          oobChannel.handleSubchannelStateChange(newState);
        }
      }

      final InternalSubchannel internalSubchannel = new InternalSubchannel(
          addressGroup,
          authority, userAgent, backoffPolicyProvider, oobTransportFactory,
          oobTransportFactory.getScheduledExecutorService(), stopwatchSupplier, syncContext,
          // All callback methods are run from syncContext
          new ManagedOobChannelCallback(),
          channelz,
          callTracerFactory.create(),
          subchannelTracer,
          subchannelLogId,
          subchannelLogger,
          transportFilters);
      oobChannelTracer.reportEvent(new ChannelTrace.Event.Builder()
          .setDescription("Child Subchannel created")
          .setSeverity(ChannelTrace.Event.Severity.CT_INFO)
          .setTimestampNanos(oobChannelCreationTime)
          .setSubchannelRef(internalSubchannel)
          .build());
      channelz.addSubchannel(oobChannel);
      channelz.addSubchannel(internalSubchannel);
      oobChannel.setSubchannel(internalSubchannel);
      final class AddOobChannel implements Runnable {
        @Override
        public void run() {
          if (terminating) {
            oobChannel.shutdown();
          }
          if (!terminated) {
            // If channel has not terminated, it will track the subchannel and block termination
            // for it.
            oobChannels.add(oobChannel);
          }
        }
      }

      syncContext.execute(new AddOobChannel());
      return oobChannel;
    }

    @Deprecated
    @Override
    public ManagedChannelBuilder<?> createResolvingOobChannelBuilder(String target) {
      return createResolvingOobChannelBuilder(target, new DefaultChannelCreds())
          // Override authority to keep the old behavior.
          // createResolvingOobChannelBuilder(String target) will be deleted soon.
          .overrideAuthority(getAuthority());
    }

    // TODO(creamsoup) prevent main channel to shutdown if oob channel is not terminated
    // TODO(zdapeng) register the channel as a subchannel of the parent channel in channelz.
    @Override
    public ManagedChannelBuilder<?> createResolvingOobChannelBuilder(
        final String target, final ChannelCredentials channelCreds) {
      checkNotNull(channelCreds, "channelCreds");

      final class ResolvingOobChannelBuilder
          extends ForwardingChannelBuilder2<ResolvingOobChannelBuilder> {
        final ManagedChannelBuilder<?> delegate;

        ResolvingOobChannelBuilder() {
          final ClientTransportFactory transportFactory;
          CallCredentials callCredentials;
          if (channelCreds instanceof DefaultChannelCreds) {
            transportFactory = originalTransportFactory;
            callCredentials = null;
          } else {
            SwapChannelCredentialsResult swapResult =
                originalTransportFactory.swapChannelCredentials(channelCreds);
            if (swapResult == null) {
              delegate = Grpc.newChannelBuilder(target, channelCreds);
              return;
            } else {
              transportFactory = swapResult.transportFactory;
              callCredentials = swapResult.callCredentials;
            }
          }
          ClientTransportFactoryBuilder transportFactoryBuilder =
              new ClientTransportFactoryBuilder() {
                @Override
                public ClientTransportFactory buildClientTransportFactory() {
                  return transportFactory;
                }
              };
          delegate = new ManagedChannelImplBuilder(
              target,
              channelCreds,
              callCredentials,
              transportFactoryBuilder,
              new FixedPortProvider(nameResolverArgs.getDefaultPort()))
              .nameResolverRegistry(nameResolverRegistry);
        }

        @Override
        protected ManagedChannelBuilder<?> delegate() {
          return delegate;
        }
      }

      checkState(!terminated, "Channel is terminated");

      @SuppressWarnings("deprecation")
      ResolvingOobChannelBuilder builder = new ResolvingOobChannelBuilder();

      return builder
          // TODO(zdapeng): executors should not outlive the parent channel.
          .executor(executor)
          .offloadExecutor(offloadExecutorHolder.getExecutor())
          .maxTraceEvents(maxTraceEvents)
          .proxyDetector(nameResolverArgs.getProxyDetector())
          .userAgent(userAgent);
    }

    @Override
    public ChannelCredentials getUnsafeChannelCredentials() {
      if (originalChannelCreds == null) {
        return new DefaultChannelCreds();
      }
      return originalChannelCreds;
    }

    @Override
    public void updateOobChannelAddresses(ManagedChannel channel, EquivalentAddressGroup eag) {
      updateOobChannelAddresses(channel, Collections.singletonList(eag));
    }

    @Override
    public void updateOobChannelAddresses(ManagedChannel channel,
        List<EquivalentAddressGroup> eag) {
      checkArgument(channel instanceof OobChannel,
          "channel must have been returned from createOobChannel");
      ((OobChannel) channel).updateAddresses(eag);
    }

    @Override
    public String getAuthority() {
      return ManagedChannelImpl.this.authority();
    }

    @Override
    public String getChannelTarget() {
      return targetUri.toString();
    }

    @Override
    public SynchronizationContext getSynchronizationContext() {
      return syncContext;
    }

    @Override
    public ScheduledExecutorService getScheduledExecutorService() {
      return scheduledExecutor;
    }

    @Override
    public ChannelLogger getChannelLogger() {
      return channelLogger;
    }

    @Override
    public NameResolver.Args getNameResolverArgs() {
      return nameResolverArgs;
    }

    @Override
    public NameResolverRegistry getNameResolverRegistry() {
      return nameResolverRegistry;
    }

    @Override
    public MetricRecorder getMetricRecorder() {
      return metricRecorder;
    }

    /**
     * A placeholder for channel creds if user did not specify channel creds for the channel.
     */
    // TODO(zdapeng): get rid of this class and let all ChannelBuilders always provide a non-null
    //     channel creds.
    final class DefaultChannelCreds extends ChannelCredentials {
      @Override
      public ChannelCredentials withoutBearerTokens() {
        return this;
      }
    }
  }

  final class NameResolverListener extends NameResolver.Listener2 {
    final LbHelperImpl helper;
    final NameResolver resolver;

    NameResolverListener(LbHelperImpl helperImpl, NameResolver resolver) {
      this.helper = checkNotNull(helperImpl, "helperImpl");
      this.resolver = checkNotNull(resolver, "resolver");
    }

    @Override
    public void onResult(final ResolutionResult resolutionResult) {
      final class NamesResolved implements Runnable {

        @Override
        public void run() {
          Status status = onResult2(resolutionResult);
          ResolutionResultListener resolutionResultListener = resolutionResult.getAttributes()
              .get(RetryingNameResolver.RESOLUTION_RESULT_LISTENER_KEY);
          resolutionResultListener.resolutionAttempted(status);
        }
      }

      syncContext.execute(new NamesResolved());
    }

    @SuppressWarnings("ReferenceEquality")
    @Override
    public Status onResult2(final ResolutionResult resolutionResult) {
      syncContext.throwIfNotInThisSynchronizationContext();
      if (ManagedChannelImpl.this.nameResolver != resolver) {
        return Status.OK;
      }

      List<EquivalentAddressGroup> servers = resolutionResult.getAddresses();
      channelLogger.log(
          ChannelLogLevel.DEBUG,
          "Resolved address: {0}, config={1}",
          servers,
          resolutionResult.getAttributes());

      if (lastResolutionState != ResolutionState.SUCCESS) {
        channelLogger.log(ChannelLogLevel.INFO, "Address resolved: {0}", servers);
        lastResolutionState = ResolutionState.SUCCESS;
      }

      ConfigOrError configOrError = resolutionResult.getServiceConfig();
      InternalConfigSelector resolvedConfigSelector =
          resolutionResult.getAttributes().get(InternalConfigSelector.KEY);
      ManagedChannelServiceConfig validServiceConfig =
          configOrError != null && configOrError.getConfig() != null
              ? (ManagedChannelServiceConfig) configOrError.getConfig()
              : null;
      Status serviceConfigError = configOrError != null ? configOrError.getError() : null;

      ManagedChannelServiceConfig effectiveServiceConfig;
      if (!lookUpServiceConfig) {
        if (validServiceConfig != null) {
          channelLogger.log(
              ChannelLogLevel.INFO,
              "Service config from name resolver discarded by channel settings");
        }
        effectiveServiceConfig =
            defaultServiceConfig == null ? EMPTY_SERVICE_CONFIG : defaultServiceConfig;
        if (resolvedConfigSelector != null) {
          channelLogger.log(
              ChannelLogLevel.INFO,
              "Config selector from name resolver discarded by channel settings");
        }
        realChannel.updateConfigSelector(effectiveServiceConfig.getDefaultConfigSelector());
      } else {
        // Try to use config if returned from name resolver
        // Otherwise, try to use the default config if available
        if (validServiceConfig != null) {
          effectiveServiceConfig = validServiceConfig;
          if (resolvedConfigSelector != null) {
            realChannel.updateConfigSelector(resolvedConfigSelector);
            if (effectiveServiceConfig.getDefaultConfigSelector() != null) {
              channelLogger.log(
                  ChannelLogLevel.DEBUG,
                  "Method configs in service config will be discarded due to presence of"
                      + "config-selector");
            }
          } else {
            realChannel.updateConfigSelector(effectiveServiceConfig.getDefaultConfigSelector());
          }
        } else if (defaultServiceConfig != null) {
          effectiveServiceConfig = defaultServiceConfig;
          realChannel.updateConfigSelector(effectiveServiceConfig.getDefaultConfigSelector());
          channelLogger.log(
              ChannelLogLevel.INFO,
              "Received no service config, using default service config");
        } else if (serviceConfigError != null) {
          if (!serviceConfigUpdated) {
            // First DNS lookup has invalid service config, and cannot fall back to default
            channelLogger.log(
                ChannelLogLevel.INFO,
                "Fallback to error due to invalid first service config without default config");
            // This error could be an "inappropriate" control plane error that should not bleed
            // through to client code using gRPC. We let them flow through here to the LB as
            // we later check for these error codes when investigating pick results in
            // GrpcUtil.getTransportFromPickResult().
            onError(configOrError.getError());
            return configOrError.getError();
          } else {
            effectiveServiceConfig = lastServiceConfig;
          }
        } else {
          effectiveServiceConfig = EMPTY_SERVICE_CONFIG;
          realChannel.updateConfigSelector(null);
        }
        if (!effectiveServiceConfig.equals(lastServiceConfig)) {
          channelLogger.log(
              ChannelLogLevel.INFO,
              "Service config changed{0}",
              effectiveServiceConfig == EMPTY_SERVICE_CONFIG ? " to empty" : "");
          lastServiceConfig = effectiveServiceConfig;
          transportProvider.throttle = effectiveServiceConfig.getRetryThrottling();
        }

        try {
          // TODO(creamsoup): when `servers` is empty and lastResolutionStateCopy == SUCCESS
          //  and lbNeedAddress, it shouldn't call the handleServiceConfigUpdate. But,
          //  lbNeedAddress is not deterministic
          serviceConfigUpdated = true;
        } catch (RuntimeException re) {
          logger.log(
              Level.WARNING,
              "[" + getLogId() + "] Unexpected exception from parsing service config",
              re);
        }
      }

      Attributes effectiveAttrs = resolutionResult.getAttributes();
      // Call LB only if it's not shutdown.  If LB is shutdown, lbHelper won't match.
      if (NameResolverListener.this.helper == ManagedChannelImpl.this.lbHelper) {
        Attributes.Builder attrBuilder =
            effectiveAttrs.toBuilder().discard(InternalConfigSelector.KEY);
        Map<String, ?> healthCheckingConfig =
            effectiveServiceConfig.getHealthCheckingConfig();
        if (healthCheckingConfig != null) {
          attrBuilder
              .set(LoadBalancer.ATTR_HEALTH_CHECKING_CONFIG, healthCheckingConfig)
              .build();
        }
        Attributes attributes = attrBuilder.build();

        return helper.lb.tryAcceptResolvedAddresses(
            ResolvedAddresses.newBuilder()
                .setAddresses(servers)
                .setAttributes(attributes)
                .setLoadBalancingPolicyConfig(effectiveServiceConfig.getLoadBalancingConfig())
                .build());
      }
      return Status.OK;
    }

    @Override
    public void onError(final Status error) {
      checkArgument(!error.isOk(), "the error status must not be OK");
      final class NameResolverErrorHandler implements Runnable {
        @Override
        public void run() {
          handleErrorInSyncContext(error);
        }
      }

      syncContext.execute(new NameResolverErrorHandler());
    }

    private void handleErrorInSyncContext(Status error) {
      logger.log(Level.WARNING, "[{0}] Failed to resolve name. status={1}",
          new Object[] {getLogId(), error});
      realChannel.onConfigError();
      if (lastResolutionState != ResolutionState.ERROR) {
        channelLogger.log(ChannelLogLevel.WARNING, "Failed to resolve name: {0}", error);
        lastResolutionState = ResolutionState.ERROR;
      }
      // Call LB only if it's not shutdown.  If LB is shutdown, lbHelper won't match.
      if (NameResolverListener.this.helper != ManagedChannelImpl.this.lbHelper) {
        return;
      }

      helper.lb.handleNameResolutionError(error);
    }
  }

  private final class SubchannelImpl extends AbstractSubchannel {
    final CreateSubchannelArgs args;
    final InternalLogId subchannelLogId;
    final ChannelLoggerImpl subchannelLogger;
    final ChannelTracer subchannelTracer;
    List<EquivalentAddressGroup> addressGroups;
    InternalSubchannel subchannel;
    boolean started;
    boolean shutdown;
    ScheduledHandle delayedShutdownTask;

    SubchannelImpl(CreateSubchannelArgs args) {
      checkNotNull(args, "args");
      addressGroups = args.getAddresses();
      if (authorityOverride != null) {
        List<EquivalentAddressGroup> eagsWithoutOverrideAttr =
            stripOverrideAuthorityAttributes(args.getAddresses());
        args = args.toBuilder().setAddresses(eagsWithoutOverrideAttr).build();
      }
      this.args = args;
      subchannelLogId = InternalLogId.allocate("Subchannel", /*details=*/ authority());
      subchannelTracer = new ChannelTracer(
          subchannelLogId, maxTraceEvents, timeProvider.currentTimeNanos(),
          "Subchannel for " + args.getAddresses());
      subchannelLogger = new ChannelLoggerImpl(subchannelTracer, timeProvider);
    }

    @Override
    public void start(final SubchannelStateListener listener) {
      syncContext.throwIfNotInThisSynchronizationContext();
      checkState(!started, "already started");
      checkState(!shutdown, "already shutdown");
      checkState(!terminating, "Channel is being terminated");
      started = true;
      final class ManagedInternalSubchannelCallback extends InternalSubchannel.Callback {
        // All callbacks are run in syncContext
        @Override
        void onTerminated(InternalSubchannel is) {
          subchannels.remove(is);
          channelz.removeSubchannel(is);
          maybeTerminateChannel();
        }

        @Override
        void onStateChange(InternalSubchannel is, ConnectivityStateInfo newState) {
          checkState(listener != null, "listener is null");
          listener.onSubchannelState(newState);
        }

        @Override
        void onInUse(InternalSubchannel is) {
          inUseStateAggregator.updateObjectInUse(is, true);
        }

        @Override
        void onNotInUse(InternalSubchannel is) {
          inUseStateAggregator.updateObjectInUse(is, false);
        }
      }

      final InternalSubchannel internalSubchannel = new InternalSubchannel(
          args.getAddresses(),
          authority(),
          userAgent,
          backoffPolicyProvider,
          transportFactory,
          transportFactory.getScheduledExecutorService(),
          stopwatchSupplier,
          syncContext,
          new ManagedInternalSubchannelCallback(),
          channelz,
          callTracerFactory.create(),
          subchannelTracer,
          subchannelLogId,
          subchannelLogger,
          transportFilters);

      channelTracer.reportEvent(new ChannelTrace.Event.Builder()
          .setDescription("Child Subchannel started")
          .setSeverity(ChannelTrace.Event.Severity.CT_INFO)
          .setTimestampNanos(timeProvider.currentTimeNanos())
          .setSubchannelRef(internalSubchannel)
          .build());

      this.subchannel = internalSubchannel;
      channelz.addSubchannel(internalSubchannel);
      subchannels.add(internalSubchannel);
    }

    @Override
    InternalInstrumented<ChannelStats> getInstrumentedInternalSubchannel() {
      checkState(started, "not started");
      return subchannel;
    }

    @Override
    public void shutdown() {
      syncContext.throwIfNotInThisSynchronizationContext();
      if (subchannel == null) {
        // start() was not successful
        shutdown = true;
        return;
      }
      if (shutdown) {
        if (terminating && delayedShutdownTask != null) {
          // shutdown() was previously called when terminating == false, thus a delayed shutdown()
          // was scheduled.  Now since terminating == true, We should expedite the shutdown.
          delayedShutdownTask.cancel();
          delayedShutdownTask = null;
          // Will fall through to the subchannel.shutdown() at the end.
        } else {
          return;
        }
      } else {
        shutdown = true;
      }
      // Add a delay to shutdown to deal with the race between 1) a transport being picked and
      // newStream() being called on it, and 2) its Subchannel is shut down by LoadBalancer (e.g.,
      // because of address change, or because LoadBalancer is shutdown by Channel entering idle
      // mode). If (2) wins, the app will see a spurious error. We work around this by delaying
      // shutdown of Subchannel for a few seconds here.
      //
      // TODO(zhangkun83): consider a better approach
      // (https://github.com/grpc/grpc-java/issues/2562).
      if (!terminating) {
        final class ShutdownSubchannel implements Runnable {
          @Override
          public void run() {
            subchannel.shutdown(SUBCHANNEL_SHUTDOWN_STATUS);
          }
        }

        delayedShutdownTask = syncContext.schedule(
            new LogExceptionRunnable(new ShutdownSubchannel()),
            SUBCHANNEL_SHUTDOWN_DELAY_SECONDS, TimeUnit.SECONDS,
            transportFactory.getScheduledExecutorService());
        return;
      }
      // When terminating == true, no more real streams will be created. It's safe and also
      // desirable to shutdown timely.
      subchannel.shutdown(SHUTDOWN_STATUS);
    }

    @Override
    public void requestConnection() {
      syncContext.throwIfNotInThisSynchronizationContext();
      checkState(started, "not started");
      subchannel.obtainActiveTransport();
    }

    @Override
    public List<EquivalentAddressGroup> getAllAddresses() {
      syncContext.throwIfNotInThisSynchronizationContext();
      checkState(started, "not started");
      return addressGroups;
    }

    @Override
    public Attributes getAttributes() {
      return args.getAttributes();
    }

    @Override
    public String toString() {
      return subchannelLogId.toString();
    }

    @Override
    public Channel asChannel() {
      checkState(started, "not started");
      return new SubchannelChannel(
          subchannel, balancerRpcExecutorHolder.getExecutor(),
          transportFactory.getScheduledExecutorService(),
          callTracerFactory.create(),
          new AtomicReference<InternalConfigSelector>(null));
    }

    @Override
    public Object getInternalSubchannel() {
      checkState(started, "Subchannel is not started");
      return subchannel;
    }

    @Override
    public ChannelLogger getChannelLogger() {
      return subchannelLogger;
    }

    @Override
    public void updateAddresses(List<EquivalentAddressGroup> addrs) {
      syncContext.throwIfNotInThisSynchronizationContext();
      addressGroups = addrs;
      if (authorityOverride != null) {
        addrs = stripOverrideAuthorityAttributes(addrs);
      }
      subchannel.updateAddresses(addrs);
    }

    private List<EquivalentAddressGroup> stripOverrideAuthorityAttributes(
        List<EquivalentAddressGroup> eags) {
      List<EquivalentAddressGroup> eagsWithoutOverrideAttr = new ArrayList<>();
      for (EquivalentAddressGroup eag : eags) {
        EquivalentAddressGroup eagWithoutOverrideAttr = new EquivalentAddressGroup(
            eag.getAddresses(),
            eag.getAttributes().toBuilder().discard(ATTR_AUTHORITY_OVERRIDE).build());
        eagsWithoutOverrideAttr.add(eagWithoutOverrideAttr);
      }
      return Collections.unmodifiableList(eagsWithoutOverrideAttr);
    }
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .add("logId", logId.getId())
        .add("target", target)
        .toString();
  }

  /**
   * Called from syncContext.
   */
  private final class DelayedTransportListener implements ManagedClientTransport.Listener {
    @Override
    public void transportShutdown(Status s) {
      checkState(shutdown.get(), "Channel must have been shut down");
    }

    @Override
    public void transportReady() {
      // Don't care
    }

    @Override
    public Attributes filterTransport(Attributes attributes) {
      return attributes;
    }

    @Override
    public void transportInUse(final boolean inUse) {
      inUseStateAggregator.updateObjectInUse(delayedTransport, inUse);
      if (inUse) {
        // It's possible to be in idle mode while inUseStateAggregator is in-use, if one of the
        // subchannels is in use. But we should never be in idle mode when delayed transport is in
        // use.
        exitIdleMode();
      }
    }

    @Override
    public void transportTerminated() {
      checkState(shutdown.get(), "Channel must have been shut down");
      terminating = true;
      shutdownNameResolverAndLoadBalancer(false);
      // No need to call channelStateManager since we are already in SHUTDOWN state.
      // Until LoadBalancer is shutdown, it may still create new subchannels.  We catch them
      // here.
      maybeShutdownNowSubchannels();
      maybeTerminateChannel();
    }
  }

  /**
   * Must be accessed from syncContext.
   */
  private final class IdleModeStateAggregator extends InUseStateAggregator<Object> {
    @Override
    protected void handleInUse() {
      exitIdleMode();
    }

    @Override
    protected void handleNotInUse() {
      if (shutdown.get()) {
        return;
      }
      rescheduleIdleTimer();
    }
  }

  /**
   * Lazily request for Executor from an executor pool.
   * Also act as an Executor directly to simply run a cmd
   */
  @VisibleForTesting
  static final class ExecutorHolder implements Executor {
    private final ObjectPool<? extends Executor> pool;
    private Executor executor;

    ExecutorHolder(ObjectPool<? extends Executor> executorPool) {
      this.pool = checkNotNull(executorPool, "executorPool");
    }

    synchronized Executor getExecutor() {
      if (executor == null) {
        executor = checkNotNull(pool.getObject(), "%s.getObject()", executor);
      }
      return executor;
    }

    synchronized void release() {
      if (executor != null) {
        executor = pool.returnObject(executor);
      }
    }

    @Override
    public void execute(Runnable command) {
      getExecutor().execute(command);
    }
  }

  private static final class RestrictedScheduledExecutor implements ScheduledExecutorService {
    final ScheduledExecutorService delegate;

    private RestrictedScheduledExecutor(ScheduledExecutorService delegate) {
      this.delegate = checkNotNull(delegate, "delegate");
    }

    @Override
    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
      return delegate.schedule(callable, delay, unit);
    }

    @Override
    public ScheduledFuture<?> schedule(Runnable cmd, long delay, TimeUnit unit) {
      return delegate.schedule(cmd, delay, unit);
    }

    @Override
    public ScheduledFuture<?> scheduleAtFixedRate(
        Runnable command, long initialDelay, long period, TimeUnit unit) {
      return delegate.scheduleAtFixedRate(command, initialDelay, period, unit);
    }

    @Override
    public ScheduledFuture<?> scheduleWithFixedDelay(
        Runnable command, long initialDelay, long delay, TimeUnit unit) {
      return delegate.scheduleWithFixedDelay(command, initialDelay, delay, unit);
    }

    @Override
    public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
      return delegate.awaitTermination(timeout, unit);
    }

    @Override
    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException {
      return delegate.invokeAll(tasks);
    }

    @Override
    public <T> List<Future<T>> invokeAll(
        Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
        throws InterruptedException {
      return delegate.invokeAll(tasks, timeout, unit);
    }

    @Override
    public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException {
      return delegate.invokeAny(tasks);
    }

    @Override
    public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
      return delegate.invokeAny(tasks, timeout, unit);
    }

    @Override
    public boolean isShutdown() {
      return delegate.isShutdown();
    }

    @Override
    public boolean isTerminated() {
      return delegate.isTerminated();
    }

    @Override
    public void shutdown() {
      throw new UnsupportedOperationException("Restricted: shutdown() is not allowed");
    }

    @Override
    public List<Runnable> shutdownNow() {
      throw new UnsupportedOperationException("Restricted: shutdownNow() is not allowed");
    }

    @Override
    public <T> Future<T> submit(Callable<T> task) {
      return delegate.submit(task);
    }

    @Override
    public Future<?> submit(Runnable task) {
      return delegate.submit(task);
    }

    @Override
    public <T> Future<T> submit(Runnable task, T result) {
      return delegate.submit(task, result);
    }

    @Override
    public void execute(Runnable command) {
      delegate.execute(command);
    }
  }

  /**
   * A ResolutionState indicates the status of last name resolution.
   */
  enum ResolutionState {
    NO_RESOLUTION,
    SUCCESS,
    ERROR
  }
}

grpc / grpc-java / #19390

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