#19680

Committed 05 Feb 2025 10:18PM CUT coverage: 88.617% (+0.03%) from 88.584%

Build # #19680

Build Type

push

github

Committed by

web-flow

Commit Message

core: updates the backoff range as per the A6 redefinition (#11858)

* core: updates the backoff range being used from [0, 1] to [0.8, 1.2] as per the A6 redefinition

* adds a flag for experimental jitter

* xds: Allow FaultFilter's interceptor to be reused

This is the only usage of PickSubchannelArgs when creating a filter's
ClientInterceptor, and a follow-up commit will remove the argument and
actually reuse the interceptors. Other filter's interceptors can
already be reused.

There doesn't seem to be any significant loss of legibility by making
FaultFilter a more ordinary interceptor, but the change does cause the
ForwardingClientCall to be present when faultDelay is configured,
independent of whether the fault delay ends up being triggered.

Reusing interceptors will move more state management out of the RPC path
which will be more relevant with RLQS.

* netty: Removed 4096 min buffer size (#11856)

* netty: Removed 4096 min buffer size

* turns the flag in a var for better efficiency

---------

Co-authored-by: Eric Anderson <ejona@google.com>

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95.5

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

/*
 * Copyright 2017 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 com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Objects;
import com.google.errorprone.annotations.CheckReturnValue;
import com.google.errorprone.annotations.concurrent.GuardedBy;
import io.grpc.Attributes;
import io.grpc.ClientStreamTracer;
import io.grpc.Compressor;
import io.grpc.Deadline;
import io.grpc.DecompressorRegistry;
import io.grpc.Metadata;
import io.grpc.MethodDescriptor;
import io.grpc.Status;
import io.grpc.SynchronizationContext;
import io.grpc.internal.ClientStreamListener.RpcProgress;
import java.io.InputStream;
import java.lang.Thread.UncaughtExceptionHandler;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Random;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import javax.annotation.CheckForNull;
import javax.annotation.Nullable;

/** A logical {@link ClientStream} that is retriable. */
abstract class RetriableStream<ReqT> implements ClientStream {
  @VisibleForTesting
  static final Metadata.Key<String> GRPC_PREVIOUS_RPC_ATTEMPTS =
      Metadata.Key.of("grpc-previous-rpc-attempts", Metadata.ASCII_STRING_MARSHALLER);

  @VisibleForTesting
  static final Metadata.Key<String> GRPC_RETRY_PUSHBACK_MS =
      Metadata.Key.of("grpc-retry-pushback-ms", Metadata.ASCII_STRING_MARSHALLER);

  private static final Status CANCELLED_BECAUSE_COMMITTED =
      Status.CANCELLED.withDescription("Stream thrown away because RetriableStream committed");

  private final MethodDescriptor<ReqT, ?> method;
  private final Executor callExecutor;
  private final Executor listenerSerializeExecutor = new SynchronizationContext(
      new UncaughtExceptionHandler() {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
          throw Status.fromThrowable(e)
              .withDescription("Uncaught exception in the SynchronizationContext. Re-thrown.")
              .asRuntimeException();
        }
      }
  );
  private final ScheduledExecutorService scheduledExecutorService;
  // Must not modify it.
  private final Metadata headers;
  @Nullable
  private final RetryPolicy retryPolicy;
  @Nullable
  private final HedgingPolicy hedgingPolicy;
  private final boolean isHedging;

  /** Must be held when updating state, accessing state.buffer, or certain substream attributes. */
  private final Object lock = new Object();

  private final ChannelBufferMeter channelBufferUsed;
  private final long perRpcBufferLimit;
  private final long channelBufferLimit;
  @Nullable
  private final Throttle throttle;
  @GuardedBy("lock")
  private final InsightBuilder closedSubstreamsInsight = new InsightBuilder();

  private volatile State state = new State(
      new ArrayList<BufferEntry>(8), Collections.<Substream>emptyList(), null, null, false, false,
      false, 0);

  /**
   * Either non-local transparent retry happened or reached server's application logic.
   *
   * <p>Note that local-only transparent retries are unlimited.
   */
  private final AtomicBoolean noMoreTransparentRetry = new AtomicBoolean();
  private final AtomicInteger localOnlyTransparentRetries = new AtomicInteger();
  private final AtomicInteger inFlightSubStreams = new AtomicInteger();
  private SavedCloseMasterListenerReason savedCloseMasterListenerReason;

  // Used for recording the share of buffer used for the current call out of the channel buffer.
  // This field would not be necessary if there is no channel buffer limit.
  @GuardedBy("lock")
  private long perRpcBufferUsed;

  private ClientStreamListener masterListener;
  @GuardedBy("lock")
  private FutureCanceller scheduledRetry;
  @GuardedBy("lock")
  private FutureCanceller scheduledHedging;
  private long nextBackoffIntervalNanos;
  private Status cancellationStatus;
  private boolean isClosed;

  RetriableStream(
      MethodDescriptor<ReqT, ?> method, Metadata headers,
      ChannelBufferMeter channelBufferUsed, long perRpcBufferLimit, long channelBufferLimit,
      Executor callExecutor, ScheduledExecutorService scheduledExecutorService,
      @Nullable RetryPolicy retryPolicy, @Nullable HedgingPolicy hedgingPolicy,
      @Nullable Throttle throttle) {
    this.method = method;
    this.channelBufferUsed = channelBufferUsed;
    this.perRpcBufferLimit = perRpcBufferLimit;
    this.channelBufferLimit = channelBufferLimit;
    this.callExecutor = callExecutor;
    this.scheduledExecutorService = scheduledExecutorService;
    this.headers = headers;
    this.retryPolicy = retryPolicy;
    if (retryPolicy != null) {
      this.nextBackoffIntervalNanos = retryPolicy.initialBackoffNanos;
    }
    this.hedgingPolicy = hedgingPolicy;
    checkArgument(
        retryPolicy == null || hedgingPolicy == null,
        "Should not provide both retryPolicy and hedgingPolicy");
    this.isHedging = hedgingPolicy != null;
    this.throttle = throttle;
  }

  @SuppressWarnings("GuardedBy")  // TODO(b/145386688) this.lock==ScheduledCancellor.lock so ok
  @Nullable // null if already committed
  @CheckReturnValue
  private Runnable commit(final Substream winningSubstream) {
    synchronized (lock) {
      if (state.winningSubstream != null) {
        return null;
      }
      final Collection<Substream> savedDrainedSubstreams = state.drainedSubstreams;

      state = state.committed(winningSubstream);

      // subtract the share of this RPC from channelBufferUsed.
      channelBufferUsed.addAndGet(-perRpcBufferUsed);

      final boolean wasCancelled = (scheduledRetry != null) ? scheduledRetry.isCancelled() : false;
      final Future<?> retryFuture;
      if (scheduledRetry != null) {
        retryFuture = scheduledRetry.markCancelled();
        scheduledRetry = null;
      } else {
        retryFuture = null;
      }
      // cancel the scheduled hedging if it is scheduled prior to the commitment
      final Future<?> hedgingFuture;
      if (scheduledHedging != null) {
        hedgingFuture = scheduledHedging.markCancelled();
        scheduledHedging = null;
      } else {
        hedgingFuture = null;
      }

      class CommitTask implements Runnable {
        @Override
        public void run() {
          // For hedging only, not needed for normal retry
          for (Substream substream : savedDrainedSubstreams) {
            if (substream != winningSubstream) {
              substream.stream.cancel(CANCELLED_BECAUSE_COMMITTED);
            }
          }
          if (retryFuture != null) {
            retryFuture.cancel(false);
            if (!wasCancelled && inFlightSubStreams.decrementAndGet() == Integer.MIN_VALUE) {
              assert savedCloseMasterListenerReason != null;
              listenerSerializeExecutor.execute(
                  new Runnable() {
                    @Override
                    public void run() {
                      isClosed = true;
                      masterListener.closed(savedCloseMasterListenerReason.status,
                          savedCloseMasterListenerReason.progress,
                          savedCloseMasterListenerReason.metadata);
                    }
                  });
            }
          }

          if (hedgingFuture != null) {
            hedgingFuture.cancel(false);
          }

          postCommit();
        }
      }

      return new CommitTask();
    }
  }

  abstract void postCommit();

  /**
   * Calls commit() and if successful runs the post commit task. Post commit task will be non-null
   * for only once. The post commit task cancels other non-winning streams on separate transport
   * threads, thus it must be run on the callExecutor to prevent deadlocks between multiple stream
   * transports.(issues/10314)
   * This method should be called only in subListener callbacks. This guarantees callExecutor
   * schedules tasks before master listener closes, which is protected by the inFlightSubStreams
   * decorative. That is because:
   * For a successful winning stream, other streams won't attempt to close master listener.
   * For a cancelled winning stream (noop), other stream won't attempt to close master listener.
   * For a failed/closed winning stream, the last closed stream closes the master listener, and
   * callExecutor scheduling happens-before that.
   */
  private void commitAndRun(Substream winningSubstream) {
    Runnable postCommitTask = commit(winningSubstream);

    if (postCommitTask != null) {
      callExecutor.execute(postCommitTask);
    }
  }

  // returns null means we should not create new sub streams, e.g. cancelled or
  // other close condition is met for retriableStream.
  @Nullable
  private Substream createSubstream(int previousAttemptCount, boolean isTransparentRetry) {
    int inFlight;
    do {
      inFlight = inFlightSubStreams.get();
      if (inFlight < 0) {
        return null;
      }
    } while (!inFlightSubStreams.compareAndSet(inFlight, inFlight + 1));
    Substream sub = new Substream(previousAttemptCount);
    // one tracer per substream
    final ClientStreamTracer bufferSizeTracer = new BufferSizeTracer(sub);
    ClientStreamTracer.Factory tracerFactory = new ClientStreamTracer.Factory() {
      @Override
      public ClientStreamTracer newClientStreamTracer(
          ClientStreamTracer.StreamInfo info, Metadata headers) {
        return bufferSizeTracer;
      }
    };

    Metadata newHeaders = updateHeaders(headers, previousAttemptCount);
    // NOTICE: This set _must_ be done before stream.start() and it actually is.
    sub.stream = newSubstream(newHeaders, tracerFactory, previousAttemptCount, isTransparentRetry);
    return sub;
  }

  /**
   * Creates a new physical ClientStream that represents a retry/hedging attempt. The returned
   * Client stream is not yet started.
   */
  abstract ClientStream newSubstream(
      Metadata headers, ClientStreamTracer.Factory tracerFactory, int previousAttempts,
      boolean isTransparentRetry);

  /** Adds grpc-previous-rpc-attempts in the headers of a retry/hedging RPC. */
  @VisibleForTesting
  final Metadata updateHeaders(
      Metadata originalHeaders, int previousAttemptCount) {
    Metadata newHeaders = new Metadata();
    newHeaders.merge(originalHeaders);
    if (previousAttemptCount > 0) {
      newHeaders.put(GRPC_PREVIOUS_RPC_ATTEMPTS, String.valueOf(previousAttemptCount));
    }
    return newHeaders;
  }

  private void drain(Substream substream) {
    int index = 0;
    int chunk = 0x80;
    List<BufferEntry> list = null;
    boolean streamStarted = false;
    Runnable onReadyRunnable = null;

    while (true) {
      State savedState;

      synchronized (lock) {
        savedState = state;
        if (savedState.winningSubstream != null && savedState.winningSubstream != substream) {
          // committed but not me, to be cancelled
          break;
        }
        if (savedState.cancelled) {
          break;
        }
        if (index == savedState.buffer.size()) { // I'm drained
          state = savedState.substreamDrained(substream);
          if (!isReady()) {
            return;
          }
          onReadyRunnable = new Runnable() {
            @Override
            public void run() {
              if (!isClosed) {
                masterListener.onReady();
              }
            }
          };
          break;
        }

        if (substream.closed) {
          return;
        }

        int stop = Math.min(index + chunk, savedState.buffer.size());
        if (list == null) {
          list = new ArrayList<>(savedState.buffer.subList(index, stop));
        } else {
          list.clear();
          list.addAll(savedState.buffer.subList(index, stop));
        }
        index = stop;
      }

      for (BufferEntry bufferEntry : list) {
        bufferEntry.runWith(substream);
        if (bufferEntry instanceof RetriableStream.StartEntry) {
          streamStarted = true;
        }
        savedState = state;
        if (savedState.winningSubstream != null && savedState.winningSubstream != substream) {
          // committed but not me, to be cancelled
          break;
        }
        if (savedState.cancelled) {
          break;
        }
      }
    }

    if (onReadyRunnable != null) {
      listenerSerializeExecutor.execute(onReadyRunnable);
      return;
    }

    if (!streamStarted) {
      // Start stream so inFlightSubStreams is decremented in Sublistener.closed()
      substream.stream.start(new Sublistener(substream));
    }
    substream.stream.cancel(
        state.winningSubstream == substream ? cancellationStatus : CANCELLED_BECAUSE_COMMITTED);
  }

  /**
   * Runs pre-start tasks. Returns the Status of shutdown if the channel is shutdown.
   */
  @CheckReturnValue
  @Nullable
  abstract Status prestart();

  class StartEntry implements BufferEntry {
    @Override
    public void runWith(Substream substream) {
      substream.stream.start(new Sublistener(substream));
    }
  }

  /** Starts the first PRC attempt. */
  @Override
  public final void start(ClientStreamListener listener) {
    masterListener = listener;

    Status shutdownStatus = prestart();

    if (shutdownStatus != null) {
      cancel(shutdownStatus);
      return;
    }

    synchronized (lock) {
      state.buffer.add(new StartEntry());
    }

    Substream substream = createSubstream(0, false);
    if (substream == null) {
      return;
    }
    if (isHedging) {
      FutureCanceller scheduledHedgingRef = null;

      synchronized (lock) {
        state = state.addActiveHedge(substream);
        if (hasPotentialHedging(state)
            && (throttle == null || throttle.isAboveThreshold())) {
          scheduledHedging = scheduledHedgingRef = new FutureCanceller(lock);
        }
      }

      if (scheduledHedgingRef != null) {
        scheduledHedgingRef.setFuture(
            scheduledExecutorService.schedule(
                new HedgingRunnable(scheduledHedgingRef),
                hedgingPolicy.hedgingDelayNanos,
                TimeUnit.NANOSECONDS));
      }
    }

    drain(substream);
  }

  @SuppressWarnings("GuardedBy")  // TODO(b/145386688) this.lock==ScheduledCancellor.lock so ok
  private void pushbackHedging(@Nullable Integer delayMillis) {
    if (delayMillis == null) {
      return;
    }
    if (delayMillis < 0) {
      freezeHedging();
      return;
    }

    // Cancels the current scheduledHedging and reschedules a new one.
    FutureCanceller future;
    Future<?> futureToBeCancelled;

    synchronized (lock) {
      if (scheduledHedging == null) {
        return;
      }

      futureToBeCancelled = scheduledHedging.markCancelled();
      scheduledHedging = future = new FutureCanceller(lock);
    }

    if (futureToBeCancelled != null) {
      futureToBeCancelled.cancel(false);
    }
    future.setFuture(scheduledExecutorService.schedule(
        new HedgingRunnable(future), delayMillis, TimeUnit.MILLISECONDS));
  }

  private final class HedgingRunnable implements Runnable {

    // Need to hold a ref to the FutureCanceller in case RetriableStrea.scheduledHedging is renewed
    // by a positive push-back just after newSubstream is instantiated, so that we can double check.
    final FutureCanceller scheduledHedgingRef;

    HedgingRunnable(FutureCanceller scheduledHedging) {
      scheduledHedgingRef = scheduledHedging;
    }

    @Override
    public void run() {
      // It's safe to read state.hedgingAttemptCount here.
      // If this run is not cancelled, the value of state.hedgingAttemptCount won't change
      // until state.addActiveHedge() is called subsequently, even the state could possibly
      // change.
      Substream newSubstream = createSubstream(state.hedgingAttemptCount, false);
      if (newSubstream == null) {
        return;
      }
      callExecutor.execute(
          new Runnable() {
            @SuppressWarnings("GuardedBy")  //TODO(b/145386688) lock==ScheduledCancellor.lock so ok
            @Override
            public void run() {
              boolean cancelled = false;
              FutureCanceller future = null;

              synchronized (lock) {
                if (scheduledHedgingRef.isCancelled()) {
                  cancelled = true;
                } else {
                  state = state.addActiveHedge(newSubstream);
                  if (hasPotentialHedging(state)
                      && (throttle == null || throttle.isAboveThreshold())) {
                    scheduledHedging = future = new FutureCanceller(lock);
                  } else {
                    state = state.freezeHedging();
                    scheduledHedging = null;
                  }
                }
              }

              if (cancelled) {
                // Start stream so inFlightSubStreams is decremented in Sublistener.closed()
                newSubstream.stream.start(new Sublistener(newSubstream));
                newSubstream.stream.cancel(Status.CANCELLED.withDescription("Unneeded hedging"));
                return;
              }
              if (future != null) {
                future.setFuture(
                    scheduledExecutorService.schedule(
                        new HedgingRunnable(future),
                        hedgingPolicy.hedgingDelayNanos,
                        TimeUnit.NANOSECONDS));
              }
              drain(newSubstream);
            }
          });
    }
  }

  @Override
  public final void cancel(final Status reason) {
    Substream noopSubstream = new Substream(0 /* previousAttempts doesn't matter here */);
    noopSubstream.stream = new NoopClientStream();
    Runnable runnable = commit(noopSubstream);

    if (runnable != null) {
      synchronized (lock) {
        state = state.substreamDrained(noopSubstream);
      }
      runnable.run();
      safeCloseMasterListener(reason, RpcProgress.PROCESSED, new Metadata());
      return;
    }

    Substream winningSubstreamToCancel = null;
    synchronized (lock) {
      if (state.drainedSubstreams.contains(state.winningSubstream)) {
        winningSubstreamToCancel = state.winningSubstream;
      } else { // the winningSubstream will be cancelled while draining
        cancellationStatus = reason;
      }
      state = state.cancelled();
    }
    if (winningSubstreamToCancel != null) {
      winningSubstreamToCancel.stream.cancel(reason);
    }
  }

  private void delayOrExecute(BufferEntry bufferEntry) {
    Collection<Substream> savedDrainedSubstreams;
    synchronized (lock) {
      if (!state.passThrough) {
        state.buffer.add(bufferEntry);
      }
      savedDrainedSubstreams = state.drainedSubstreams;
    }

    for (Substream substream : savedDrainedSubstreams) {
      bufferEntry.runWith(substream);
    }
  }

  /**
   * Do not use it directly. Use {@link #sendMessage(Object)} instead because we don't use
   * InputStream for buffering.
   */
  @Override
  public final void writeMessage(InputStream message) {
    throw new IllegalStateException("RetriableStream.writeMessage() should not be called directly");
  }

  final void sendMessage(final ReqT message) {
    State savedState = state;
    if (savedState.passThrough) {
      savedState.winningSubstream.stream.writeMessage(method.streamRequest(message));
      return;
    }

    class SendMessageEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.writeMessage(method.streamRequest(message));
        // TODO(ejona): Workaround Netty memory leak. Message writes always need to be followed by
        // flushes (or half close), but retry appears to have a code path that the flushes may
        // not happen. The code needs to be fixed and this removed. See #9340.
        substream.stream.flush();
      }
    }

    delayOrExecute(new SendMessageEntry());
  }

  @Override
  public final void request(final int numMessages) {
    State savedState = state;
    if (savedState.passThrough) {
      savedState.winningSubstream.stream.request(numMessages);
      return;
    }

    class RequestEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.request(numMessages);
      }
    }

    delayOrExecute(new RequestEntry());
  }

  @Override
  public final void flush() {
    State savedState = state;
    if (savedState.passThrough) {
      savedState.winningSubstream.stream.flush();
      return;
    }

    class FlushEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.flush();
      }
    }

    delayOrExecute(new FlushEntry());
  }

  @Override
  public final boolean isReady() {
    for (Substream substream : state.drainedSubstreams) {
      if (substream.stream.isReady()) {
        return true;
      }
    }
    return false;
  }

  @Override
  public void optimizeForDirectExecutor() {
    class OptimizeDirectEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.optimizeForDirectExecutor();
      }
    }

    delayOrExecute(new OptimizeDirectEntry());
  }

  @Override
  public final void setCompressor(final Compressor compressor) {
    class CompressorEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setCompressor(compressor);
      }
    }

    delayOrExecute(new CompressorEntry());
  }

  @Override
  public final void setFullStreamDecompression(final boolean fullStreamDecompression) {
    class FullStreamDecompressionEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setFullStreamDecompression(fullStreamDecompression);
      }
    }

    delayOrExecute(new FullStreamDecompressionEntry());
  }

  @Override
  public final void setMessageCompression(final boolean enable) {
    class MessageCompressionEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setMessageCompression(enable);
      }
    }

    delayOrExecute(new MessageCompressionEntry());
  }

  @Override
  public final void halfClose() {
    class HalfCloseEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.halfClose();
      }
    }

    delayOrExecute(new HalfCloseEntry());
  }

  @Override
  public final void setAuthority(final String authority) {
    class AuthorityEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setAuthority(authority);
      }
    }

    delayOrExecute(new AuthorityEntry());
  }

  @Override
  public final void setDecompressorRegistry(final DecompressorRegistry decompressorRegistry) {
    class DecompressorRegistryEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setDecompressorRegistry(decompressorRegistry);
      }
    }

    delayOrExecute(new DecompressorRegistryEntry());
  }

  @Override
  public final void setMaxInboundMessageSize(final int maxSize) {
    class MaxInboundMessageSizeEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setMaxInboundMessageSize(maxSize);
      }
    }

    delayOrExecute(new MaxInboundMessageSizeEntry());
  }

  @Override
  public final void setMaxOutboundMessageSize(final int maxSize) {
    class MaxOutboundMessageSizeEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setMaxOutboundMessageSize(maxSize);
      }
    }

    delayOrExecute(new MaxOutboundMessageSizeEntry());
  }

  @Override
  public final void setDeadline(final Deadline deadline) {
    class DeadlineEntry implements BufferEntry {
      @Override
      public void runWith(Substream substream) {
        substream.stream.setDeadline(deadline);
      }
    }

    delayOrExecute(new DeadlineEntry());
  }

  @Override
  public final Attributes getAttributes() {
    if (state.winningSubstream != null) {
      return state.winningSubstream.stream.getAttributes();
    }
    return Attributes.EMPTY;
  }

  @Override
  public void appendTimeoutInsight(InsightBuilder insight) {
    State currentState;
    synchronized (lock) {
      insight.appendKeyValue("closed", closedSubstreamsInsight);
      currentState = state;
    }
    if (currentState.winningSubstream != null) {
      // TODO(zhangkun83): in this case while other drained substreams have been cancelled in favor
      // of the winning substream, they may not have received closed() notifications yet, thus they
      // may be missing from closedSubstreamsInsight.  This may be a little confusing to the user.
      // We need to figure out how to include them.
      InsightBuilder substreamInsight = new InsightBuilder();
      currentState.winningSubstream.stream.appendTimeoutInsight(substreamInsight);
      insight.appendKeyValue("committed", substreamInsight);
    } else {
      InsightBuilder openSubstreamsInsight = new InsightBuilder();
      // drainedSubstreams doesn't include all open substreams.  Those which have just been created
      // and are still catching up with buffered requests (in other words, still draining) will not
      // show up.  We think this is benign, because the draining should be typically fast, and it'd
      // be indistinguishable from the case where those streams are to be created a little late due
      // to delays in the timer.
      for (Substream sub : currentState.drainedSubstreams) {
        InsightBuilder substreamInsight = new InsightBuilder();
        sub.stream.appendTimeoutInsight(substreamInsight);
        openSubstreamsInsight.append(substreamInsight);
      }
      insight.appendKeyValue("open", openSubstreamsInsight);
    }
  }

  private static Random random = new Random();

  @VisibleForTesting
  static void setRandom(Random random) {
    RetriableStream.random = random;
  }

  /**
   * Whether there is any potential hedge at the moment. A false return value implies there is
   * absolutely no potential hedge. At least one of the hedges will observe a false return value
   * when calling this method, unless otherwise the rpc is committed.
   */
  // only called when isHedging is true
  @GuardedBy("lock")
  private boolean hasPotentialHedging(State state) {
    return state.winningSubstream == null
        && state.hedgingAttemptCount < hedgingPolicy.maxAttempts
        && !state.hedgingFrozen;
  }

  @SuppressWarnings("GuardedBy")  // TODO(b/145386688) this.lock==ScheduledCancellor.lock so ok
  private void freezeHedging() {
    Future<?> futureToBeCancelled = null;
    synchronized (lock) {
      if (scheduledHedging != null) {
        futureToBeCancelled = scheduledHedging.markCancelled();
        scheduledHedging = null;
      }
      state = state.freezeHedging();
    }

    if (futureToBeCancelled != null) {
      futureToBeCancelled.cancel(false);
    }
  }

  private void safeCloseMasterListener(Status status, RpcProgress progress, Metadata metadata) {
    savedCloseMasterListenerReason = new SavedCloseMasterListenerReason(status, progress,
        metadata);
    if (inFlightSubStreams.addAndGet(Integer.MIN_VALUE) == Integer.MIN_VALUE) {
      listenerSerializeExecutor.execute(
          new Runnable() {
            @Override
            public void run() {
              isClosed = true;
              masterListener.closed(status, progress, metadata);
            }
          });
    }
  }

  private static final boolean isExperimentalRetryJitterEnabled = GrpcUtil
          .getFlag("GRPC_EXPERIMENTAL_XDS_RLS_LB", true);

  public static long intervalWithJitter(long intervalNanos) {
    double inverseJitterFactor = isExperimentalRetryJitterEnabled
            ? 0.8 * random.nextDouble() + 0.4 : random.nextDouble();
    return (long) (intervalNanos * inverseJitterFactor);
  }

  private static final class SavedCloseMasterListenerReason {
    private final Status status;
    private final RpcProgress progress;
    private final Metadata metadata;

    SavedCloseMasterListenerReason(Status status, RpcProgress progress, Metadata metadata) {
      this.status = status;
      this.progress = progress;
      this.metadata = metadata;
    }
  }

  private interface BufferEntry {
    /** Replays the buffer entry with the given stream. */
    void runWith(Substream substream);
  }

  private final class Sublistener implements ClientStreamListener {
    final Substream substream;

    Sublistener(Substream substream) {
      this.substream = substream;
    }

    @Override
    public void headersRead(final Metadata headers) {
      if (substream.previousAttemptCount > 0) {
        headers.discardAll(GRPC_PREVIOUS_RPC_ATTEMPTS);
        headers.put(GRPC_PREVIOUS_RPC_ATTEMPTS, String.valueOf(substream.previousAttemptCount));
      }
      commitAndRun(substream);
      if (state.winningSubstream == substream) {
        if (throttle != null) {
          throttle.onSuccess();
        }
        listenerSerializeExecutor.execute(
            new Runnable() {
              @Override
              public void run() {
                masterListener.headersRead(headers);
              }
            });
      }
    }

    @Override
    public void closed(
        final Status status, final RpcProgress rpcProgress, final Metadata trailers) {
      synchronized (lock) {
        state = state.substreamClosed(substream);
        closedSubstreamsInsight.append(status.getCode());
      }

      if (inFlightSubStreams.decrementAndGet() == Integer.MIN_VALUE) {
        assert savedCloseMasterListenerReason != null;
        listenerSerializeExecutor.execute(
            new Runnable() {
              @Override
              public void run() {
                isClosed = true;
                masterListener.closed(savedCloseMasterListenerReason.status,
                    savedCloseMasterListenerReason.progress,
                    savedCloseMasterListenerReason.metadata);
              }
            });
        return;
      }

      // handle a race between buffer limit exceeded and closed, when setting
      // substream.bufferLimitExceeded = true happens before state.substreamClosed(substream).
      if (substream.bufferLimitExceeded) {
        commitAndRun(substream);
        if (state.winningSubstream == substream) {
          safeCloseMasterListener(status, rpcProgress, trailers);
        }
        return;
      }
      if (rpcProgress == RpcProgress.MISCARRIED
          && localOnlyTransparentRetries.incrementAndGet() > 1_000) {
        commitAndRun(substream);
        if (state.winningSubstream == substream) {
          Status tooManyTransparentRetries = Status.INTERNAL
              .withDescription("Too many transparent retries. Might be a bug in gRPC")
              .withCause(status.asRuntimeException());
          safeCloseMasterListener(tooManyTransparentRetries, rpcProgress, trailers);
        }
        return;
      }

      if (state.winningSubstream == null) {
        if (rpcProgress == RpcProgress.MISCARRIED
            || (rpcProgress == RpcProgress.REFUSED
                && noMoreTransparentRetry.compareAndSet(false, true))) {
          // transparent retry
          final Substream newSubstream = createSubstream(substream.previousAttemptCount, true);
          if (newSubstream == null) {
            return;
          }
          if (isHedging) {
            synchronized (lock) {
              // Although this operation is not done atomically with
              // noMoreTransparentRetry.compareAndSet(false, true), it does not change the size() of
              // activeHedges, so neither does it affect the commitment decision of other threads,
              // nor do the commitment decision making threads affect itself.
              state = state.replaceActiveHedge(substream, newSubstream);
            }
          }

          callExecutor.execute(new Runnable() {
            @Override
            public void run() {
              drain(newSubstream);
            }
          });
          return;
        } else if (rpcProgress == RpcProgress.DROPPED) {
          // For normal retry, nothing need be done here, will just commit.
          // For hedging, cancel scheduled hedge that is scheduled prior to the drop
          if (isHedging) {
            freezeHedging();
          }
        } else {
          noMoreTransparentRetry.set(true);

          if (isHedging) {
            HedgingPlan hedgingPlan = makeHedgingDecision(status, trailers);
            if (hedgingPlan.isHedgeable) {
              pushbackHedging(hedgingPlan.hedgingPushbackMillis);
            }
            synchronized (lock) {
              state = state.removeActiveHedge(substream);
              // The invariant is whether or not #(Potential Hedge + active hedges) > 0.
              // Once hasPotentialHedging(state) is false, it will always be false, and then
              // #(state.activeHedges) will be decreasing. This guarantees that even there may be
              // multiple concurrent hedges, one of the hedges will end up committed.
              if (hedgingPlan.isHedgeable) {
                if (hasPotentialHedging(state) || !state.activeHedges.isEmpty()) {
                  return;
                }
                // else, no activeHedges, no new hedges possible, try to commit
              } // else, isHedgeable is false, try to commit
            }
          } else {
            RetryPlan retryPlan = makeRetryDecision(status, trailers);
            if (retryPlan.shouldRetry) {
              // retry
              Substream newSubstream = createSubstream(substream.previousAttemptCount + 1, false);
              if (newSubstream == null) {
                return;
              }
              // The check state.winningSubstream == null, checking if is not already committed, is
              // racy, but is still safe b/c the retry will also handle committed/cancellation
              FutureCanceller scheduledRetryCopy;
              synchronized (lock) {
                scheduledRetry = scheduledRetryCopy = new FutureCanceller(lock);
              }

              class RetryBackoffRunnable implements Runnable {
                @Override
                @SuppressWarnings("FutureReturnValueIgnored")
                public void run() {
                  synchronized (scheduledRetryCopy.lock) {
                    if (scheduledRetryCopy.isCancelled()) {
                      return;
                    } else {
                      scheduledRetryCopy.markCancelled();
                    }
                  }

                  callExecutor.execute(
                      new Runnable() {
                        @Override
                        public void run() {
                          drain(newSubstream);
                        }
                      });
                }
              }

              scheduledRetryCopy.setFuture(
                  scheduledExecutorService.schedule(
                      new RetryBackoffRunnable(),
                      retryPlan.backoffNanos,
                      TimeUnit.NANOSECONDS));
              return;
            }
          }
        }
      }

      commitAndRun(substream);
      if (state.winningSubstream == substream) {
        safeCloseMasterListener(status, rpcProgress, trailers);
      }
    }

    /**
     * Decides in current situation whether or not the RPC should retry and if it should retry how
     * long the backoff should be. The decision does not take the commitment status into account, so
     * caller should check it separately. It also updates the throttle. It does not change state.
     */
    private RetryPlan makeRetryDecision(Status status, Metadata trailer) {
      if (retryPolicy == null) {
        return new RetryPlan(false, 0);
      }
      boolean shouldRetry = false;
      long backoffNanos = 0L;
      boolean isRetryableStatusCode = retryPolicy.retryableStatusCodes.contains(status.getCode());
      Integer pushbackMillis = getPushbackMills(trailer);
      boolean isThrottled = false;
      if (throttle != null) {
        if (isRetryableStatusCode || (pushbackMillis != null && pushbackMillis < 0)) {
          isThrottled = !throttle.onQualifiedFailureThenCheckIsAboveThreshold();
        }
      }

      if (retryPolicy.maxAttempts > substream.previousAttemptCount + 1 && !isThrottled) {
        if (pushbackMillis == null) {
          if (isRetryableStatusCode) {
            shouldRetry = true;
            backoffNanos = intervalWithJitter(nextBackoffIntervalNanos);
            nextBackoffIntervalNanos = Math.min(
                (long) (nextBackoffIntervalNanos * retryPolicy.backoffMultiplier),
                retryPolicy.maxBackoffNanos);
          } // else no retry
        } else if (pushbackMillis >= 0) {
          shouldRetry = true;
          backoffNanos = TimeUnit.MILLISECONDS.toNanos(pushbackMillis);
          nextBackoffIntervalNanos = retryPolicy.initialBackoffNanos;
        } // else no retry
      } // else no retry

      return new RetryPlan(shouldRetry, backoffNanos);
    }

    private HedgingPlan makeHedgingDecision(Status status, Metadata trailer) {
      Integer pushbackMillis = getPushbackMills(trailer);
      boolean isFatal = !hedgingPolicy.nonFatalStatusCodes.contains(status.getCode());
      boolean isThrottled = false;
      if (throttle != null) {
        if (!isFatal || (pushbackMillis != null && pushbackMillis < 0)) {
          isThrottled = !throttle.onQualifiedFailureThenCheckIsAboveThreshold();
        }
      }
      if (!isFatal && !isThrottled && !status.isOk()
          && (pushbackMillis != null && pushbackMillis > 0)) {
        pushbackMillis = 0; // We want the retry after a nonfatal error to be immediate
      }
      return new HedgingPlan(!isFatal && !isThrottled, pushbackMillis);
    }

    @Nullable
    private Integer getPushbackMills(Metadata trailer) {
      String pushbackStr = trailer.get(GRPC_RETRY_PUSHBACK_MS);
      Integer pushbackMillis = null;
      if (pushbackStr != null) {
        try {
          pushbackMillis = Integer.valueOf(pushbackStr);
        } catch (NumberFormatException e) {
          pushbackMillis = -1;
        }
      }
      return pushbackMillis;
    }

    @Override
    public void messagesAvailable(final MessageProducer producer) {
      State savedState = state;
      checkState(
          savedState.winningSubstream != null, "Headers should be received prior to messages.");
      if (savedState.winningSubstream != substream) {
        GrpcUtil.closeQuietly(producer);
        return;
      }
      listenerSerializeExecutor.execute(
          new Runnable() {
            @Override
            public void run() {
              masterListener.messagesAvailable(producer);
            }
          });
    }

    @Override
    public void onReady() {
      // FIXME(#7089): hedging case is broken.
      if (!isReady()) {
        return;
      }
      listenerSerializeExecutor.execute(
          new Runnable() {
            @Override
            public void run() {
              if (!isClosed) {
                masterListener.onReady();
              }
            }
          });
    }
  }

  private static final class State {
    /** Committed and the winning substream drained. */
    final boolean passThrough;

    /** A list of buffered ClientStream runnables. Set to Null once passThrough. */
    @Nullable final List<BufferEntry> buffer;

    /**
     * Unmodifiable collection of all the open substreams that are drained. Singleton once
     * passThrough; Empty if committed but not passTrough.
     */
    final Collection<Substream> drainedSubstreams;

    /**
     * Unmodifiable collection of all the active hedging substreams.
     *
     * <p>A substream even with the attribute substream.closed being true may be considered still
     * "active" at the moment as long as it is in this collection.
     */
    final Collection<Substream> activeHedges; // not null once isHedging = true

    final int hedgingAttemptCount;

    /** Null until committed. */
    @Nullable final Substream winningSubstream;

    /** Not required to set to true when cancelled, but can short-circuit the draining process. */
    final boolean cancelled;

    /** No more hedging due to events like drop or pushback. */
    final boolean hedgingFrozen;

    State(
        @Nullable List<BufferEntry> buffer,
        Collection<Substream> drainedSubstreams,
        Collection<Substream> activeHedges,
        @Nullable Substream winningSubstream,
        boolean cancelled,
        boolean passThrough,
        boolean hedgingFrozen,
        int hedgingAttemptCount) {
      this.buffer = buffer;
      this.drainedSubstreams =
          checkNotNull(drainedSubstreams, "drainedSubstreams");
      this.winningSubstream = winningSubstream;
      this.activeHedges = activeHedges;
      this.cancelled = cancelled;
      this.passThrough = passThrough;
      this.hedgingFrozen = hedgingFrozen;
      this.hedgingAttemptCount = hedgingAttemptCount;

      checkState(!passThrough || buffer == null, "passThrough should imply buffer is null");
      checkState(
          !passThrough || winningSubstream != null,
          "passThrough should imply winningSubstream != null");
      checkState(
          !passThrough
              || (drainedSubstreams.size() == 1 && drainedSubstreams.contains(winningSubstream))
              || (drainedSubstreams.size() == 0 && winningSubstream.closed),
          "passThrough should imply winningSubstream is drained");
      checkState(!cancelled || winningSubstream != null, "cancelled should imply committed");
    }

    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    State cancelled() {
      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, true, passThrough,
          hedgingFrozen, hedgingAttemptCount);
    }

    /** The given substream is drained. */
    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    State substreamDrained(Substream substream) {
      checkState(!passThrough, "Already passThrough");

      Collection<Substream> drainedSubstreams;
      
      if (substream.closed) {
        drainedSubstreams = this.drainedSubstreams;
      } else if (this.drainedSubstreams.isEmpty()) {
        // optimize for 0-retry, which is most of the cases.
        drainedSubstreams = Collections.singletonList(substream);
      } else {
        drainedSubstreams = new ArrayList<>(this.drainedSubstreams);
        drainedSubstreams.add(substream);
        drainedSubstreams = Collections.unmodifiableCollection(drainedSubstreams);
      }

      boolean passThrough = winningSubstream != null;

      List<BufferEntry> buffer = this.buffer;
      if (passThrough) {
        checkState(
            winningSubstream == substream, "Another RPC attempt has already committed");
        buffer = null;
      }

      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
          hedgingFrozen, hedgingAttemptCount);
    }

    /** The given substream is closed. */
    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    State substreamClosed(Substream substream) {
      substream.closed = true;
      if (this.drainedSubstreams.contains(substream)) {
        Collection<Substream> drainedSubstreams = new ArrayList<>(this.drainedSubstreams);
        drainedSubstreams.remove(substream);
        drainedSubstreams = Collections.unmodifiableCollection(drainedSubstreams);
        return new State(
            buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
            hedgingFrozen, hedgingAttemptCount);
      } else {
        return this;
      }
    }

    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    State committed(Substream winningSubstream) {
      checkState(this.winningSubstream == null, "Already committed");

      boolean passThrough = false;
      List<BufferEntry> buffer = this.buffer;
      Collection<Substream> drainedSubstreams;

      if (this.drainedSubstreams.contains(winningSubstream)) {
        passThrough = true;
        buffer = null;
        drainedSubstreams = Collections.singleton(winningSubstream);
      } else {
        drainedSubstreams = Collections.emptyList();
      }

      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
          hedgingFrozen, hedgingAttemptCount);
    }

    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    State freezeHedging() {
      if (hedgingFrozen) {
        return this;
      }
      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
          true, hedgingAttemptCount);
    }

    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    // state.hedgingAttemptCount is modified only here.
    // The method is only called in RetriableStream.start() and HedgingRunnable.run()
    State addActiveHedge(Substream substream) {
      // hasPotentialHedging must be true
      checkState(!hedgingFrozen, "hedging frozen");
      checkState(winningSubstream == null, "already committed");

      Collection<Substream> activeHedges;
      if (this.activeHedges == null) {
        activeHedges = Collections.singleton(substream);
      } else {
        activeHedges = new ArrayList<>(this.activeHedges);
        activeHedges.add(substream);
        activeHedges = Collections.unmodifiableCollection(activeHedges);
      }

      int hedgingAttemptCount = this.hedgingAttemptCount + 1;
      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
          hedgingFrozen, hedgingAttemptCount);
    }

    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    // The method is only called in Sublistener.closed()
    State removeActiveHedge(Substream substream) {
      Collection<Substream> activeHedges = new ArrayList<>(this.activeHedges);
      activeHedges.remove(substream);
      activeHedges = Collections.unmodifiableCollection(activeHedges);

      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
          hedgingFrozen, hedgingAttemptCount);
    }

    @CheckReturnValue
    // GuardedBy RetriableStream.lock
    // The method is only called for transparent retry.
    State replaceActiveHedge(Substream oldOne, Substream newOne) {
      Collection<Substream> activeHedges = new ArrayList<>(this.activeHedges);
      activeHedges.remove(oldOne);
      activeHedges.add(newOne);
      activeHedges = Collections.unmodifiableCollection(activeHedges);

      return new State(
          buffer, drainedSubstreams, activeHedges, winningSubstream, cancelled, passThrough,
          hedgingFrozen, hedgingAttemptCount);
    }
  }

  /**
   * A wrapper of a physical stream of a retry/hedging attempt, that comes with some useful
   *  attributes.
   */
  private static final class Substream {
    ClientStream stream;

    // GuardedBy RetriableStream.lock
    boolean closed;

    // setting to true must be GuardedBy RetriableStream.lock
    boolean bufferLimitExceeded;

    final int previousAttemptCount;

    Substream(int previousAttemptCount) {
      this.previousAttemptCount = previousAttemptCount;
    }
  }


  /**
   * Traces the buffer used by a substream.
   */
  class BufferSizeTracer extends ClientStreamTracer {
    // Each buffer size tracer is dedicated to one specific substream.
    private final Substream substream;

    @GuardedBy("lock")
    long bufferNeeded;

    BufferSizeTracer(Substream substream) {
      this.substream = substream;
    }

    /**
     * A message is sent to the wire, so its reference would be released if no retry or
     * hedging were involved. So at this point we have to hold the reference of the message longer
     * for retry, and we need to increment {@code substream.bufferNeeded}.
     */
    @Override
    public void outboundWireSize(long bytes) {
      if (state.winningSubstream != null) {
        return;
      }

      Runnable postCommitTask = null;

      // TODO(zdapeng): avoid using the same lock for both in-bound and out-bound.
      synchronized (lock) {
        if (state.winningSubstream != null || substream.closed) {
          return;
        }
        bufferNeeded += bytes;
        if (bufferNeeded <= perRpcBufferUsed) {
          return;
        }

        if (bufferNeeded > perRpcBufferLimit) {
          substream.bufferLimitExceeded = true;
        } else {
          // Only update channelBufferUsed when perRpcBufferUsed is not exceeding perRpcBufferLimit.
          long savedChannelBufferUsed =
              channelBufferUsed.addAndGet(bufferNeeded - perRpcBufferUsed);
          perRpcBufferUsed = bufferNeeded;

          if (savedChannelBufferUsed > channelBufferLimit) {
            substream.bufferLimitExceeded = true;
          }
        }

        if (substream.bufferLimitExceeded) {
          postCommitTask = commit(substream);
        }
      }

      if (postCommitTask != null) {
        postCommitTask.run();
      }
    }
  }

  /**
   *  Used to keep track of the total amount of memory used to buffer retryable or hedged RPCs for
   *  the Channel. There should be a single instance of it for each channel.
   */
  static final class ChannelBufferMeter {
    private final AtomicLong bufferUsed = new AtomicLong();

    @VisibleForTesting
    long addAndGet(long newBytesUsed) {
      return bufferUsed.addAndGet(newBytesUsed);
    }
  }

  /**
   * Used for retry throttling.
   */
  static final class Throttle {

    private static final int THREE_DECIMAL_PLACES_SCALE_UP = 1000;

    /**
     * 1000 times the maxTokens field of the retryThrottling policy in service config.
     * The number of tokens starts at maxTokens. The token_count will always be between 0 and
     * maxTokens.
     */
    final int maxTokens;

    /**
     * Half of {@code maxTokens}.
     */
    final int threshold;

    /**
     * 1000 times the tokenRatio field of the retryThrottling policy in service config.
     */
    final int tokenRatio;

    final AtomicInteger tokenCount = new AtomicInteger();

    Throttle(float maxTokens, float tokenRatio) {
      // tokenRatio is up to 3 decimal places
      this.tokenRatio = (int) (tokenRatio * THREE_DECIMAL_PLACES_SCALE_UP);
      this.maxTokens = (int) (maxTokens * THREE_DECIMAL_PLACES_SCALE_UP);
      this.threshold = this.maxTokens / 2;
      tokenCount.set(this.maxTokens);
    }

    @VisibleForTesting
    boolean isAboveThreshold() {
      return tokenCount.get() > threshold;
    }

    /**
     * Counts down the token on qualified failure and checks if it is above the threshold
     * atomically. Qualified failure is a failure with a retryable or non-fatal status code or with
     * a not-to-retry pushback.
     */
    @VisibleForTesting
    boolean onQualifiedFailureThenCheckIsAboveThreshold() {
      while (true) {
        int currentCount = tokenCount.get();
        if (currentCount == 0) {
          return false;
        }
        int decremented = currentCount - (1 * THREE_DECIMAL_PLACES_SCALE_UP);
        boolean updated = tokenCount.compareAndSet(currentCount, Math.max(decremented, 0));
        if (updated) {
          return decremented > threshold;
        }
      }
    }

    @VisibleForTesting
    void onSuccess() {
      while (true) {
        int currentCount = tokenCount.get();
        if (currentCount == maxTokens) {
          break;
        }
        int incremented = currentCount + tokenRatio;
        boolean updated = tokenCount.compareAndSet(currentCount, Math.min(incremented, maxTokens));
        if (updated) {
          break;
        }
      }
    }

    @Override
    public boolean equals(Object o) {
      if (this == o) {
        return true;
      }
      if (!(o instanceof Throttle)) {
        return false;
      }
      Throttle that = (Throttle) o;
      return maxTokens == that.maxTokens && tokenRatio == that.tokenRatio;
    }

    @Override
    public int hashCode() {
      return Objects.hashCode(maxTokens, tokenRatio);
    }
  }

  private static final class RetryPlan {
    final boolean shouldRetry;
    final long backoffNanos;

    RetryPlan(boolean shouldRetry, long backoffNanos) {
      this.shouldRetry = shouldRetry;
      this.backoffNanos = backoffNanos;
    }
  }

  private static final class HedgingPlan {
    final boolean isHedgeable;
    @Nullable
    final Integer hedgingPushbackMillis;

    public HedgingPlan(
        boolean isHedgeable, @Nullable Integer hedgingPushbackMillis) {
      this.isHedgeable = isHedgeable;
      this.hedgingPushbackMillis = hedgingPushbackMillis;
    }
  }

  /** Allows cancelling a Future without racing with setting the future. */
  private static final class FutureCanceller {

    final Object lock;
    @GuardedBy("lock")
    Future<?> future;
    @GuardedBy("lock")
    boolean cancelled;

    FutureCanceller(Object lock) {
      this.lock = lock;
    }

    void setFuture(Future<?> future) {
      boolean wasCancelled;
      synchronized (lock) {
        wasCancelled = cancelled;
        if (!wasCancelled) {
          this.future = future;
        }
      }
      if (wasCancelled) {
        future.cancel(false);
      }
    }

    @GuardedBy("lock")
    @CheckForNull // Must cancel the returned future if not null.
    Future<?> markCancelled() {
      cancelled = true;
      return future;
    }

    @GuardedBy("lock")
    boolean isCancelled() {
      return cancelled;
    }
  }
}

grpc / grpc-java / #19680

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