#20160

Committed 28 Jan 2026 12:21AM UTC coverage: 88.71% (+0.04%) from 88.666%

Build # #20160

Build Type

push

github

Committed by

ejona86

Commit Message

xds: Normalize weights before combining endpoint and locality weights

Previously, the number of endpoints in a locality would skew how much
traffic was sent to that locality. Also, if endpoints in localities had
wildly different weights, that would impact cross-locality weighting.

For example, consider:
  LocalityA weight=1 endpointWeights=[100, 100, 100, 100]
  LocalityB weight=1 endpointWeights=[1]

The endpoint in LocalityB should have an endpoint weight that is half
the total sum of endpoint weights, in order to receive half the traffic.
But the multiple endpoints in LocalityA would cause it to get 4x the
traffic and the endpoint weights in LocalityA causes them to get 100x
the traffic.

See gRFC A113

Run Details

35415 of 39922 relevant lines covered (88.71%)

0.89 hits per line

Source File
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98.07

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

/*
 * Copyright 2023 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.checkNotNull;
import static io.grpc.ConnectivityState.CONNECTING;
import static io.grpc.ConnectivityState.IDLE;
import static io.grpc.ConnectivityState.READY;
import static io.grpc.ConnectivityState.SHUTDOWN;
import static io.grpc.ConnectivityState.TRANSIENT_FAILURE;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;
import com.google.errorprone.annotations.CheckReturnValue;
import io.grpc.Attributes;
import io.grpc.ConnectivityState;
import io.grpc.ConnectivityStateInfo;
import io.grpc.EquivalentAddressGroup;
import io.grpc.InternalEquivalentAddressGroup;
import io.grpc.LoadBalancer;
import io.grpc.Status;
import io.grpc.SynchronizationContext.ScheduledHandle;
import java.net.Inet4Address;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.Nullable;

/**
 * A {@link LoadBalancer} that provides no load-balancing over the addresses from the {@link
 * io.grpc.NameResolver}. The channel's default behavior is used, which is walking down the address
 * list and sticking to the first that works.
 */
final class PickFirstLeafLoadBalancer extends LoadBalancer {
  private static final Logger log = Logger.getLogger(PickFirstLeafLoadBalancer.class.getName());
  @VisibleForTesting
  static final int CONNECTION_DELAY_INTERVAL_MS = 250;
  private final boolean enableHappyEyeballs = !isSerializingRetries()
      && PickFirstLoadBalancerProvider.isEnabledHappyEyeballs();
  static boolean weightedShuffling =
      GrpcUtil.getFlag("GRPC_EXPERIMENTAL_PF_WEIGHTED_SHUFFLING", true);
  private final Helper helper;
  private final Map<SocketAddress, SubchannelData> subchannels = new HashMap<>();
  private final Index addressIndex = new Index(ImmutableList.of(), this.enableHappyEyeballs);
  private int numTf = 0;
  private boolean firstPass = true;
  @Nullable
  private ScheduledHandle scheduleConnectionTask = null;
  private ConnectivityState rawConnectivityState = IDLE;
  private ConnectivityState concludedState = IDLE;
  private boolean notAPetiolePolicy = true; // means not under a petiole policy
  private final BackoffPolicy.Provider bkoffPolProvider = new ExponentialBackoffPolicy.Provider();
  private BackoffPolicy reconnectPolicy;
  @Nullable
  private ScheduledHandle reconnectTask = null;
  private final boolean serializingRetries = isSerializingRetries();

  PickFirstLeafLoadBalancer(Helper helper) {
    this.helper = checkNotNull(helper, "helper");
  }

  static boolean isSerializingRetries() {
    return GrpcUtil.getFlag("GRPC_SERIALIZE_RETRIES", false);
  }

  @Override
  public Status acceptResolvedAddresses(ResolvedAddresses resolvedAddresses) {
    if (rawConnectivityState == SHUTDOWN) {
      return Status.FAILED_PRECONDITION.withDescription("Already shut down");
    }

    // Check whether this is a petiole policy, which is based off of an address attribute
    Boolean isPetiolePolicy = resolvedAddresses.getAttributes().get(IS_PETIOLE_POLICY);
    this.notAPetiolePolicy = isPetiolePolicy == null || !isPetiolePolicy;

    List<EquivalentAddressGroup> servers = resolvedAddresses.getAddresses();

    // Validate the address list
    if (servers.isEmpty()) {
      Status unavailableStatus = Status.UNAVAILABLE.withDescription(
              "NameResolver returned no usable address. addrs=" + resolvedAddresses.getAddresses()
                      + ", attrs=" + resolvedAddresses.getAttributes());
      handleNameResolutionError(unavailableStatus);
      return unavailableStatus;
    }
    for (EquivalentAddressGroup eag : servers) {
      if (eag == null) {
        Status unavailableStatus = Status.UNAVAILABLE.withDescription(
            "NameResolver returned address list with null endpoint. addrs="
                + resolvedAddresses.getAddresses() + ", attrs="
                + resolvedAddresses.getAttributes());
        handleNameResolutionError(unavailableStatus);
        return unavailableStatus;
      }
    }

    // Since we have a new set of addresses, we are again at first pass
    firstPass = true;

    List<EquivalentAddressGroup> cleanServers = deDupAddresses(servers);

    // We can optionally be configured to shuffle the address list. This can help better distribute
    // the load.
    if (resolvedAddresses.getLoadBalancingPolicyConfig()
        instanceof PickFirstLeafLoadBalancerConfig) {
      PickFirstLeafLoadBalancerConfig config
          = (PickFirstLeafLoadBalancerConfig) resolvedAddresses.getLoadBalancingPolicyConfig();
      if (config.shuffleAddressList != null && config.shuffleAddressList) {
        cleanServers = shuffle(
            cleanServers, config.randomSeed != null ? new Random(config.randomSeed) : new Random());
      }
    }

    final ImmutableList<EquivalentAddressGroup> newImmutableAddressGroups =
        ImmutableList.copyOf(cleanServers);

    if (rawConnectivityState == READY
        || (rawConnectivityState == CONNECTING
          && (!enableHappyEyeballs || addressIndex.isValid()))) {
      // If the previous ready (or connecting) subchannel exists in new address list,
      // keep this connection and don't create new subchannels. Happy Eyeballs is excluded when
      // connecting, because it allows multiple attempts simultaneously, thus is fine to start at
      // the beginning.
      SocketAddress previousAddress = addressIndex.getCurrentAddress();
      addressIndex.updateGroups(newImmutableAddressGroups);
      if (addressIndex.seekTo(previousAddress)) {
        SubchannelData subchannelData = subchannels.get(previousAddress);
        subchannelData.getSubchannel().updateAddresses(addressIndex.getCurrentEagAsList());
        shutdownRemovedAddresses(newImmutableAddressGroups);
        return Status.OK;
      }
      // Previous ready subchannel not in the new list of addresses
    } else {
      addressIndex.updateGroups(newImmutableAddressGroups);
    }

    // No old addresses means first time through, so we will do an explicit move to CONNECTING
    // which is what we implicitly started with
    boolean noOldAddrs = shutdownRemovedAddresses(newImmutableAddressGroups);

    if (noOldAddrs) {
      // Make tests happy; they don't properly assume starting in CONNECTING
      rawConnectivityState = CONNECTING;
      updateBalancingState(CONNECTING, new FixedResultPicker(PickResult.withNoResult()));
    }

    if (rawConnectivityState == READY) {
      // connect from beginning when prompted
      rawConnectivityState = IDLE;
      updateBalancingState(IDLE, new RequestConnectionPicker(this));

    } else if (rawConnectivityState == CONNECTING || rawConnectivityState == TRANSIENT_FAILURE) {
      // start connection attempt at first address
      cancelScheduleTask();
      requestConnection();
    }

    return Status.OK;
  }

  /**
   * Compute the difference between the flattened new addresses and the old addresses that had been
   * made into subchannels and then shutdown the matching subchannels.
   * @return true if there were no old addresses
   */
  private boolean shutdownRemovedAddresses(
      ImmutableList<EquivalentAddressGroup> newImmutableAddressGroups) {

    Set<SocketAddress> oldAddrs = new HashSet<>(subchannels.keySet());

    // Flatten the new EAGs addresses
    Set<SocketAddress> newAddrs = new HashSet<>();
    for (EquivalentAddressGroup endpoint : newImmutableAddressGroups) {
      newAddrs.addAll(endpoint.getAddresses());
    }

    // Shut them down and remove them
    for (SocketAddress oldAddr : oldAddrs) {
      if (!newAddrs.contains(oldAddr)) {
        subchannels.remove(oldAddr).getSubchannel().shutdown();
      }
    }
    return oldAddrs.isEmpty();
  }

  private static List<EquivalentAddressGroup> deDupAddresses(List<EquivalentAddressGroup> groups) {
    Set<SocketAddress> seenAddresses = new HashSet<>();
    List<EquivalentAddressGroup> newGroups = new ArrayList<>();

    for (EquivalentAddressGroup group : groups) {
      List<SocketAddress> addrs = new ArrayList<>();
      for (SocketAddress addr : group.getAddresses()) {
        if (seenAddresses.add(addr)) {
          addrs.add(addr);
        }
      }
      if (!addrs.isEmpty()) {
        newGroups.add(new EquivalentAddressGroup(addrs, group.getAttributes()));
      }
    }

    return newGroups;
  }

  // Also used by PickFirstLoadBalancer
  @CheckReturnValue
  static List<EquivalentAddressGroup> shuffle(List<EquivalentAddressGroup> eags, Random random) {
    if (weightedShuffling) {
      List<WeightEntry> weightedEntries = new ArrayList<>(eags.size());
      for (EquivalentAddressGroup eag : eags) {
        weightedEntries.add(new WeightEntry(eag, eagToWeight(eag, random)));
      }
      Collections.sort(weightedEntries, Collections.reverseOrder() /* descending */);
      return Lists.transform(weightedEntries, entry -> entry.eag);
    } else {
      List<EquivalentAddressGroup> eagsCopy = new ArrayList<>(eags);
      Collections.shuffle(eagsCopy, random);
      return eagsCopy;
    }
  }

  private static double eagToWeight(EquivalentAddressGroup eag, Random random) {
    Long weight = eag.getAttributes().get(InternalEquivalentAddressGroup.ATTR_WEIGHT);
    if (weight == null) {
      weight = 1L;
    }
    return Math.pow(random.nextDouble(), 1.0 / weight);
  }

  private static final class WeightEntry implements Comparable<WeightEntry> {
    final EquivalentAddressGroup eag;
    final double weight;

    public WeightEntry(EquivalentAddressGroup eag, double weight) {
      this.eag = eag;
      this.weight = weight;
    }

    @Override
    public int compareTo(WeightEntry entry) {
      return Double.compare(this.weight, entry.weight);
    }
  }

  @Override
  public void handleNameResolutionError(Status error) {
    if (rawConnectivityState == SHUTDOWN) {
      return;
    }

    for (SubchannelData subchannelData : subchannels.values()) {
      subchannelData.getSubchannel().shutdown();
    }
    subchannels.clear();
    addressIndex.updateGroups(ImmutableList.of());
    rawConnectivityState = TRANSIENT_FAILURE;
    updateBalancingState(TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(error)));
  }

  void processSubchannelState(SubchannelData subchannelData, ConnectivityStateInfo stateInfo) {
    ConnectivityState newState = stateInfo.getState();

    // Shutdown channels/previously relevant subchannels can still callback with state updates.
    // To prevent pickers from returning these obsolete subchannels, this logic
    // is included to check if the current list of active subchannels includes this subchannel.
    if (subchannelData != subchannels.get(getAddress(subchannelData.subchannel))) {
      return;
    }

    if (newState == SHUTDOWN) {
      return;
    }

    if (newState == IDLE && subchannelData.state == READY) {
      helper.refreshNameResolution();
    }

    // If we are transitioning from a TRANSIENT_FAILURE to CONNECTING or IDLE we ignore this state
    // transition and still keep the LB in TRANSIENT_FAILURE state. This is referred to as "sticky
    // transient failure". Only a subchannel state change to READY will get the LB out of
    // TRANSIENT_FAILURE. If the state is IDLE we additionally request a new connection so that we
    // keep retrying for a connection.

    // With the new pick first implementation, individual subchannels will have their own backoff
    // on a per-address basis. Thus, iterative requests for connections will not be requested
    // once the first pass through is complete.
    // However, every time there is an address update, we will perform a pass through for the new
    // addresses in the updated list.
    subchannelData.updateState(newState);
    if (rawConnectivityState == TRANSIENT_FAILURE || concludedState == TRANSIENT_FAILURE)  {
      if (newState == CONNECTING) {
        // each subchannel is responsible for its own backoff
        return;
      } else if (newState == IDLE) {
        requestConnection();
        return;
      }
    }

    switch (newState) {
      case IDLE:
        // Shutdown when ready: connect from beginning when prompted
        addressIndex.reset();
        rawConnectivityState = IDLE;
        updateBalancingState(IDLE, new RequestConnectionPicker(this));
        break;

      case CONNECTING:
        rawConnectivityState = CONNECTING;
        updateBalancingState(CONNECTING, new FixedResultPicker(PickResult.withNoResult()));
        break;

      case READY:
        shutdownRemaining(subchannelData);
        addressIndex.seekTo(getAddress(subchannelData.subchannel));
        rawConnectivityState = READY;
        updateHealthCheckedState(subchannelData);
        break;

      case TRANSIENT_FAILURE:
        // If we are looking at current channel, request a connection if possible
        if (addressIndex.isValid()
            && subchannels.get(addressIndex.getCurrentAddress()) == subchannelData) {
          if (addressIndex.increment()) {
            cancelScheduleTask();
            requestConnection(); // is recursive so might hit the end of the addresses
          } else {
            if (subchannels.size() >= addressIndex.size()) {
              scheduleBackoff();
            } else {
              // We must have done a seek to the middle of the list lets start over from the
              // beginning
              addressIndex.reset();
              requestConnection();
            }
          }
        }

        if (isPassComplete()) {
          rawConnectivityState = TRANSIENT_FAILURE;
          updateBalancingState(TRANSIENT_FAILURE,
              new FixedResultPicker(PickResult.withError(stateInfo.getStatus())));

          // Refresh Name Resolution, but only when all 3 conditions are met
          // * We are at the end of addressIndex
          // * have had status reported for all subchannels.
          // * And one of the following conditions:
          //    * Have had enough TF reported since we completed first pass
          //    * Just completed the first pass
          if (++numTf >= addressIndex.size() || firstPass) {
            firstPass = false;
            numTf = 0;
            helper.refreshNameResolution();
          }
        }
        break;

      default:
        throw new IllegalArgumentException("Unsupported state:" + newState);
    }
  }

  /**
   * Only called after all addresses attempted and failed (TRANSIENT_FAILURE).
   */
  private void scheduleBackoff() {
    if (!serializingRetries) {
      return;
    }

    class EndOfCurrentBackoff implements Runnable {
      @Override
      public void run() {
        reconnectTask = null;
        addressIndex.reset();
        requestConnection();
      }
    }

    // Just allow the previous one to trigger when ready if we're already in backoff
    if (reconnectTask != null) {
      return;
    }

    if (reconnectPolicy == null) {
      reconnectPolicy = bkoffPolProvider.get();
    }
    long delayNanos = reconnectPolicy.nextBackoffNanos();
    reconnectTask = helper.getSynchronizationContext().schedule(
        new EndOfCurrentBackoff(),
        delayNanos,
        TimeUnit.NANOSECONDS,
        helper.getScheduledExecutorService());
  }

  private void updateHealthCheckedState(SubchannelData subchannelData) {
    if (subchannelData.state != READY) {
      return;
    }

    if (notAPetiolePolicy || subchannelData.getHealthState() == READY) {
      updateBalancingState(READY,
          new FixedResultPicker(PickResult.withSubchannel(subchannelData.subchannel)));
    } else if (subchannelData.getHealthState() == TRANSIENT_FAILURE) {
      updateBalancingState(TRANSIENT_FAILURE, new FixedResultPicker(PickResult.withError(
          subchannelData.healthStateInfo.getStatus())));
    } else if (concludedState != TRANSIENT_FAILURE) {
      updateBalancingState(subchannelData.getHealthState(),
          new FixedResultPicker(PickResult.withNoResult()));
    }
  }

  private void updateBalancingState(ConnectivityState state, SubchannelPicker picker) {
    // an optimization: de-dup IDLE or CONNECTING notification.
    if (state == concludedState && (state == IDLE || state == CONNECTING)) {
      return;
    }
    concludedState = state;
    helper.updateBalancingState(state, picker);
  }

  @Override
  public void shutdown() {
    log.log(Level.FINE,
        "Shutting down, currently have {} subchannels created", subchannels.size());
    rawConnectivityState = SHUTDOWN;
    concludedState = SHUTDOWN;
    cancelScheduleTask();
    if (reconnectTask != null) {
      reconnectTask.cancel();
      reconnectTask = null;
    }
    reconnectPolicy = null;

    for (SubchannelData subchannelData : subchannels.values()) {
      subchannelData.getSubchannel().shutdown();
    }

    subchannels.clear();
  }

  /**
  * Shuts down remaining subchannels. Called when a subchannel becomes ready, which means
  * that all other subchannels must be shutdown.
  */
  private void shutdownRemaining(SubchannelData activeSubchannelData) {
    if (reconnectTask != null) {
      reconnectTask.cancel();
      reconnectTask = null;
    }
    reconnectPolicy = null;

    cancelScheduleTask();
    for (SubchannelData subchannelData : subchannels.values()) {
      if (!subchannelData.getSubchannel().equals(activeSubchannelData.subchannel)) {
        subchannelData.getSubchannel().shutdown();
      }
    }
    subchannels.clear();
    activeSubchannelData.updateState(READY);
    subchannels.put(getAddress(activeSubchannelData.subchannel), activeSubchannelData);
  }

  /**
   * Requests a connection to the next applicable address' subchannel, creating one if necessary.
   * Schedules a connection to next address in list as well.
   * If the current channel has already attempted a connection, we attempt a connection
   * to the next address/subchannel in our list.  We assume that createNewSubchannel will never
   * return null.
   */
  @Override
  public void requestConnection() {
    if (!addressIndex.isValid() || rawConnectivityState == SHUTDOWN) {
      return;
    }

    SocketAddress currentAddress = addressIndex.getCurrentAddress();
    SubchannelData subchannelData = subchannels.get(currentAddress);
    if (subchannelData == null) {
      subchannelData = createNewSubchannel(currentAddress, addressIndex.getCurrentEagAttributes());
    }

    ConnectivityState subchannelState = subchannelData.getState();
    switch (subchannelState) {
      case IDLE:
        subchannelData.subchannel.requestConnection();
        subchannelData.updateState(CONNECTING);
        scheduleNextConnection();
        break;
      case CONNECTING:
        scheduleNextConnection();
        break;
      case TRANSIENT_FAILURE:
        if (!serializingRetries) {
          addressIndex.increment();
          requestConnection();
        } else {
          if (!addressIndex.isValid()) {
            scheduleBackoff();
          } else {
            subchannelData.subchannel.requestConnection();
            subchannelData.updateState(CONNECTING);
          }
        }
        break;
      default:
        // Wait for current subchannel to change state
    }
  }


  /**
  * Happy Eyeballs
  * Schedules connection attempt to happen after a delay to the next available address.
  */
  private void scheduleNextConnection() {
    if (!enableHappyEyeballs
        || (scheduleConnectionTask != null && scheduleConnectionTask.isPending())) {
      return;
    }

    class StartNextConnection implements Runnable {
      @Override
      public void run() {
        scheduleConnectionTask = null;
        if (addressIndex.increment()) {
          requestConnection();
        }
      }
    }

    scheduleConnectionTask = helper.getSynchronizationContext().schedule(
        new StartNextConnection(),
        CONNECTION_DELAY_INTERVAL_MS,
        TimeUnit.MILLISECONDS,
        helper.getScheduledExecutorService());
  }

  private void cancelScheduleTask() {
    if (scheduleConnectionTask != null) {
      scheduleConnectionTask.cancel();
      scheduleConnectionTask = null;
    }
  }

  private SubchannelData createNewSubchannel(SocketAddress addr, Attributes attrs) {
    HealthListener hcListener = new HealthListener();
    final Subchannel subchannel = helper.createSubchannel(
        CreateSubchannelArgs.newBuilder()
            .setAddresses(Lists.newArrayList(
                new EquivalentAddressGroup(addr, attrs)))
            .addOption(HEALTH_CONSUMER_LISTENER_ARG_KEY, hcListener)
            .addOption(LoadBalancer.DISABLE_SUBCHANNEL_RECONNECT_KEY, serializingRetries)
            .build());
    if (subchannel == null) {
      log.warning("Was not able to create subchannel for " + addr);
      throw new IllegalStateException("Can't create subchannel");
    }
    SubchannelData subchannelData = new SubchannelData(subchannel, IDLE);
    hcListener.subchannelData = subchannelData;
    subchannels.put(addr, subchannelData);
    Attributes scAttrs = subchannel.getAttributes();
    if (notAPetiolePolicy || scAttrs.get(LoadBalancer.HAS_HEALTH_PRODUCER_LISTENER_KEY) == null) {
      subchannelData.healthStateInfo = ConnectivityStateInfo.forNonError(READY);
    }
    subchannel.start(stateInfo -> processSubchannelState(subchannelData, stateInfo));
    return subchannelData;
  }

  private boolean isPassComplete() {
    if (subchannels.size() < addressIndex.size()) {
      return false;
    }
    for (SubchannelData sc : subchannels.values()) {
      if (!sc.isCompletedConnectivityAttempt() ) {
        return false;
      }
    }
    return true;
  }

  private final class HealthListener implements SubchannelStateListener {
    private SubchannelData subchannelData;

    @Override
    public void onSubchannelState(ConnectivityStateInfo newState) {
      if (notAPetiolePolicy) {
        log.log(Level.WARNING,
            "Ignoring health status {0} for subchannel {1} as this is not under a petiole policy",
            new Object[]{newState, subchannelData.subchannel});
        return;
      }

      log.log(Level.FINE, "Received health status {0} for subchannel {1}",
          new Object[]{newState, subchannelData.subchannel});
      subchannelData.healthStateInfo = newState;
      if (addressIndex.isValid()
          && subchannelData == subchannels.get(addressIndex.getCurrentAddress())) {
        updateHealthCheckedState(subchannelData);
      }
    }
  }

  private SocketAddress getAddress(Subchannel subchannel) {
    return subchannel.getAddresses().getAddresses().get(0);
  }

  @VisibleForTesting
  ConnectivityState getConcludedConnectivityState() {
    return this.concludedState;
  }

  /**
   * Picker that requests connection during the first pick, and returns noResult.
   */
  private final class RequestConnectionPicker extends SubchannelPicker {
    private final PickFirstLeafLoadBalancer pickFirstLeafLoadBalancer;
    private final AtomicBoolean connectionRequested = new AtomicBoolean(false);

    RequestConnectionPicker(PickFirstLeafLoadBalancer pickFirstLeafLoadBalancer) {
      this.pickFirstLeafLoadBalancer =
          checkNotNull(pickFirstLeafLoadBalancer, "pickFirstLeafLoadBalancer");
    }

    @Override
    public PickResult pickSubchannel(PickSubchannelArgs args) {
      if (connectionRequested.compareAndSet(false, true)) {
        helper.getSynchronizationContext().execute(pickFirstLeafLoadBalancer::requestConnection);
      }
      return PickResult.withNoResult();
    }
  }

  /**
   * This contains both an ordered list of addresses and a pointer(i.e. index) to the current entry.
   * All updates should be done in a synchronization context.
   */
  @VisibleForTesting
  static final class Index {
    private List<UnwrappedEag> orderedAddresses;
    private int activeElement = 0;
    private boolean enableHappyEyeballs;

    Index(List<EquivalentAddressGroup> groups, boolean enableHappyEyeballs) {
      this.enableHappyEyeballs = enableHappyEyeballs;
      updateGroups(groups);
    }

    public boolean isValid() {
      return activeElement < orderedAddresses.size();
    }

    public boolean isAtBeginning() {
      return activeElement == 0;
    }

    /**
     * Move to next address in group.  If last address in group move to first address of next group.
     * @return false if went off end of the list, otherwise true
     */
    public boolean increment() {
      if (!isValid()) {
        return false;
      }

      activeElement++;

      return isValid();
    }

    public void reset() {
      activeElement = 0;
    }

    public SocketAddress getCurrentAddress() {
      if (!isValid()) {
        throw new IllegalStateException("Index is past the end of the address group list");
      }
      return orderedAddresses.get(activeElement).address;
    }

    public Attributes getCurrentEagAttributes() {
      if (!isValid()) {
        throw new IllegalStateException("Index is off the end of the address group list");
      }
      return orderedAddresses.get(activeElement).attributes;
    }

    public List<EquivalentAddressGroup> getCurrentEagAsList() {
      return Collections.singletonList(getCurrentEag());
    }

    private EquivalentAddressGroup getCurrentEag() {
      if (!isValid()) {
        throw new IllegalStateException("Index is past the end of the address group list");
      }
      return orderedAddresses.get(activeElement).asEag();
    }

    /**
     * Update to new groups, resetting the current index.
     */
    public void updateGroups(List<EquivalentAddressGroup> newGroups) {
      checkNotNull(newGroups, "newGroups");
      orderedAddresses = enableHappyEyeballs
                             ? updateGroupsHE(newGroups)
                             : updateGroupsNonHE(newGroups);
      reset();
    }

    /**
     * Returns false if the needle was not found and the current index was left unchanged.
     */
    public boolean seekTo(SocketAddress needle) {
      checkNotNull(needle, "needle");
      for (int i = 0; i < orderedAddresses.size(); i++) {
        if (orderedAddresses.get(i).address.equals(needle)) {
          this.activeElement = i;
          return true;
        }
      }
      return false;
    }

    public int size() {
      return orderedAddresses.size();
    }

    private List<UnwrappedEag> updateGroupsNonHE(List<EquivalentAddressGroup> newGroups) {
      List<UnwrappedEag> entries = new ArrayList<>();
      for (int g = 0; g < newGroups.size(); g++) {
        EquivalentAddressGroup eag = newGroups.get(g);
        for (int a = 0; a < eag.getAddresses().size(); a++) {
          SocketAddress addr = eag.getAddresses().get(a);
          entries.add(new UnwrappedEag(eag.getAttributes(), addr));
        }
      }

      return entries;
    }

    private List<UnwrappedEag> updateGroupsHE(List<EquivalentAddressGroup> newGroups) {
      Boolean firstIsV6 = null;
      List<UnwrappedEag> v4Entries = new ArrayList<>();
      List<UnwrappedEag> v6Entries = new ArrayList<>();
      for (int g = 0; g <  newGroups.size(); g++) {
        EquivalentAddressGroup eag = newGroups.get(g);
        for (int a = 0; a < eag.getAddresses().size(); a++) {
          SocketAddress addr = eag.getAddresses().get(a);
          boolean isIpV4 = addr instanceof InetSocketAddress
              && ((InetSocketAddress) addr).getAddress() instanceof Inet4Address;
          if (isIpV4) {
            if (firstIsV6 == null) {
              firstIsV6 = false;
            }
            v4Entries.add(new UnwrappedEag(eag.getAttributes(), addr));
          } else {
            if (firstIsV6 == null) {
              firstIsV6 = true;
            }
            v6Entries.add(new UnwrappedEag(eag.getAttributes(), addr));
          }
        }
      }

      return firstIsV6 != null && firstIsV6
          ? interleave(v6Entries, v4Entries)
          : interleave(v4Entries, v6Entries);
    }

    private List<UnwrappedEag> interleave(List<UnwrappedEag> firstFamily,
                                          List<UnwrappedEag> secondFamily) {
      if (firstFamily.isEmpty()) {
        return secondFamily;
      }
      if (secondFamily.isEmpty()) {
        return firstFamily;
      }

      List<UnwrappedEag> result = new ArrayList<>(firstFamily.size() + secondFamily.size());
      for (int i = 0; i < Math.max(firstFamily.size(), secondFamily.size()); i++) {
        if (i < firstFamily.size()) {
          result.add(firstFamily.get(i));
        }
        if (i < secondFamily.size()) {
          result.add(secondFamily.get(i));
        }
      }
      return result;
    }

    private static final class UnwrappedEag {
      private final Attributes attributes;
      private final SocketAddress address;

      public UnwrappedEag(Attributes attributes, SocketAddress address) {
        this.attributes = attributes;
        this.address = address;
      }

      private EquivalentAddressGroup asEag() {
        return new EquivalentAddressGroup(address, attributes);
      }
    }
  }

  @VisibleForTesting
  int getIndexLocation() {
    return addressIndex.activeElement;
  }

  @VisibleForTesting
  boolean isIndexValid() {
    return addressIndex.isValid();
  }

  private static final class SubchannelData {
    private final Subchannel subchannel;
    private ConnectivityState state;
    private boolean completedConnectivityAttempt = false;
    private ConnectivityStateInfo healthStateInfo = ConnectivityStateInfo.forNonError(IDLE);

    public SubchannelData(Subchannel subchannel, ConnectivityState state) {
      this.subchannel = subchannel;
      this.state = state;
    }

    public Subchannel getSubchannel() {
      return this.subchannel;
    }

    public ConnectivityState getState() {
      return this.state;
    }

    public boolean isCompletedConnectivityAttempt() {
      return completedConnectivityAttempt;
    }

    private void updateState(ConnectivityState newState) {
      this.state = newState;
      if (newState == READY || newState == TRANSIENT_FAILURE) {
        completedConnectivityAttempt = true;
      } else if (newState == IDLE) {
        completedConnectivityAttempt = false;
      }
    }

    private ConnectivityState getHealthState() {
      return healthStateInfo.getState();
    }
  }

  public static final class PickFirstLeafLoadBalancerConfig {

    @Nullable
    public final Boolean shuffleAddressList;

    // For testing purposes only, not meant to be parsed from a real config.
    @Nullable
    final Long randomSeed;

    public PickFirstLeafLoadBalancerConfig(@Nullable Boolean shuffleAddressList) {
      this(shuffleAddressList, null);
    }

    PickFirstLeafLoadBalancerConfig(@Nullable Boolean shuffleAddressList,
        @Nullable Long randomSeed) {
      this.shuffleAddressList = shuffleAddressList;
      this.randomSeed = randomSeed;
    }
  }

}

grpc / grpc-java / #20160

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