/*
    * Copyright (C) 2016, Google Inc.
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
    *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or
   * without modification, are permitted provided that the following
   * conditions are met:
   *
   * - Redistributions of source code must retain the above copyright
   *   notice, this list of conditions and the following disclaimer.
   *
   * - Redistributions in binary form must reproduce the above
   *   copyright notice, this list of conditions and the following
   *   disclaimer in the documentation and/or other materials provided
   *   with the distribution.
   *
   * - Neither the name of the Eclipse Foundation, Inc. nor the
   *   names of its contributors may be used to endorse or promote
   *   products derived from this software without specific prior
   *   written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
   * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
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  package org.eclipse.jgit.internal.ketch;
  
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.CommitSpeed.BATCHED;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.CommitSpeed.FAST;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.CURRENT;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.LAGGING;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.OFFLINE;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.UNKNOWN;
  import static org.eclipse.jgit.lib.Constants.HEAD;
  import static org.eclipse.jgit.lib.FileMode.TYPE_GITLINK;
  import static org.eclipse.jgit.transport.ReceiveCommand.Result.NOT_ATTEMPTED;
  import static org.eclipse.jgit.transport.ReceiveCommand.Result.OK;
  import static org.eclipse.jgit.transport.ReceiveCommand.Type.CREATE;
  
  import java.io.IOException;
  import java.lang.ref.WeakReference;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.concurrent.Callable;
  import java.util.concurrent.Future;
  
  import org.eclipse.jgit.annotations.NonNull;
  import org.eclipse.jgit.annotations.Nullable;
  import org.eclipse.jgit.internal.storage.reftree.RefTree;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.Ref;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.transport.ReceiveCommand;
  import org.eclipse.jgit.treewalk.TreeWalk;
  import org.eclipse.jgit.util.SystemReader;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  /**
   * A Ketch replica, either {@link org.eclipse.jgit.internal.ketch.LocalReplica}
   * or {@link org.eclipse.jgit.internal.ketch.RemoteGitReplica}.
   * <p>
   * Replicas can be either a stock Git replica, or a Ketch-aware replica.
   * <p>
   * A stock Git replica has no special knowledge of Ketch and simply stores
   * objects and references. Ketch communicates with the stock Git replica using
   * the Git push wire protocol. The
   * {@link org.eclipse.jgit.internal.ketch.KetchLeader} commits an agreed upon
   * state by pushing all references to the Git replica, for example
   * {@code "refs/heads/master"} is pushed during commit. Stock Git replicas use
   * {@link org.eclipse.jgit.internal.ketch.KetchReplica.CommitMethod#ALL_REFS} to
   * record the final state.
   * <p>
   * Ketch-aware replicas understand the {@code RefTree} sent during the proposal
  * and during commit are able to update their own reference space to match the
  * state represented by the {@code RefTree}. Ketch-aware replicas typically use
  * a {@link org.eclipse.jgit.internal.storage.reftree.RefTreeDatabase} and
  * {@link org.eclipse.jgit.internal.ketch.KetchReplica.CommitMethod#TXN_COMMITTED}
  * to record the final state.
  * <p>
  * KetchReplica instances are tightly coupled with a single
  * {@link org.eclipse.jgit.internal.ketch.KetchLeader}. Some state may be
  * accessed by the leader thread and uses the leader's own
  * {@link org.eclipse.jgit.internal.ketch.KetchLeader#lock} to protect shared
  * data.
  */
 public abstract class KetchReplica {
 	static final Logger log = LoggerFactory.getLogger(KetchReplica.class);
 	private static final byte[] PEEL = { ' ', '^' };
 
 	/** Participation of a replica in establishing consensus. */
 	public enum Participation {
 		/** Replica can vote. */
 		FULL,
 
 		/** Replica does not vote, but tracks leader. */
 		FOLLOWER_ONLY;
 	}
 
 	/** How this replica wants to receive Ketch commit operations. */
 	public enum CommitMethod {
 		/** All references are pushed to the peer as standard Git. */
 		ALL_REFS,
 
 		/** Only {@code refs/txn/committed} is written/updated. */
 		TXN_COMMITTED;
 	}
 
 	/** Delay before committing to a replica. */
 	public enum CommitSpeed {
 		/**
 		 * Send the commit immediately, even if it could be batched with the
 		 * next proposal.
 		 */
 		FAST,
 
 		/**
 		 * If the next proposal is available, batch the commit with it,
 		 * otherwise just send the commit. This generates less network use, but
 		 * may provide slower consistency on the replica.
 		 */
 		BATCHED;
 	}
 
 	/** Current state of a replica. */
 	public enum State {
 		/** Leader has not yet contacted the replica. */
 		UNKNOWN,
 
 		/** Replica is behind the consensus. */
 		LAGGING,
 
 		/** Replica matches the consensus. */
 		CURRENT,
 
 		/** Replica has a different (or unknown) history. */
 		DIVERGENT,
 
 		/** Replica's history contains the leader's history. */
 		AHEAD,
 
 		/** Replica can not be contacted. */
 		OFFLINE;
 	}
 
 	private final KetchLeader leader;
 	private final String replicaName;
 	private final Participation participation;
 	private final CommitMethod commitMethod;
 	private final CommitSpeed commitSpeed;
 	private final long minRetryMillis;
 	private final long maxRetryMillis;
 	private final Map<ObjectId, List<ReceiveCommand>> staged;
 	private final Map<String, ReceiveCommand> running;
 	private final Map<String, ReceiveCommand> waiting;
 	private final List<ReplicaPushRequest> queued;
 
 	/**
 	 * Value known for {@code "refs/txn/accepted"}.
 	 * <p>
 	 * Raft literature refers to this as {@code matchIndex}.
 	 */
 	private ObjectId txnAccepted;
 
 	/**
 	 * Value known for {@code "refs/txn/committed"}.
 	 * <p>
 	 * Raft literature refers to this as {@code commitIndex}. In traditional
 	 * Raft this is a state variable inside the follower implementation, but
 	 * Ketch keeps it in the leader.
 	 */
 	private ObjectId txnCommitted;
 
 	/** What is happening with this replica. */
 	private State state = UNKNOWN;
 	private String error;
 
 	/** Scheduled retry due to communication failure. */
 	private Future<?> retryFuture;
 	private long lastRetryMillis;
 	private long retryAtMillis;
 
 	/**
 	 * Configure a replica representation.
 	 *
 	 * @param leader
 	 *            instance this replica follows.
 	 * @param name
 	 *            unique-ish name identifying this replica for debugging.
 	 * @param cfg
 	 *            how Ketch should treat the replica.
 	 */
 	protected KetchReplica(KetchLeader leader, String name, ReplicaConfig cfg) {
 		this.leader = leader;
 		this.replicaName = name;
 		this.participation = cfg.getParticipation();
 		this.commitMethod = cfg.getCommitMethod();
 		this.commitSpeed = cfg.getCommitSpeed();
 		this.minRetryMillis = cfg.getMinRetry(MILLISECONDS);
 		this.maxRetryMillis = cfg.getMaxRetry(MILLISECONDS);
 		this.staged = new HashMap<>();
 		this.running = new HashMap<>();
 		this.waiting = new HashMap<>();
 		this.queued = new ArrayList<>(4);
 	}
 
 	/**
 	 * Get system configuration.
 	 *
 	 * @return system configuration.
 	 */
 	public KetchSystem getSystem() {
 		return getLeader().getSystem();
 	}
 
 	/**
 	 * Get leader instance this replica follows.
 	 *
 	 * @return leader instance this replica follows.
 	 */
 	public KetchLeader getLeader() {
 		return leader;
 	}
 
 	/**
 	 * Get unique-ish name for debugging.
 	 *
 	 * @return unique-ish name for debugging.
 	 */
 	public String getName() {
 		return replicaName;
 	}
 
 	/**
 	 * Get description of this replica for error/debug logging purposes.
 	 *
 	 * @return description of this replica for error/debug logging purposes.
 	 */
 	protected String describeForLog() {
 		return getName();
 	}
 
 	/**
 	 * Get how the replica participates in this Ketch system.
 	 *
 	 * @return how the replica participates in this Ketch system.
 	 */
 	public Participation getParticipation() {
 		return participation;
 	}
 
 	/**
 	 * Get how Ketch will commit to the repository.
 	 *
 	 * @return how Ketch will commit to the repository.
 	 */
 	public CommitMethod getCommitMethod() {
 		return commitMethod;
 	}
 
 	/**
 	 * Get when Ketch will commit to the repository.
 	 *
 	 * @return when Ketch will commit to the repository.
 	 */
 	public CommitSpeed getCommitSpeed() {
 		return commitSpeed;
 	}
 
 	/**
 	 * Called by leader to perform graceful shutdown.
 	 * <p>
 	 * Default implementation cancels any scheduled retry. Subclasses may add
 	 * additional logic before or after calling {@code super.shutdown()}.
 	 * <p>
 	 * Called with {@link org.eclipse.jgit.internal.ketch.KetchLeader#lock} held
 	 * by caller.
 	 */
 	protected void shutdown() {
 		Future<?> f = retryFuture;
 		if (f != null) {
 			retryFuture = null;
 			f.cancel(true);
 		}
 	}
 
 	ReplicaSnapshot snapshot() {
 		ReplicaSnapshot s = new ReplicaSnapshot(this);
 		s.accepted = txnAccepted;
 		s.committed = txnCommitted;
 		s.state = state;
 		s.error = error;
 		s.retryAtMillis = waitingForRetry() ? retryAtMillis : 0;
 		return s;
 	}
 
 	/**
 	 * Update the leader's view of the replica after a poll.
 	 * <p>
 	 * Called with {@link KetchLeader#lock} held by caller.
 	 *
 	 * @param refs
 	 *            map of refs from the replica.
 	 */
 	void initialize(Map<String, Ref> refs) {
 		if (txnAccepted == null) {
 			txnAccepted = getId(refs.get(getSystem().getTxnAccepted()));
 		}
 		if (txnCommitted == null) {
 			txnCommitted = getId(refs.get(getSystem().getTxnCommitted()));
 		}
 	}
 
 	ObjectId getTxnAccepted() {
 		return txnAccepted;
 	}
 
 	boolean hasAccepted(LogIndex id) {
 		return equals(txnAccepted, id);
 	}
 
 	private static boolean equals(@Nullable ObjectId a, LogIndex b) {
 		return a != null && b != null && AnyObjectId.equals(a, b);
 	}
 
 	/**
 	 * Schedule a proposal round with the replica.
 	 * <p>
 	 * Called with {@link KetchLeader#lock} held by caller.
 	 *
 	 * @param round
 	 *            current round being run by the leader.
 	 */
 	void pushTxnAcceptedAsync(Round round) {
 		List<ReceiveCommand> cmds = new ArrayList<>();
 		if (commitSpeed == BATCHED) {
 			LogIndex committedIndex = leader.getCommitted();
 			if (equals(txnAccepted, committedIndex)
 					&& !equals(txnCommitted, committedIndex)) {
 				prepareTxnCommitted(cmds, committedIndex);
 			}
 		}
 
 		// TODO(sop) Lagging replicas should build accept on the fly.
 		if (round.stageCommands != null) {
 			for (ReceiveCommand cmd : round.stageCommands) {
 				// TODO(sop): Do not send certain object graphs to replica.
 				cmds.add(copy(cmd));
 			}
 		}
 		cmds.add(new ReceiveCommand(
 				round.acceptedOldIndex, round.acceptedNewIndex,
 				getSystem().getTxnAccepted()));
 		pushAsync(new ReplicaPushRequest(this, cmds));
 	}
 
 	private static ReceiveCommand copy(ReceiveCommand c) {
 		return new ReceiveCommand(c.getOldId(), c.getNewId(), c.getRefName());
 	}
 
 	boolean shouldPushUnbatchedCommit(LogIndex committed, boolean leaderIdle) {
 		return (leaderIdle || commitSpeed == FAST) && hasAccepted(committed);
 	}
 
 	void pushCommitAsync(LogIndex committed) {
 		List<ReceiveCommand> cmds = new ArrayList<>();
 		prepareTxnCommitted(cmds, committed);
 		pushAsync(new ReplicaPushRequest(this, cmds));
 	}
 
 	private void prepareTxnCommitted(List<ReceiveCommand> cmds,
 			ObjectId committed) {
 		removeStaged(cmds, committed);
 		cmds.add(new ReceiveCommand(
 				txnCommitted, committed,
 				getSystem().getTxnCommitted()));
 	}
 
 	private void removeStaged(List<ReceiveCommand> cmds, ObjectId committed) {
 		List<ReceiveCommand> a = staged.remove(committed);
 		if (a != null) {
 			delete(cmds, a);
 		}
 		if (staged.isEmpty() || !(committed instanceof LogIndex)) {
 			return;
 		}
 
 		LogIndex committedIndex = (LogIndex) committed;
 		Iterator<Map.Entry<ObjectId, List<ReceiveCommand>>> itr = staged
 				.entrySet().iterator();
 		while (itr.hasNext()) {
 			Map.Entry<ObjectId, List<ReceiveCommand>> e = itr.next();
 			if (e.getKey() instanceof LogIndex) {
 				LogIndex stagedIndex = (LogIndex) e.getKey();
 				if (stagedIndex.isBefore(committedIndex)) {
 					delete(cmds, e.getValue());
 					itr.remove();
 				}
 			}
 		}
 	}
 
 	private static void delete(List<ReceiveCommand> cmds,
 			List<ReceiveCommand> createCmds) {
 		for (ReceiveCommand cmd : createCmds) {
 			ObjectId id = cmd.getNewId();
 			String name = cmd.getRefName();
 			cmds.add(new ReceiveCommand(id, ObjectId.zeroId(), name));
 		}
 	}
 
 	/**
 	 * Determine the next push for this replica (if any) and start it.
 	 * <p>
 	 * If the replica has successfully accepted the committed state of the
 	 * leader, this method will push all references to the replica using the
 	 * configured {@link CommitMethod}.
 	 * <p>
 	 * If the replica is {@link State#LAGGING} this method will begin catch up
 	 * by sending a more recent {@code refs/txn/accepted}.
 	 * <p>
 	 * Must be invoked with {@link KetchLeader#lock} held by caller.
 	 */
 	private void runNextPushRequest() {
 		LogIndex committed = leader.getCommitted();
 		if (!equals(txnCommitted, committed)
 				&& shouldPushUnbatchedCommit(committed, leader.isIdle())) {
 			pushCommitAsync(committed);
 		}
 
 		if (queued.isEmpty() || !running.isEmpty() || waitingForRetry()) {
 			return;
 		}
 
 		// Collapse all queued requests into a single request.
 		Map<String, ReceiveCommand> cmdMap = new HashMap<>();
 		for (ReplicaPushRequest req : queued) {
 			for (ReceiveCommand cmd : req.getCommands()) {
 				String name = cmd.getRefName();
 				ReceiveCommand old = cmdMap.remove(name);
 				if (old != null) {
 					cmd = new ReceiveCommand(
 							old.getOldId(), cmd.getNewId(),
 							name);
 				}
 				cmdMap.put(name, cmd);
 			}
 		}
 		queued.clear();
 		waiting.clear();
 
 		List<ReceiveCommand> next = new ArrayList<>(cmdMap.values());
 		for (ReceiveCommand cmd : next) {
 			running.put(cmd.getRefName(), cmd);
 		}
 		startPush(new ReplicaPushRequest(this, next));
 	}
 
 	private void pushAsync(ReplicaPushRequest req) {
 		if (defer(req)) {
 			// TODO(sop) Collapse during long retry outage.
 			for (ReceiveCommand cmd : req.getCommands()) {
 				waiting.put(cmd.getRefName(), cmd);
 			}
 			queued.add(req);
 		} else {
 			for (ReceiveCommand cmd : req.getCommands()) {
 				running.put(cmd.getRefName(), cmd);
 			}
 			startPush(req);
 		}
 	}
 
 	private boolean defer(ReplicaPushRequest req) {
 		if (waitingForRetry()) {
 			// Prior communication failure; everything is deferred.
 			return true;
 		}
 
 		for (ReceiveCommand nextCmd : req.getCommands()) {
 			ReceiveCommand priorCmd = waiting.get(nextCmd.getRefName());
 			if (priorCmd == null) {
 				priorCmd = running.get(nextCmd.getRefName());
 			}
 			if (priorCmd != null) {
 				// Another request pending on same ref; that must go first.
 				// Verify priorCmd.newId == nextCmd.oldId?
 				return true;
 			}
 		}
 		return false;
 	}
 
 	private boolean waitingForRetry() {
 		Future<?> f = retryFuture;
 		return f != null && !f.isDone();
 	}
 
 	private void retryLater(ReplicaPushRequest req) {
 		Collection<ReceiveCommand> cmds = req.getCommands();
 		for (ReceiveCommand cmd : cmds) {
 			cmd.setResult(NOT_ATTEMPTED, null);
 			if (!waiting.containsKey(cmd.getRefName())) {
 				waiting.put(cmd.getRefName(), cmd);
 			}
 		}
 		queued.add(0, new ReplicaPushRequest(this, cmds));
 
 		if (!waitingForRetry()) {
 			long delay = KetchSystem.delay(
 					lastRetryMillis,
 					minRetryMillis, maxRetryMillis);
 			if (log.isDebugEnabled()) {
 				log.debug("Retrying {} after {} ms", //$NON-NLS-1$
 						describeForLog(), Long.valueOf(delay));
 			}
 			lastRetryMillis = delay;
 			retryAtMillis = SystemReader.getInstance().getCurrentTime() + delay;
 			retryFuture = getSystem().getExecutor()
 					.schedule(new WeakRetryPush(this), delay, MILLISECONDS);
 		}
 	}
 
 	/** Weakly holds a retrying replica, allowing it to garbage collect. */
 	static class WeakRetryPush extends WeakReference<KetchReplica>
 			implements Callable<Void> {
 		WeakRetryPush(KetchReplica r) {
 			super(r);
 		}
 
 		@Override
 		public Void call() throws Exception {
 			KetchReplica r = get();
 			if (r != null) {
 				r.doRetryPush();
 			}
 			return null;
 		}
 	}
 
 	private void doRetryPush() {
 		leader.lock.lock();
 		try {
 			retryFuture = null;
 			runNextPushRequest();
 		} finally {
 			leader.lock.unlock();
 		}
 	}
 
 	/**
 	 * Begin executing a single push.
 	 * <p>
 	 * This method must move processing onto another thread. Called with
 	 * {@link org.eclipse.jgit.internal.ketch.KetchLeader#lock} held by caller.
 	 *
 	 * @param req
 	 *            the request to send to the replica.
 	 */
 	protected abstract void startPush(ReplicaPushRequest req);
 
 	/**
 	 * Callback from {@link ReplicaPushRequest} upon success or failure.
 	 * <p>
 	 * Acquires the {@link KetchLeader#lock} and updates the leader's internal
 	 * knowledge about this replica to reflect what has been learned during a
 	 * push to the replica. In some cases of divergence this method may take
 	 * some time to determine how the replica has diverged; to reduce contention
 	 * this is evaluated before acquiring the leader lock.
 	 *
 	 * @param repo
 	 *            local repository instance used by the push thread.
 	 * @param req
 	 *            push request just attempted.
 	 */
 	void afterPush(@Nullable Repository repo, ReplicaPushRequest req) {
 		ReceiveCommand acceptCmd = null;
 		ReceiveCommand commitCmd = null;
 		List<ReceiveCommand> stages = null;
 
 		for (ReceiveCommand cmd : req.getCommands()) {
 			String name = cmd.getRefName();
 			if (name.equals(getSystem().getTxnAccepted())) {
 				acceptCmd = cmd;
 			} else if (name.equals(getSystem().getTxnCommitted())) {
 				commitCmd = cmd;
 			} else if (cmd.getResult() == OK && cmd.getType() == CREATE
 					&& name.startsWith(getSystem().getTxnStage())) {
 				if (stages == null) {
 					stages = new ArrayList<>();
 				}
 				stages.add(cmd);
 			}
 		}
 
 		State newState = null;
 		ObjectId acceptId = readId(req, acceptCmd);
 		if (repo != null && acceptCmd != null && acceptCmd.getResult() != OK
 				&& req.getException() == null) {
 			try (LagCheck lag = new LagCheck(this, repo)) {
 				newState = lag.check(acceptId, acceptCmd);
 				acceptId = lag.getRemoteId();
 			}
 		}
 
 		leader.lock.lock();
 		try {
 			for (ReceiveCommand cmd : req.getCommands()) {
 				running.remove(cmd.getRefName());
 			}
 
 			Throwable err = req.getException();
 			if (err != null) {
 				state = OFFLINE;
 				error = err.toString();
 				retryLater(req);
 				leader.onReplicaUpdate(this);
 				return;
 			}
 
 			lastRetryMillis = 0;
 			error = null;
 			updateView(req, acceptId, commitCmd);
 
 			if (acceptCmd != null && acceptCmd.getResult() == OK) {
 				state = hasAccepted(leader.getHead()) ? CURRENT : LAGGING;
 				if (stages != null) {
 					staged.put(acceptCmd.getNewId(), stages);
 				}
 			} else if (newState != null) {
 				state = newState;
 			}
 
 			leader.onReplicaUpdate(this);
 			runNextPushRequest();
 		} finally {
 			leader.lock.unlock();
 		}
 	}
 
 	private void updateView(ReplicaPushRequest req, @Nullable ObjectId acceptId,
 			ReceiveCommand commitCmd) {
 		if (acceptId != null) {
 			txnAccepted = acceptId;
 		}
 
 		ObjectId committed = readId(req, commitCmd);
 		if (committed != null) {
 			txnCommitted = committed;
 		} else if (acceptId != null && txnCommitted == null) {
 			// Initialize during first conversation.
 			Map<String, Ref> adv = req.getRefs();
 			if (adv != null) {
 				Ref refs = adv.get(getSystem().getTxnCommitted());
 				txnCommitted = getId(refs);
 			}
 		}
 	}
 
 	@Nullable
 	private static ObjectId readId(ReplicaPushRequest req,
 			@Nullable ReceiveCommand cmd) {
 		if (cmd == null) {
 			// Ref was not in the command list, do not trust advertisement.
 			return null;
 
 		} else if (cmd.getResult() == OK) {
 			// Currently at newId.
 			return cmd.getNewId();
 		}
 
 		Map<String, Ref> refs = req.getRefs();
 		return refs != null ? getId(refs.get(cmd.getRefName())) : null;
 	}
 
 	/**
 	 * Fetch objects from the remote using the calling thread.
 	 * <p>
 	 * Called without {@link org.eclipse.jgit.internal.ketch.KetchLeader#lock}.
 	 *
 	 * @param repo
 	 *            local repository to fetch objects into.
 	 * @param req
 	 *            the request to fetch from a replica.
 	 * @throws java.io.IOException
 	 *             communication with the replica was not possible.
 	 */
 	protected abstract void blockingFetch(Repository repo,
 			ReplicaFetchRequest req) throws IOException;
 
 	/**
 	 * Build a list of commands to commit
 	 * {@link org.eclipse.jgit.internal.ketch.KetchReplica.CommitMethod#ALL_REFS}.
 	 *
 	 * @param git
 	 *            local leader repository to read committed state from.
 	 * @param current
 	 *            all known references in the replica's repository. Typically
 	 *            this comes from a push advertisement.
 	 * @param committed
 	 *            state being pushed to {@code refs/txn/committed}.
 	 * @return commands to update during commit.
 	 * @throws java.io.IOException
 	 *             cannot read the committed state.
 	 */
 	protected Collection<ReceiveCommand> prepareCommit(Repository git,
 			Map<String, Ref> current, ObjectId committed) throws IOException {
 		List<ReceiveCommand> delta = new ArrayList<>();
 		Map<String, Ref> remote = new HashMap<>(current);
 		try (RevWalk rw = new RevWalk(git);
 				TreeWalk tw = new TreeWalk(rw.getObjectReader())) {
 			tw.setRecursive(true);
 			tw.addTree(rw.parseCommit(committed).getTree());
 			while (tw.next()) {
 				if (tw.getRawMode(0) != TYPE_GITLINK
 						|| tw.isPathSuffix(PEEL, 2)) {
 					// Symbolic references cannot be pushed.
 					// Caching peeled values is handled remotely.
 					continue;
 				}
 
 				// TODO(sop) Do not send certain ref names to replica.
 				String name = RefTree.refName(tw.getPathString());
 				Ref oldRef = remote.remove(name);
 				ObjectId oldId = getId(oldRef);
 				ObjectId newId = tw.getObjectId(0);
 				if (!AnyObjectId.equals(oldId, newId)) {
 					delta.add(new ReceiveCommand(oldId, newId, name));
 				}
 			}
 		}
 
 		// Delete any extra references not in the committed state.
 		for (Ref ref : remote.values()) {
 			if (canDelete(ref)) {
 				delta.add(new ReceiveCommand(
 					ref.getObjectId(), ObjectId.zeroId(),
 					ref.getName()));
 			}
 		}
 		return delta;
 	}
 
 	boolean canDelete(Ref ref) {
 		String name = ref.getName();
 		if (HEAD.equals(name)) {
 			return false;
 		}
 		if (name.startsWith(getSystem().getTxnNamespace())) {
 			return false;
 		}
 		// TODO(sop) Do not delete precious names from replica.
 		return true;
 	}
 
 	@NonNull
 	static ObjectId getId(@Nullable Ref ref) {
 		if (ref != null) {
 			ObjectId id = ref.getObjectId();
 			if (id != null) {
 				return id;
 			}
 		}
 		return ObjectId.zeroId();
 	}
 }