com.google.gerrit.server.git.HackPushNegotiateHook.java Source code

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Here is the source code for com.google.gerrit.server.git.HackPushNegotiateHook.java

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// Copyright (C) 2010 The Android Open Source Project
//
// 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 com.google.gerrit.server.git;

import static org.eclipse.jgit.lib.RefDatabase.ALL;

import com.google.common.collect.Sets;
import java.io.IOException;
import java.util.Collection;
import java.util.Collections;
import java.util.Map;
import java.util.Set;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.Ref;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevWalk;
import org.eclipse.jgit.transport.AdvertiseRefsHook;
import org.eclipse.jgit.transport.BaseReceivePack;
import org.eclipse.jgit.transport.ServiceMayNotContinueException;
import org.eclipse.jgit.transport.UploadPack;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Advertises part of history to git push clients.
 *
 * <p>This is a hack to work around the lack of negotiation in the send-pack/receive-pack wire
 * protocol.
 *
 * <p>When the server is frequently advancing master by creating merge commits, the client may not
 * be able to discover a common ancestor during push. Attempting to push will re-upload a very large
 * amount of history. This hook hacks in a fake negotiation replacement by walking history and
 * sending recent commits as {@code ".have"} lines in the wire protocol, allowing the client to find
 * a common ancestor.
 */
public class HackPushNegotiateHook implements AdvertiseRefsHook {
    private static final Logger log = LoggerFactory.getLogger(HackPushNegotiateHook.class);

    /** Size of an additional ".have" line. */
    private static final int HAVE_LINE_LEN = 4 + Constants.OBJECT_ID_STRING_LENGTH + 1 + 5 + 1;

    /**
     * Maximum number of bytes to "waste" in the advertisement with a peek at this repository's
     * current reachable history.
     */
    private static final int MAX_EXTRA_BYTES = 8192;

    /**
     * Number of recent commits to advertise immediately, hoping to show a client a nearby merge base.
     */
    private static final int BASE_COMMITS = 64;

    /** Number of commits to skip once base has already been shown. */
    private static final int STEP_COMMITS = 16;

    /** Total number of commits to extract from the history. */
    private static final int MAX_HISTORY = MAX_EXTRA_BYTES / HAVE_LINE_LEN;

    @Override
    public void advertiseRefs(UploadPack us) {
        throw new UnsupportedOperationException("HackPushNegotiateHook cannot be used for UploadPack");
    }

    @Override
    public void advertiseRefs(BaseReceivePack rp) throws ServiceMayNotContinueException {
        Map<String, Ref> r = rp.getAdvertisedRefs();
        if (r == null) {
            try {
                r = rp.getRepository().getRefDatabase().getRefs(ALL);
            } catch (ServiceMayNotContinueException e) {
                throw e;
            } catch (IOException e) {
                ServiceMayNotContinueException ex = new ServiceMayNotContinueException();
                ex.initCause(e);
                throw ex;
            }
        }
        rp.setAdvertisedRefs(r, history(r.values(), rp));
    }

    private Set<ObjectId> history(Collection<Ref> refs, BaseReceivePack rp) {
        Set<ObjectId> alreadySending = rp.getAdvertisedObjects();
        if (alreadySending.isEmpty()) {
            alreadySending = idsOf(refs);
        }

        int max = MAX_HISTORY - Math.max(0, alreadySending.size() - refs.size());
        if (max <= 0) {
            return Collections.emptySet();
        }

        // Scan history until the advertisement is full.
        RevWalk rw = rp.getRevWalk();
        rw.reset();
        try {
            for (Ref ref : refs) {
                try {
                    if (ref.getObjectId() != null) {
                        rw.markStart(rw.parseCommit(ref.getObjectId()));
                    }
                } catch (IOException badCommit) {
                    continue;
                }
            }

            Set<ObjectId> history = Sets.newHashSetWithExpectedSize(max);
            try {
                int stepCnt = 0;
                for (RevCommit c; history.size() < max && (c = rw.next()) != null;) {
                    if (c.getParentCount() <= 1 && !alreadySending.contains(c)
                            && (history.size() < BASE_COMMITS || (++stepCnt % STEP_COMMITS) == 0)) {
                        history.add(c);
                    }
                }
            } catch (IOException err) {
                log.error("error trying to advertise history", err);
            }
            return history;
        } finally {
            rw.reset();
        }
    }

    private static Set<ObjectId> idsOf(Collection<Ref> refs) {
        Set<ObjectId> r = Sets.newHashSetWithExpectedSize(refs.size());
        for (Ref ref : refs) {
            if (ref.getObjectId() != null) {
                r.add(ref.getObjectId());
            }
        }
        return r;
    }
}