gerrit-server/src/main/java/com/google/gerrit/server/git/ChainedReceiveCommands.java

// Copyright (C) 2016 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 com.google.common.base.MoreObjects.firstNonNull;
import static com.google.common.base.Preconditions.checkArgument;

import org.eclipse.jgit.lib.BatchRefUpdate;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.Ref;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.transport.ReceiveCommand;

import java.io.IOException;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

/**
 * Collection of {@link ReceiveCommand}s that supports multiple updates per ref.
 * <p>
 * The underlying behavior of {@link BatchRefUpdate} is undefined (an
 * implementations vary) when more than one command per ref is added. This class
 * works around that limitation by allowing multiple updates per ref, as long as
 * the previous new SHA-1 matches the next old SHA-1.
 */
public class ChainedReceiveCommands {
  private final Map<String, ReceiveCommand> commands = new LinkedHashMap<>();
  private final Map<String, ObjectId> oldIds = new HashMap<>();

  public boolean isEmpty() {
    return commands.isEmpty();
  }

  /**
   * Add a command.
   *
   * @param cmd command to add. If a command has been previously added for the
   *     same ref, the new SHA-1 of the most recent previous command must match
   *     the old SHA-1 of this command.
   */
  public void add(ReceiveCommand cmd) {
    checkArgument(!cmd.getOldId().equals(cmd.getNewId()),
        "ref update is a no-op: %s", cmd);
    ReceiveCommand old = commands.get(cmd.getRefName());
    if (old == null) {
      commands.put(cmd.getRefName(), cmd);
      return;
    }
    checkArgument(old.getResult() == ReceiveCommand.Result.NOT_ATTEMPTED,
        "cannot chain ref update %s after update %s with result %s",
        cmd, old, old.getResult());
    checkArgument(cmd.getOldId().equals(old.getNewId()),
        "cannot chain ref update %s after update %s with different new ID",
        cmd, old);
    commands.put(cmd.getRefName(), new ReceiveCommand(
        old.getOldId(), cmd.getNewId(), cmd.getRefName()));
  }

  /**
   * Get the latest value of a ref according to this sequence of commands.
   * <p>
   * Once the value for a ref is read once, it is cached in this instance, so
   * that multiple callers using this instance for lookups see a single
   * consistent snapshot.
   *
   * @param repo repository to read from, if result is not cached.
   * @param refName name of the ref.
   * @return value of the ref, taking into account commands that have already
   *     been added to this instance. Null if the ref is deleted, matching the
   *     behavior of {@link Repository#exactRef(String)}.
   */
  public ObjectId getObjectId(Repository repo, String refName)
      throws IOException {
    ReceiveCommand cmd = commands.get(refName);
    if (cmd != null) {
      return zeroToNull(cmd.getNewId());
    }
    ObjectId old = oldIds.get(refName);
    if (old != null) {
      return zeroToNull(old);
    }
    Ref ref = repo.exactRef(refName);
    ObjectId id = ref != null ? ref.getObjectId() : null;
    // Cache missing ref as zeroId to match value in commands map.
    oldIds.put(refName, firstNonNull(id, ObjectId.zeroId()));
    return id;
  }

  private static ObjectId zeroToNull(ObjectId id) {
    if (id == null || id.equals(ObjectId.zeroId())) {
      return null;
    }
    return id;
  }

  /**
   * Add commands from this instance to a native JGit batch update.
   * <p>
   * Exactly one command per ref will be added to the update. The old SHA-1 will
   * be the old SHA-1 of the first command added to this instance for that ref;
   * the new SHA-1 will be the new SHA-1 of the last command.
   *
   * @param bru batch update
   */
  public void addTo(BatchRefUpdate bru) {
    for (ReceiveCommand cmd : commands.values()) {
      bru.addCommand(cmd);
    }
  }
}