Move BatchUpdate to a new package

This class is a whole framework for executing updates in a mixed
ReviewDb/NoteDb environment. Move it to some place more specific and
applicable than "git", along with some auxiliary classes.

Change-Id: Ibcea72b8900d259e51d3311b3db8bc70218ca148

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

@@ -0,0 +1,125 @@
+// 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.update;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.gerrit.server.git.RefCache;
+import com.google.gerrit.server.git.RepoRefCache;
+import java.io.IOException;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Optional;
+import org.eclipse.jgit.lib.BatchRefUpdate;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.transport.ReceiveCommand;
+
+/**
+ * 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 implements RefCache {
+  private final Map<String, ReceiveCommand> commands = new LinkedHashMap<>();
+  private final RepoRefCache refCache;
+
+  public ChainedReceiveCommands(Repository repo) {
+    this(new RepoRefCache(repo));
+  }
+
+  public ChainedReceiveCommands(RepoRefCache refCache) {
+    this.refCache = checkNotNull(refCache);
+  }
+
+  public RepoRefCache getRepoRefCache() {
+    return refCache;
+  }
+
+  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>After the value for a ref is read from the repo once, it is cached as in {@link
+   * RepoRefCache}.
+   *
+   * @see RefCache#get(String)
+   */
+  @Override
+  public Optional<ObjectId> get(String refName) throws IOException {
+    ReceiveCommand cmd = commands.get(refName);
+    if (cmd != null) {
+      return !cmd.getNewId().equals(ObjectId.zeroId())
+          ? Optional.of(cmd.getNewId())
+          : Optional.empty();
+    }
+    return refCache.get(refName);
+  }
+
+  /**
+   * 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);
+    }
+  }
+
+  /** @return an unmodifiable view of commands. */
+  public Map<String, ReceiveCommand> getCommands() {
+    return Collections.unmodifiableMap(commands);
+  }
+}