Ketch: Simple map of running leader instances

Supports use in basic daemons where RepositoryCache is also effective.

Change-Id: Ia736b32c2c710b5a802233badc0d30818de763cd

org.eclipse.jgit/src/org/eclipse/jgit/internal/ketch/KetchLeaderCache.java

@@ -0,0 +1,126 @@
+/*
+ * 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,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.internal.ketch;
+
+import java.net.URISyntaxException;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+import org.eclipse.jgit.internal.storage.dfs.DfsRepository;
+import org.eclipse.jgit.lib.Repository;
+
+/**
+ * A cache of live leader instances, keyed by repository.
+ * <p>
+ * Ketch only assigns a leader to a repository when needed. If
+ * {@link #get(Repository)} is called for a repository that does not have a
+ * leader, the leader is created and added to the cache.
+ */
+public class KetchLeaderCache {
+	private final KetchSystem system;
+	private final ConcurrentMap<String, KetchLeader> leaders;
+	private final Lock startLock;
+
+	/**
+	 * Initialize a new leader cache.
+	 *
+	 * @param system
+	 *            system configuration for the leaders
+	 */
+	public KetchLeaderCache(KetchSystem system) {
+		this.system = system;
+		leaders = new ConcurrentHashMap<>();
+		startLock = new ReentrantLock(true /* fair */);
+	}
+
+	/**
+	 * Lookup the leader instance for a given repository.
+	 *
+	 * @param repo
+	 *            repository to get the leader for.
+	 * @return the leader instance for the repository.
+	 * @throws URISyntaxException
+	 *             remote configuration contains an invalid URL.
+	 */
+	public KetchLeader get(Repository repo)
+			throws URISyntaxException {
+		String key = computeKey(repo);
+		KetchLeader leader = leaders.get(key);
+		if (leader != null) {
+			return leader;
+		}
+		return startLeader(key, repo);
+	}
+
+	private KetchLeader startLeader(String key, Repository repo)
+			throws URISyntaxException {
+		startLock.lock();
+		try {
+			KetchLeader leader = leaders.get(key);
+			if (leader != null) {
+				return leader;
+			}
+			leader = system.createLeader(repo);
+			leaders.put(key, leader);
+			return leader;
+		} finally {
+			startLock.unlock();
+		}
+	}
+
+	private static String computeKey(Repository repo) {
+		if (repo instanceof DfsRepository) {
+			DfsRepository dfs = (DfsRepository) repo;
+			return dfs.getDescription().getRepositoryName();
+		}
+
+		if (repo.getDirectory() != null) {
+			return repo.getDirectory().toURI().toString();
+		}
+
+		throw new IllegalArgumentException();
+	}
+}