Provide helper for unlocking a file

This will allow recovery from a LockFailedException where
the file associated with an exception is passed to FileUtils.unlock
to attempt an unlock on the file so the operation can be retried

Change-Id: I580166d386126bfb54a318a65253070a6e325936

org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/LockFileTest.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2012, GitHub 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.storage.file;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+import org.eclipse.jgit.api.Git;
+import org.eclipse.jgit.api.errors.JGitInternalException;
+import org.eclipse.jgit.errors.LockFailedException;
+import org.eclipse.jgit.lib.RepositoryTestCase;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.junit.Test;
+
+/**
+ * Unit tests of {@link LockFile}
+ */
+public class LockFileTest extends RepositoryTestCase {
+
+	@Test
+	public void lockFailedExceptionRecovery() throws Exception {
+		Git git = new Git(db);
+		writeTrashFile("file.txt", "content");
+		git.add().addFilepattern("file.txt").call();
+		RevCommit commit1 = git.commit().setMessage("create file").call();
+
+		assertNotNull(commit1);
+		writeTrashFile("file.txt", "content2");
+		git.add().addFilepattern("file.txt").call();
+		assertNotNull(git.commit().setMessage("edit file").call());
+
+		assertTrue(new LockFile(db.getIndexFile(), db.getFS()).lock());
+		try {
+			git.checkout().setName(commit1.name()).call();
+			fail("JGitInternalException not thrown");
+		} catch (JGitInternalException e) {
+			assertTrue(e.getCause() instanceof LockFailedException);
+			LockFile.unlock(((LockFailedException) e.getCause()).getFile());
+			git.checkout().setName(commit1.name()).call();
+		}
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/LockFile.java

@@ -57,6 +57,7 @@ import java.nio.channels.FileChannel;
 import java.text.MessageFormat;
 
 import org.eclipse.jgit.JGitText;
+import org.eclipse.jgit.errors.LockFailedException;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.util.FS;
@@ -75,6 +76,37 @@ import org.eclipse.jgit.util.FileUtils;
 public class LockFile {
 	static final String SUFFIX = ".lock"; //$NON-NLS-1$
 
+	/**
+	 * Unlock the given file.
+	 * <p>
+	 * This method can be used for recovering from a thrown
+	 * {@link LockFailedException} . This method does not validate that the lock
+	 * is or is not currently held before attempting to unlock it.
+	 *
+	 * @param file
+	 * @return true if unlocked, false if unlocking failed
+	 */
+	public static boolean unlock(final File file) {
+		final File lockFile = getLockFile(file);
+		final int flags = FileUtils.RETRY | FileUtils.SKIP_MISSING;
+		try {
+			FileUtils.delete(lockFile, flags);
+		} catch (IOException ignored) {
+			// Ignore and return whether lock file still exists
+		}
+		return !lockFile.exists();
+	}
+
+	/**
+	 * Get the lock file corresponding to the given file.
+	 *
+	 * @param file
+	 * @return lock file
+	 */
+	static File getLockFile(File file) {
+		return new File(file.getParentFile(), file.getName() + SUFFIX);
+	}
+
 	/** Filter to skip over active lock files when listing a directory. */
 	static final FilenameFilter FILTER = new FilenameFilter() {
 		public boolean accept(File dir, String name) {
@@ -107,9 +139,9 @@ public class LockFile {
 	 *            the file system abstraction which will be necessary to perform
 	 *            certain file system operations.
 	 */
-	public LockFile(final File f, FS fs) {
+	public LockFile(final File f, final FS fs) {
 		ref = f;
-		lck = new File(ref.getParentFile(), ref.getName() + SUFFIX);
+		lck = getLockFile(ref);
 		this.fs = fs;
 	}