Fix concurrent creation of fan-out object directories

If multiple threads attempted to insert loose objects into the same new
fan-out directory, the creation of that directory was subject to a race
condition that could lead to an unnecessary IOException being thrown -
because an inserter could not 'create' a directory that had just been
generated by a different thread. All we require is that the directory
does indeed *exist*, so not being able to _create_ it is not actually a
fatal problem. Setting 'skipExisting' to 'true' on the call to mkdir()
fixes the issue.

I found this issue as a real world occurrence while working on The BFG
Repo Cleaner (https://github.com/rtyley/bfg-repo-cleaner), a tool which
concurrently performs a lot of object creation.

In order to demonstrate the problem here I've added a small test case
which reliably reproduces the issue on the few different hardware
systems I've tried. The error thrown when the race-condition arises is
this:

java.io.IOException: Creating directory /home/roberto/repo.git/objects/e6 failed
at org.eclipse.jgit.util.FileUtils.mkdir(FileUtils.java:182)
at org.eclipse.jgit.storage.file.ObjectDirectory.insertUnpackedObject(ObjectDirectory.java:590)
at org.eclipse.jgit.storage.file.ObjectDirectoryInserter.insertOneObject(ObjectDirectoryInserter.java:113)
at org.eclipse.jgit.storage.file.ObjectDirectoryInserter.insert(ObjectDirectoryInserter.java:91)
at org.eclipse.jgit.lib.ObjectInserter.insert(ObjectInserter.java:329)

Change-Id: I88eac49bc600c56ba9ad290e6133d8a7113125ab

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

@@ -0,0 +1,77 @@
+/*
+ * Copyright (C) 2012, Roberto Tyley <roberto.tyley@gmail.com>
+ *
+ * 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 org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.RepositoryTestCase;
+import org.junit.Test;
+
+import java.util.*;
+import java.util.concurrent.*;
+
+public class ObjectDirectoryTest extends RepositoryTestCase {
+
+	@Test
+	public void testConcurrentInsertionOfBlobsToTheSameNewFanOutDirectory()
+			throws Exception {
+		ExecutorService e = Executors.newCachedThreadPool();
+		for (int i=0; i < 100; ++i) {
+			ObjectDirectory db = createBareRepository().getObjectDatabase();
+			for (Future f : e.invokeAll(blobInsertersForTheSameFanOutDir(db))) {
+				f.get();
+			}
+		}
+	}
+
+	private Collection<Callable<ObjectId>> blobInsertersForTheSameFanOutDir(
+			final ObjectDirectory db) {
+		Callable<ObjectId> callable = new Callable<ObjectId>() {
+			public ObjectId call() throws Exception {
+				return db.newInserter().insert(Constants.OBJ_BLOB, new byte[0]);
+			}
+		};
+		return Collections.nCopies(4, callable);
+	}
+
+}

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

@@ -577,7 +577,7 @@ public class ObjectDirectory extends FileObjectDatabase {
 		// directories are always lazily created. Note that we
 		// try the rename first as the directory likely does exist.
 		//
-		FileUtils.mkdir(dst.getParentFile());
+		FileUtils.mkdir(dst.getParentFile(), true);
 		if (tmp.renameTo(dst)) {
 			dst.setReadOnly();
 			unpackedObjectCache.add(id);