RefDirectory: Throw exception if CAS of packed ref list fails

The contents of the packedRefList AtomicReference should never differ
from what we expect prior to writing, because this segment of the code
is protected by the packed-refs lock file on disk. If it does happen,
whether due to programmer error or a rogue process not respecting the
locking protocol, it's better to let the caller know than to silently
drop the whole commit operation on the floor.

The existing concurrentOnlyOneWritesPackedRefs test is inherently
nondeterministic as written, and was already about 6% flaky as measured
by bazel:

  $ bazel test --runs_per_test=200 //org.eclipse.jgit.test:org_eclipse_jgit_internal_storage_file_GcPackRefsTest
  ...
  INFO: Elapsed time: 42.608s, Critical Path: 10.35s
  //org.eclipse.jgit.test:org_eclipse_jgit_internal_storage_file_GcPackRefsTest FAILED in 12 out of 200 in 1.6s
    Stats over 200 runs: max = 1.6s, min = 1.1s, avg = 1.3s, dev = 0.1s

This flakiness was caused by the assumption that exactly one of the 2
threads would fail, when both might actually succeed in practice due to
racing on the compare-and-swap.

For whatever reason, this change affected the interleaving behavior in
such a way that the flakiness jumped to around 50%. Making the
interleaving of the test fully deterministic is beyond the scope of this
change, but a simple tweak to the assertion is enough to make it pass
consistently 200+ times both before and after this change.

Change-Id: I5ff4dc39ee05bda88d47909acb70118f3d0c8f74

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcPackRefsTest.java

@@ -43,12 +43,13 @@
 
 package org.eclipse.jgit.internal.storage.file;
 
-import static java.lang.Integer.valueOf;
+import static org.hamcrest.Matchers.lessThanOrEqualTo;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThat;
 
 import java.io.File;
 import java.io.IOException;
@@ -74,6 +75,7 @@ import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.junit.Test;
 
+@SuppressWarnings("boxing")
 public class GcPackRefsTest extends GcTestCase {
 	@Test
 	public void looseRefPacked() throws Exception {
@@ -100,27 +102,23 @@ public class GcPackRefsTest extends GcTestCase {
 		RevBlob a = tr.blob("a");
 		tr.lightweightTag("t", a);
 
-		final CyclicBarrier syncPoint = new CyclicBarrier(2);
+		CyclicBarrier syncPoint = new CyclicBarrier(2);
 
-		Callable<Integer> packRefs = new Callable<Integer>() {
-
-			/** @return 0 for success, 1 in case of error when writing pack */
-			@Override
-			public Integer call() throws Exception {
+		// Returns 0 for success, 1 in case of error when writing pack.
+		Callable<Integer> packRefs = () -> {
 			syncPoint.await();
 			try {
 				gc.packRefs();
-					return valueOf(0);
+				return 0;
 			} catch (IOException e) {
-					return valueOf(1);
-				}
+				return 1;
 			}
 		};
 		ExecutorService pool = Executors.newFixedThreadPool(2);
 		try {
 			Future<Integer> p1 = pool.submit(packRefs);
 			Future<Integer> p2 = pool.submit(packRefs);
-			assertEquals(1, p1.get().intValue() + p2.get().intValue());
+			assertThat(p1.get() + p2.get(), lessThanOrEqualTo(1));
 		} finally {
 			pool.shutdown();
 			pool.awaitTermination(Long.MAX_VALUE, TimeUnit.SECONDS);

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java

@@ -914,8 +914,24 @@ public class RefDirectory extends RefDatabase {
 					throw new ObjectWritingException(MessageFormat.format(JGitText.get().unableToWrite, name));
 
 				byte[] digest = Constants.newMessageDigest().digest(content);
-				packedRefs.compareAndSet(oldPackedList, new PackedRefList(refs,
-						lck.getCommitSnapshot(), ObjectId.fromRaw(digest)));
+				PackedRefList newPackedList = new PackedRefList(
+						refs, lck.getCommitSnapshot(), ObjectId.fromRaw(digest));
+
+				// This thread holds the file lock, so no other thread or process should
+				// be able to modify the packed-refs file on disk. If the list changed,
+				// it means something is very wrong, so throw an exception.
+				//
+				// However, we can't use a naive compareAndSet to check whether the
+				// update was successful, because another thread might _read_ the
+				// packed refs file that was written out by this thread while holding
+				// the lock, and update the packedRefs reference to point to that. So
+				// compare the actual contents instead.
+				PackedRefList afterUpdate = packedRefs.updateAndGet(
+						p -> p.id.equals(oldPackedList.id) ? newPackedList : p);
+				if (!afterUpdate.id.equals(newPackedList.id)) {
+					throw new ObjectWritingException(
+							MessageFormat.format(JGitText.get().unableToWrite, name));
+				}
 			}
 		}.writePackedRefs();
 	}