Only increment mod count if packed-refs file changes

Previously if a packed-refs file was racily clean then there
was a 2.5 second window in which each call to getPackedRefs
would increment the mod count causing a RefsChangedEvent to be
fired since the FileSnapshot would report the file as modified.

If a RefsChangedListener called getRef/getRefs from the
onRefsChanged method then a StackOverflowError could occur
since the stack could be exhausted before the 2.5 second
window expired and the packed-refs file would no longer
report being modified.

Now a SHA-1 is computed of the packed-refs file and the
mod count is only incremented when the packed refs are
successfully set and the id of the new packed-refs file
does not match the id of the old packed-refs file.

Change-Id: I8cab6e5929479ed748812b8598c7628370e79697

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

@@ -60,6 +60,8 @@ import static org.junit.Assert.fail;
 import java.io.File;
 import java.io.IOException;
 import java.util.Map;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.eclipse.jgit.events.ListenerHandle;
 import org.eclipse.jgit.events.RefsChangedEvent;
@@ -1077,6 +1079,36 @@ public class RefDirectoryTest extends LocalDiskRepositoryTestCase {
 		assertSame(master_p2, refdir.peel(master_p2));
 	}
 
+	@Test
+	public void testRefsChangedStackOverflow() throws Exception {
+		final FileRepository newRepo = createBareRepository();
+		final RefDatabase refDb = newRepo.getRefDatabase();
+		File packedRefs = new File(newRepo.getDirectory(), "packed-refs");
+		assertTrue(packedRefs.createNewFile());
+		final AtomicReference<StackOverflowError> error = new AtomicReference<StackOverflowError>();
+		final AtomicReference<IOException> exception = new AtomicReference<IOException>();
+		final AtomicInteger changeCount = new AtomicInteger();
+		newRepo.getListenerList().addRefsChangedListener(
+				new RefsChangedListener() {
+
+					public void onRefsChanged(RefsChangedEvent event) {
+						try {
+							refDb.getRefs("ref");
+							changeCount.incrementAndGet();
+						} catch (StackOverflowError soe) {
+							error.set(soe);
+						} catch (IOException ioe) {
+							exception.set(ioe);
+						}
+					}
+				});
+		refDb.getRefs("ref");
+		refDb.getRefs("ref");
+		assertNull(error.get());
+		assertNull(exception.get());
+		assertEquals(1, changeCount.get());
+	}
+
 	private void writeLooseRef(String name, AnyObjectId id) throws IOException {
 		writeLooseRef(name, id.name() + "\n");
 	}

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

@@ -63,6 +63,8 @@ import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStreamReader;
+import java.security.DigestInputStream;
+import java.security.MessageDigest;
 import java.text.MessageFormat;
 import java.util.Arrays;
 import java.util.LinkedList;
@@ -626,24 +628,27 @@ public class RefDirectory extends RefDatabase {
 			return curList;
 
 		final PackedRefList newList = readPackedRefs();
-		if (packedRefs.compareAndSet(curList, newList))
+		if (packedRefs.compareAndSet(curList, newList)
+				&& !curList.id.equals(newList.id))
 			modCnt.incrementAndGet();
 		return newList;
 	}
 
-	private PackedRefList readPackedRefs()
-			throws IOException {
+	private PackedRefList readPackedRefs() throws IOException {
 		final FileSnapshot snapshot = FileSnapshot.save(packedRefsFile);
 		final BufferedReader br;
+		final MessageDigest digest = Constants.newMessageDigest();
 		try {
-			br = new BufferedReader(new InputStreamReader(new FileInputStream(
-					packedRefsFile), CHARSET));
+			br = new BufferedReader(new InputStreamReader(
+					new DigestInputStream(new FileInputStream(packedRefsFile),
+							digest), CHARSET));
 		} catch (FileNotFoundException noPackedRefs) {
 			// Ignore it and leave the new list empty.
 			return PackedRefList.NO_PACKED_REFS;
 		}
 		try {
-			return new PackedRefList(parsePackedRefs(br), snapshot);
+			return new PackedRefList(parsePackedRefs(br), snapshot,
+					ObjectId.fromRaw(digest.digest()));
 		} finally {
 			br.close();
 		}
@@ -724,8 +729,9 @@ public class RefDirectory extends RefDatabase {
 				if (!lck.commit())
 					throw new ObjectWritingException(MessageFormat.format(JGitText.get().unableToWrite, name));
 
-				packedRefs.compareAndSet(oldPackedList, new PackedRefList(
-						refs, lck.getCommitSnapshot()));
+				byte[] digest = Constants.newMessageDigest().digest(content);
+				packedRefs.compareAndSet(oldPackedList, new PackedRefList(refs,
+						lck.getCommitSnapshot(), ObjectId.fromRaw(digest)));
 			}
 		}.writePackedRefs();
 	}
@@ -899,13 +905,17 @@ public class RefDirectory extends RefDatabase {
 
 	private static class PackedRefList extends RefList<Ref> {
 		static final PackedRefList NO_PACKED_REFS = new PackedRefList(
-				RefList.emptyList(), FileSnapshot.MISSING_FILE);
+				RefList.emptyList(), FileSnapshot.MISSING_FILE,
+				ObjectId.zeroId());
 
 		final FileSnapshot snapshot;
 
-		PackedRefList(RefList<Ref> src, FileSnapshot s) {
+		final ObjectId id;
+
+		PackedRefList(RefList<Ref> src, FileSnapshot s, ObjectId i) {
 			super(src);
 			snapshot = s;
+			id = i;
 		}
 	}