Do not fake a SymbolicRef as an ObjectIdRef

When doing a detaching operation, JGit fakes a SymbolicRef as an
ObjectIdRef. This is because RefUpdate#updateImpl dereferences the
SymbolicRef when updating it. For example, assume that HEAD is
pointing to refs/heads/master. If I try to make a detached HEAD
pointing to a commit c0ffee, RefUpdate dereferences HEAD as
refs/heads/master first and changes refs/heads/master to c0ffee. The
detach argument of RefDatabase#newUpdate avoids this dereference by
faking HEAD as ObjectIdRef.

This faking is problematic for the linking operation of
DfsRefDatabase. It does a compare-and-swap operation on every
reference change because of its distributed systems nature. If a
SymbolicRef is faked as an ObjectRef, it thinks that there is a
racing change in the reference and rejects the update. Because of
this, DFS based repositories cannot change the link target of symbolic
refs. This has not been a problem for file-based repositories because
they have a file-lock based semantics instead of the CAS based one.
The reference implementation, InMemoryRepository, is not affected
because it only compares ObjectIds.

When [1] introduced this faking code, there was no way for RefUpdate
to distinguish the detaching operation. When [2] fixed the detaching
operation, it introduced a detachingSymbolicRef flag. This commit uses
this flag to control whether it needs to dereference the symbolic refs
by calling Ref#getLeaf. The same flag is used in the reflog update
operation.

This commit does not affect any operation that succeeds currently. In
some DFS repository implementations, this fixes a ref linking
operation, which is currently failing.

[1]: 01b5392cdb
[2]: 3a86868c08

Change-Id: I118f85f0414dbfad02250944e28d74dddd59469b
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsRefDatabase.java

@@ -217,10 +217,6 @@ public abstract class DfsRefDatabase extends RefDatabase {
 		else
 			detachingSymbolicRef = detach && ref.isSymbolic();
 
-		if (detachingSymbolicRef) {
-			ref = new ObjectIdRef.Unpeeled(NEW, refName, ref.getObjectId());
-		}
-
 		DfsRefUpdate update = new DfsRefUpdate(this, ref);
 		if (detachingSymbolicRef)
 			update.setDetachingSymbolicRef();

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

@@ -545,8 +545,6 @@ public class RefDirectory extends RefDatabase {
 			ref = new ObjectIdRef.Unpeeled(NEW, name, null);
 		else {
 			detachingSymbolicRef = detach && ref.isSymbolic();
-			if (detachingSymbolicRef)
-				ref = new ObjectIdRef.Unpeeled(LOOSE, name, ref.getObjectId());
 		}
 		RefDirectoryUpdate refDirUpdate = new RefDirectoryUpdate(this, ref);
 		if (detachingSymbolicRef)
@@ -579,7 +577,7 @@ public class RefDirectory extends RefDatabase {
 	}
 
 	void delete(RefDirectoryUpdate update) throws IOException {
-		Ref dst = update.getRef().getLeaf();
+		Ref dst = update.getRef();
 		String name = dst.getName();
 
 		// Write the packed-refs file using an atomic update. We might

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

@@ -56,6 +56,7 @@ import org.eclipse.jgit.lib.Repository;
 class RefDirectoryUpdate extends RefUpdate {
 	private final RefDirectory database;
 
+	private boolean shouldDeref;
 	private LockFile lock;
 
 	RefDirectoryUpdate(final RefDirectory r, final Ref ref) {
@@ -75,6 +76,7 @@ class RefDirectoryUpdate extends RefUpdate {
 
 	@Override
 	protected boolean tryLock(boolean deref) throws IOException {
+		shouldDeref = deref;
 		Ref dst = getRef();
 		if (deref)
 			dst = dst.getLeaf();
@@ -117,7 +119,7 @@ class RefDirectoryUpdate extends RefUpdate {
 						msg = strResult;
 				}
 			}
-			database.log(this, msg, true);
+			database.log(this, msg, shouldDeref);
 		}
 		if (!lock.commit())
 			return Result.LOCK_FAILURE;
@@ -140,7 +142,7 @@ class RefDirectoryUpdate extends RefUpdate {
 
 	@Override
 	protected Result doDelete(final Result status) throws IOException {
-		if (getRef().getLeaf().getStorage() != Ref.Storage.NEW)
+		if (getRef().getStorage() != Ref.Storage.NEW)
 			database.delete(this);
 		return status;
 	}

org.eclipse.jgit/src/org/eclipse/jgit/lib/RefUpdate.java

@@ -551,7 +551,9 @@ public abstract class RefUpdate {
 	 * @throws IOException
 	 */
 	public Result delete(final RevWalk walk) throws IOException {
-		final String myName = getRef().getLeaf().getName();
+		final String myName = detachingSymbolicRef
+				? getRef().getName()
+				: getRef().getLeaf().getName();
 		if (myName.startsWith(Constants.R_HEADS) && !getRepository().isBare()) {
 			// Don't allow the currently checked out branch to be deleted.
 			Ref head = getRefDatabase().getRef(Constants.HEAD);
@@ -628,7 +630,10 @@ public abstract class RefUpdate {
 		if (oldValue == null && checkConflicting && getRefDatabase().isNameConflicting(getName()))
 			return Result.LOCK_FAILURE;
 		try {
-			if (!tryLock(true))
+			// If we're detaching a symbolic reference, we should update the reference
+			// itself. Otherwise, we will update the leaf reference, which should be
+			// an ObjectIdRef.
+			if (!tryLock(!detachingSymbolicRef))
 				return Result.LOCK_FAILURE;
 			if (expValue != null) {
 				final ObjectId o;