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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>
This commit is contained in:
Masaya Suzuki 2016-07-13 16:57:03 -07:00 committed by Masaya Suzuki
parent ffbe03aa9b
commit 3e27fb3719
4 changed files with 12 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsRefDatabase.java
View File

@ -217,10 +217,6 @@ public RefUpdate newUpdate(String refName, boolean detach)
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();

4
org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java
View File

@ -545,8 +545,6 @@ public RefDirectoryUpdate newUpdate(String name, boolean detach)
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 @@ private void putLooseRef(LooseRef ref) {
}
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

6
org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectoryUpdate.java
View File

@ -56,6 +56,7 @@
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 @@ protected Repository getRepository() {
@Override
protected boolean tryLock(boolean deref) throws IOException {
shouldDeref = deref;
Ref dst = getRef();
if (deref)
dst = dst.getLeaf();
@ -117,7 +119,7 @@ protected Result doUpdate(final Result status) throws IOException {
msg = strResult;
}
}
database.log(this, msg, true);
database.log(this, msg, shouldDeref);
}
if (!lock.commit())
return Result.LOCK_FAILURE;
@ -140,7 +142,7 @@ private String toResultString(final Result status) {
@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;
}

9
org.eclipse.jgit/src/org/eclipse/jgit/lib/RefUpdate.java
View File

@ -551,7 +551,9 @@ public Result delete() throws IOException {
* @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 @@ private Result updateImpl(final RevWalk walk, final Store store)
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;