If we're overwriting a symref with a SHA-1, we need to resolve the value
of the symref (1) to check against update->old_sha1 and (2) to write to
its reflog. However, we've already read the symref itself and know its
referent. So there is no need to read the symref's value through the
symref; we can read the referent directly.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

To aid the effort, extract a new function, check_old_oid(), and use it
in the two places where the read value of the reference has to be
checked against update->old_sha1.

Update tests to reflect the improvements.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Some of the error messages will be improved in subsequent commits.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The tests already set a variable called prefix and passed its value as
the first argument to this function. The old argument handling was
overwriting the global variable with its same value rather than creating
a local variable.

So change test_update_rejected to refer to the global variable rather
than taking the prefix as an argument.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

I want to broaden the scope of this test file, so rename it accordingly.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Now lock_ref_sha1_basic() is only called with flags==REF_NODEREF. So we
don't have to handle other cases anymore.

This enables several simplifications, the most interesting of which come
from the fact that ref_lock::orig_ref_name is now always the same as
ref_lock::ref_name:

* Remove ref_lock::orig_ref_name
* Remove local variable orig_refname from lock_ref_sha1_basic()
* ref_name can be initialize once and its value reused
* commit_ref_update() never has to write to the reflog for
  lock->orig_ref_name

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

commit_ref_update() is now only called with flags=0. So remove the flags
parameter entirely.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

If a transaction includes a non-NODEREF update to a symbolic reference,
we don't have to look it up in lock_ref_for_update(). The reference will
be dereferenced anyway when the split-off update is processed.

This change requires that we store a backpointer from the split-off
update to its parent update, for two reasons:

* We still want to report the original reference name in error messages.
  So if an error occurs when checking the split-off update's old_sha1,
  walk the parent_update pointers back to find the original reference
  name, and report that one.

* We still need to write the old_sha1 of the symref to its reflog. So
  after we read the split-off update's reference value, walk the
  parent_update pointers back and fill in their old_sha1 fields.

Aside from eliminating unnecessary reads, this change fixes a
subtle (though not very serious) race condition: in the old code, the
old_sha1 of the symref was resolved before the reference that it pointed
at was locked. So it was possible that the old_sha1 value logged to the
symref's reflog could be wrong if another process changed the downstream
reference before it was locked.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

Before the previous patch, our first read of the reference happened
before the reference was locked, so we couldn't trust its value and had
to read it again. But now that our first read of the reference happens
after acquiring the lock, there is no need to read it a second time. So
move the read_ref_full() call into the (update->type & REF_ISSYMREF)
block.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>