[PATCH v4 0/3] fix corner cases with lock_ref_sha1_basic

On Tue, Jan 12, 2016 at 03:42:29PM -0500, Jeff King wrote:

Ah, I see. When calling git-symbolic-ref, we don't provide an old_sha1,
and therefore never call verify_lock(). And we get whatever value in
lock->old_oid we happened to read earlier in resolve_ref_unsafe(). Which
happened outside of a lock. Yikes. It seems like we could racily write
the wrong reflog entry in such a case.

[...]
The patch above is not quite right; I'll work up a series that takes
this approach.

OK, here it is. This replaces the top two patches of jk/symbolic-ref
(i.e., everything in this thread I've sent in the last day or two).

Besides fixing the race (which is detailed in patch 2/3), I think the
resulting 3/3 is much cleaner. Sorry for all the false starts. The more
I looked at this, the more complex it seemed to get. But I _think_ this
is the right solution. :)

  [1/3]: checkout,clone: check return value of create_symref
  [2/3]: lock_ref_sha1_basic: always fill old_oid while holding lock
  [3/3]: lock_ref_sha1_basic: handle REF_NODEREF with invalid refs

-Peff