refs: do not use cached refs in repack_without_ref

When we delete a ref that is packed, we rewrite the whole
packed-refs file and simply omit the ref that no longer
exists. However, we base the rewrite on whatever happens to
be in our refs cache, not what is necessarily on disk. That
opens us up to a race condition if another process is
simultaneously packing the refs, as we will overwrite their
newly-made pack-refs file with our potentially stale data,
losing commits.

You can demonstrate the race like this:

  # setup some repositories
  git init --bare parent &&
  (cd parent && git config core.logallrefupdates true) &&
  git clone parent child &&
  (cd child && git commit --allow-empty -m base)

  # in one terminal, repack the refs repeatedly
  cd parent &&
  while true; do
	git pack-refs --all
  done

  # in another terminal, simultaneously push updates to
  # master, and create and delete an unrelated ref
  cd child &&
  while true; do
	git push origin HEAD:newbranch &&
	git commit --allow-empty -m foo
	us=`git rev-parse master` &&
	git push origin master &&
	git push origin :newbranch &&
	them=`git --git-dir=../parent rev-parse master` &&
	if test "$them" != "$us"; then
		echo >&2 "$them" != "$us"
		exit 1
	fi
  done

In many cases the two processes will conflict over locking
the packed-refs file, and the deletion of newbranch will
simply fail.  But eventually you will hit the race, which
happens like this:

  1. We push a new commit to master. It is already packed
     (from the looping pack-refs call). We write the new
     value (let us call it B) to $GIT_DIR/refs/heads/master,
     but the old value (call it A) remains in the
     packed-refs file.

  2. We push the deletion of newbranch, spawning a
     receive-pack process. Receive-pack advertises all refs
     to the client, causing it to iterate over each ref; it
     caches the packed refs in memory, which points at the
     stale value A.

  3. Meanwhile, a separate pack-refs process is running. It
     runs to completion, updating the packed-refs file to
     point master at B, and deleting $GIT_DIR/refs/heads/master
     which also pointed at B.

  4. Back in the receive-pack process, we get the
     instruction to delete :newbranch. We take a lock on
     packed-refs (which works, as the other pack-refs
     process has already finished). We then rewrite the
     contents using the cached refs, which contain the stale
     value A.

The resulting packed-refs file points master once again at
A. The loose ref which would override it to point at B was
deleted (rightfully) in step 3. As a result, master now
points at A. The only trace that B ever existed in the
parent is in the reflog: the final entry will show master
moving from A to B, even though the ref still points at A
(so you can detect this race after the fact, because the
next reflog entry will move from A to C).

We can fix this by invalidating the packed-refs cache after
we have taken the lock. This means that we will re-read the
packed-refs file, and since we have the lock, we will be
sure that what we read will be atomically up-to-date when we
write (it may be out of date with respect to loose refs, but
that is OK, as loose refs take precedence).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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