repack_without_ref(): silence errors for dangling packed refs

Stop emitting an error message when deleting a packed reference if we
find another dangling packed reference that is overridden by a loose
reference.  See the previous commit for a longer explanation of the
issue.

We have to be careful to make sure that the invalid packed reference
really *is* overridden by a loose reference; otherwise what we have
found is repository corruption, which we *should* report.

Please note that this approach is vulnerable to a race condition
similar to the race conditions already known to affect packed
references [1]:

* Process 1 tries to peel packed reference X as part of deleting
  another packed reference.  It discovers that X does not refer to a
  valid object (because the object that it referred to has been
  garbage collected).

* Process 2 tries to delete reference X.  It starts by deleting the
  loose reference X.

* Process 1 checks whether there is a loose reference X.  There is not
  (it has just been deleted by process 2), so process 1 reports a
  spurious error "X does not point to a valid object!"

The worst case seems relatively harmless, and the fix is identical to
the fix that will be needed for the other race conditions (namely
holding a lock on the packed-refs file during *all* reference
deletions), so we leave the cleaning up of all of them as a future
project.

[1] http://thread.gmane.org/gmane.comp.version-control.git/211956

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

1 files changed, 1 insertions, 1 deletions

diff --git a/t/t3210-pack-refs.sh b/t/t3210-pack-refs.sh
index 675f06a02d..1a2080e3dc 100755
--- a/t/t3210-pack-refs.sh
+++ b/t/t3210-pack-refs.sh
@@ -140,7 +140,7 @@ test_expect_success 'delete ref with dangling packed version' '
 	test_cmp /dev/null result
 '
 
-test_expect_failure 'delete ref while another dangling packed ref' '
+test_expect_success 'delete ref while another dangling packed ref' '
 	git branch lamb &&
 	git commit --allow-empty -m "future garbage" &&
 	git pack-refs --all &&