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repack_without_ref(): silence errors for dangling packed refs

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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>
This commit is contained in:
Michael Haggerty 2013-04-22 21:52:25 +02:00 committed by Junio C Hamano
parent 0a0b8d1531
commit ab292bc4f3
2 changed files with 36 additions and 3 deletions
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37
refs.c
View File

@ -1902,8 +1902,41 @@ static int repack_without_ref_fn(struct ref_entry *entry, void *cb_data)
if (!strcmp(data->refname, entry->name))
return 0;
if (!ref_resolves_to_object(entry))
return 0; /* Skip broken refs */
if (entry->flag & REF_ISBROKEN) {
/* This shouldn't happen to packed refs. */
error("%s is broken!", entry->name);
return 0;
}
if (!has_sha1_file(entry->u.value.sha1)) {
unsigned char sha1[20];
int flags;
if (read_ref_full(entry->name, sha1, 0, &flags))
/* We should at least have found the packed ref. */
die("Internal error");
if ((flags & REF_ISSYMREF) || !(flags & REF_ISPACKED))
/*
* This packed reference is overridden by a
* loose reference, so it is OK that its value
* is no longer valid; for example, it might
* refer to an object that has been garbage
* collected. For this purpose we don't even
* care whether the loose reference itself is
* invalid, broken, symbolic, etc. Silently
* omit the packed reference from the output.
*/
return 0;
/*
* There is no overriding loose reference, so the fact
* that this reference doesn't refer to a valid object
* indicates some kind of repository corruption.
* Report the problem, then omit the reference from
* the output.
*/
error("%s does not point to a valid object!", entry->name);
return 0;
}
len = snprintf(line, sizeof(line), "%s %s\n",
sha1_to_hex(entry->u.value.sha1), entry->name);
/* this should not happen but just being defensive */

2
t/t3210-pack-refs.sh
View File

@ -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 &&