Re: [PATCH 1/2] prepare_packed_git(): refactor garbage reporting in pack directory

[Doug Kelly <dougk.ff7@gmail.com>]

On Wed, Dec 30, 2015 at 1:37 AM, Jeff King <peff@peff.net> wrote:
> On Wed, Nov 04, 2015 at 02:08:21PM -0600, Doug Kelly wrote:
>
>> On Wed, Nov 4, 2015 at 2:02 PM, Jeff King <peff@peff.net> wrote:
>> > Definitely cleaning up the .bitmap is sane and not racy (it's in the
>> > same boat as the .idx, I think).
>> >
>> > .keep files are more tricky. I'd have to go over the receive-pack code
>> > to confirm, but I think they _are_ racy. That is, receive-pack will
>> > create them as a lockfile before moving the pack into place. That's OK,
>> > though, if we use mtimes to give ourselves a grace period (I haven't
>> > looked at your series yet).
>> >
>> > But moreover, .keep files can be created manually by the user. If the
>> > pack they referenced goes away, they are not really serving any purpose.
>> > But it's possible that the user would want to salvage the content of the
>> > file, or know that it was there.
>> >
>> > So I'd argue we should leave them. Or at least leave ones that do not
>> > have the generic "{receive,fetch}-pack $pid on $host comment in them,
>> > which were clearly created as lockfiles.
>>
>> Currently there's no mtime-guarding logic (I dug up that conversation
>> earlier, though, but after I'd done the respin on this series)... OK,
>> in that case, I'll create a separate patch that tests/cleans up
>> .bitmap, but doesn't touch .keep.  This might be a small series since
>> I think the logic for finding pack garbage doesn't know anything about
>> .bitmap per-se, so it's looking like I'll extend that relevant code,
>> before adding the handling in gc and appropriate tests.
>
> I happened to be looking over your series again, and I noticed that we
> didn't end up with any mtime logic at all in what got merged.
>
> I _think_ that is probably OK, because we always write the pack,
> followed by the .idx, followed by the .bitmap (if any). And we don't
> drop .keep files (though I think we would perhaps note them as possible
> cruft?).
>
> So I don't think there are any races introduced here, but I wonder if we
> want to be a bit more conservative. Sorry to bring this up so much after
> the fact; I completely forgot about it when reviewing the patches.
>
> These changes are slated for the v2.7 release. Like I said, I don't
> think it's buggy, so we don't necessarily need to address it before the
> release. We could add an mtime check in the next cycle as a
> belt-and-suspenders safety, rather than a fix.
>
> -Peff

Yeah, I know I never got to adding the mtime logic, but for a simple (naive,
hard-coded) case, I did come up with a basic patch today.  I think this could
be extended to a configuration option(?) which would allow a default longer
than 10 seconds (an hour? a day?), then during the regression tests, we
could provide a shorter timeout to ensure the guarding both works and also
not wait forever for tests to complete.  Thoughts?

---
 builtin/gc.c     | 14 ++++++++++++--
 t/t5304-prune.sh |  2 ++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index 79e9886..a4ce616 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -51,8 +51,18 @@ static struct string_list pack_garbage =
STRING_LIST_INIT_DUP;
 static void clean_pack_garbage(void)
 {
  int i;
- for (i = 0; i < pack_garbage.nr; i++)
- unlink_or_warn(pack_garbage.items[i].string);
+ /* Define a cutoff time for "new" garbage to prevent race conditions */
+ time_t cutoff = time(NULL) - 10;
+ for (i = 0; i < pack_garbage.nr; i++) {
+ struct stat s;
+ char *garbage = pack_garbage.items[i].string;
+ if (!stat(garbage, &s)) {
+ if (s.st_mtime < cutoff)
+ unlink_or_warn(garbage);
+ } else
+ fprintf(stderr, _("stat failed on pack garbage: %s"),
+ garbage);
+ }
  string_list_clear(&pack_garbage, 0);
 }

diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh
index cbcc0c0..7b4650f 100755
--- a/t/t5304-prune.sh
+++ b/t/t5304-prune.sh
@@ -272,6 +272,7 @@ test_expect_success 'clean pack garbage with gc' '
  : >.git/objects/pack/fake6.keep &&
  : >.git/objects/pack/fake6.bitmap &&
  : >.git/objects/pack/fake6.idx &&
+ sleep 10 &&
  git gc &&
  git count-objects -v 2>stderr &&
  grep "^warning:" stderr | sort >actual &&
@@ -291,6 +292,7 @@ test_expect_success 'ensure unknown garbage kept with gc' '
  : >.git/objects/pack/foo.keep &&
  : >.git/objects/pack/fake.pack &&
  : >.git/objects/pack/fake2.foo &&
+ sleep 10 &&
  git gc &&
  git count-objects -v 2>stderr &&
  grep "^warning:" stderr | sort >actual &&
-- 
2.6.1