From: Jeff King <peff@peff.net> To: Derrick Stolee <stolee@gmail.com> Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com> Subject: Re: [PATCH] t5304: add a test for pruning with bitmaps Date: Fri, 19 Apr 2019 23:24:35 -0400 Message-ID: <20190420032435.GA3559@sigill.intra.peff.net> (raw) In-Reply-To: <ba16afb4-25c1-8148-602e-130b0e17fc89@gmail.com> On Fri, Apr 19, 2019 at 09:01:50PM -0400, Derrick Stolee wrote: > > +test_expect_success 'trivial prune with bitmaps enabled' ' > > + git repack -adb && > > + blob=$(echo bitmap-unreachable-blob | git hash-object -w --stdin) && > > + git prune --expire=now && > > + git cat-file -e HEAD && > > + test_must_fail git cat-file -e $blob > > +' > > + > > test_done > > This test does cover the 'mark_object_seen()' method, which I tested by > removing the "!" in the "if (!obj) die();" condition. > > However, my first inclination was to remove the line > > obj->flags |= SEEN; > > from the method, and I found that it still worked! This was confusing, > so I looked for places where the SEEN flag was added during bitmap walks, > and it turns out that the objects are marked as SEEN by prepare_bitmap_walk(). I think this is only _some_ objects. The full bitmap is created by reading the on-disk bitmaps, and then filling in any necessary gaps by doing a traditional traversal. We also set it on tip objects to de-dup the initial revs->pending list. Try running t5304 with this: diff --git a/reachable.c b/reachable.c index eba6f64e01..7ec73ef43f 100644 --- a/reachable.c +++ b/reachable.c @@ -191,6 +191,8 @@ static int mark_object_seen(const struct object_id *oid, if (!obj) die("unable to create object '%s'", oid_to_hex(oid)); + if (!(obj->flags & SEEN)) + die("seen flag not already there"); obj->flags |= SEEN; return 0; } which shows that we are indeed freshly setting SEEN on some objects. Interestingly, I don't _think_ you can cause an object to get pruned accidentally here, because: 1. Any object that will miss its SEEN has to be in the bitmapped pack, and not found through normal traversal. 2. git-prune only removes loose objects, not packed ones. 3. Loose objects cannot be in the bitmapped pack. Therefore, no object can hit both cases (1) and (2). Even if that were the end of the story, I don't think I'd want to rely on it here. The post-condition of the function should be that SEEN is set on all reachable objects, whether bitmaps are used or not. And we do indeed use this elsewhere: We'll later call prune_shallow(), which uses SEEN to discard unreachable entries. I'm not sure it even makes sense to have a shallow repo with bitmaps, though. Obviously we're lacking the full graph, but I wouldn't be surprised if the shallow code quietly pretends that all is well and we generate bogus bitmaps or something. Or maybe it even mostly works as long as you don't walk over the shallow cut-points. mark_reachable_objects() is also used for reflog expiration with --stale-fix. I tried generating a test that would fail with your patch, but I think it's not quite possible, because --stale-fix will do a follow-up walk of the graph to see which objects mentioned in the reflog we have and do not have. So it doesn't actually break things, it just makes them slower (because we erroneously fail to mark SEEN things that it's then forced to walk). So I don't _think_ your patch actually breaks anything user-visible, but it's a bit like leaving a live grenade in your living room for somebody to find. And I think we are indeed covering lookup_object_by_type(), etc in the t5304 test I added. So AFAICT all of the new code is covered after that? -Peff
next prev parent reply other threads:[~2019-04-20 3:24 UTC|newest] Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-02-14 4:31 [PATCH 0/3] some prune optimizations Jeff King 2019-02-14 4:35 ` [PATCH 1/3] prune: lazily perform reachability traversal Jeff King 2019-02-14 10:54 ` Eric Sunshine 2019-02-14 11:07 ` Jeff King 2019-02-14 4:37 ` [PATCH 2/3] prune: use bitmaps for " Jeff King 2019-03-09 2:49 ` bitmaps by default? [was: prune: use bitmaps for reachability traversal] Eric Wong 2019-03-10 23:39 ` Jeff King 2019-03-12 3:13 ` [PATCH] repack: enable bitmaps by default on bare repos Eric Wong 2019-03-12 9:07 ` Ævar Arnfjörð Bjarmason 2019-03-12 10:49 ` Jeff King 2019-03-12 12:05 ` Jeff King 2019-03-13 1:51 ` Eric Wong 2019-03-13 14:54 ` Jeff King 2019-03-14 9:12 ` [PATCH v3] " Eric Wong 2019-03-14 16:02 ` Jeff King 2019-03-15 6:21 ` [PATCH 0/2] enable bitmap hash-cache by default Jeff King 2019-03-15 6:22 ` [PATCH 1/2] t5310: correctly remove bitmaps for jgit test Jeff King 2019-03-15 13:25 ` SZEDER Gábor 2019-03-15 18:36 ` Jeff King 2019-03-15 6:25 ` [PATCH 2/2] pack-objects: default to writing bitmap hash-cache Jeff King 2019-04-09 15:10 ` [PATCH v3] repack: enable bitmaps by default on bare repos Ævar Arnfjörð Bjarmason 2019-04-10 22:57 ` Jeff King 2019-04-25 7:16 ` Junio C Hamano 2019-05-04 1:37 ` Jeff King 2019-05-04 6:52 ` Ævar Arnfjörð Bjarmason 2019-05-04 13:23 ` SZEDER Gábor 2019-05-08 20:17 ` Ævar Arnfjörð Bjarmason 2019-05-09 4:24 ` Junio C Hamano 2019-05-07 7:45 ` Jeff King 2019-05-07 8:12 ` Ævar Arnfjörð Bjarmason 2019-05-08 7:11 ` Jeff King 2019-05-08 14:20 ` Derrick Stolee 2019-05-08 16:13 ` Ævar Arnfjörð Bjarmason 2019-05-08 22:25 ` Jeff King 2019-05-23 11:30 ` Jeff King 2019-05-23 12:53 ` Derrick Stolee 2019-05-24 7:24 ` Jeff King 2019-05-24 10:33 ` Derrick Stolee 2019-05-23 19:26 ` Ævar Arnfjörð Bjarmason 2019-05-24 7:27 ` Jeff King 2019-05-24 7:55 ` Ævar Arnfjörð Bjarmason 2019-05-24 8:26 ` Jeff King 2019-05-24 9:01 ` Ævar Arnfjörð Bjarmason 2019-05-24 9:29 ` SZEDER Gábor 2019-05-24 11:17 ` Ævar Arnfjörð Bjarmason 2019-05-24 11:41 ` SZEDER Gábor 2019-05-24 11:58 ` Ævar Arnfjörð Bjarmason 2019-05-24 12:34 ` SZEDER Gábor 2019-05-24 13:41 ` Ævar Arnfjörð Bjarmason 2019-05-24 11:31 ` [PATCH] pack-bitmap: look for an uninteresting bitmap Derrick Stolee 2019-04-15 15:00 ` [PATCH 2/3] prune: use bitmaps for reachability traversal Derrick Stolee 2019-04-18 19:49 ` Jeff King 2019-04-18 20:08 ` [PATCH] t5304: add a test for pruning with bitmaps Jeff King 2019-04-20 1:01 ` Derrick Stolee 2019-04-20 3:24 ` Jeff King [this message] 2019-04-20 21:01 ` Derrick Stolee 2019-02-14 4:38 ` [PATCH 3/3] prune: check SEEN flag for reachability Jeff King
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