| Message ID | pull.1711.git.1712495507815.gitgitgadget@gmail.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | midx: disable replace objects | expand |
On Sun, Apr 07, 2024 at 01:11:47PM +0000, blanet via GitGitGadget wrote: > From: Xing Xin <xingxin.xx@bytedance.com> > > We observed a series of clone failures arose in a specific set of > repositories after we fully enabled the MIDX bitmap feature within our > Codebase service. These failures were accompanied with error messages > such as: > > fatal: did not receive expected object ... > fatal: fetch-pack: invalid index-pack output > > Temporarily disabling the MIDX feature eliminated the reported issues. > After some investigation we found that all repositories experiencing > failures contain replace references, which seem to be improperly > acknowledged by the MIDX bitmap generation logic. I was suspicious that this might be related to the MIDX or MIDX bitmap, but noticed something curious upon digging in. Applying the following on top of your patch: --- 8< --- diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh index 5e4cdef6a8..8543f8d097 100755 --- a/t/t5326-multi-pack-bitmaps.sh +++ b/t/t5326-multi-pack-bitmaps.sh @@ -451,9 +451,7 @@ test_expect_success 'do not follow replace objects for MIDX bitmap' ' git rev-list --objects --no-object-names $B |sort >expected && git replace $A $C && - git repack -ad && - git multi-pack-index write --bitmap && - git rev-list --objects --no-object-names --use-bitmap-index $B |sort >actual && + git rev-list --objects --no-object-names $B |sort >actual && test_cmp expected actual ) ' --- >8 --- , I can still produce the failure that you are seeing here. So I suspect that while it's entirely possible that there is a bug in the MIDX/bitmap code, that this test is not exercising it. I think the first step to demonstrate a bug in the MIDX/bitmap machinery would be to provide a reproducer that fails only when using a MIDX and/or bitmap. > @@ -273,6 +274,8 @@ int cmd_multi_pack_index(int argc, const char **argv, > }; > struct option *options = parse_options_concat(builtin_multi_pack_index_options, common_opts); > > + disable_replace_refs(); > + Supposing for a moment that this issue is in the MIDX, we know that regardless of what replace refs might be in place, the MIDX should only be storing the objects that are in the packs being indexed, not the objects which are their replacements. Are we storing objects in the MIDX that are replacements? Looking at midx.c::fill_pack_entry(), I think the answer is "no", since we're looking up packed objects by calling nth_packed_object_id(), which is just a table read into the .idx, all of which is beneath the level of replace refs. > @@ -434,6 +434,30 @@ test_expect_success 'tagged commits are selected for bitmapping' ' > ) > ' > > +test_expect_success 'do not follow replace objects for MIDX bitmap' ' > + rm -fr repo && > + git init repo && > + test_when_finished "rm -fr repo" && > + ( > + cd repo && > + > + test_commit A && > + A=$(git rev-parse HEAD) && It's possible that much of this will be moot if the current test gets rewritten, but here are a couple of suggestions for writing tests in Git's suite: - test_commit will create a tag for you, so there is no need to store "$A", "$B", and "$C". > + test_commit B && > + B=$(git rev-parse HEAD) && > + git checkout --orphan=orphan $A && > + test_commit orphan && > + C=$(git rev-parse HEAD) && > + git rev-list --objects --no-object-names $B |sort >expected && - We do not allow Git invocations on the left-hand side of a pipe, since doing so will squelch its exit code. Instead, try: git rev-list --objects --no-object-names B >expect.raw && sort expect.raw >expect && Thanks, Taylor
On Sun, Apr 07, 2024 at 10:16:28AM -0400, Taylor Blau wrote: > , I can still produce the failure that you are seeing here. So I suspect > that while it's entirely possible that there is a bug in the MIDX/bitmap > code, that this test is not exercising it. > > I think the first step to demonstrate a bug in the MIDX/bitmap machinery > would be to provide a reproducer that fails only when using a MIDX > and/or bitmap. I had a some more time to look into this, and I think that your original fix is correct. The issue is, as you suggest, due to the following (from your original patch): > After some investigation we found that all repositories experiencing > failures contain replace references, which seem to be improperly > acknowledged by the MIDX bitmap generation logic. Indeed, the pack-bitmap-write machinery does not itself call disable_replace_refs(). So when it generates a reachability bitmap, it is doing so with the replace refs in mind. You can see that this is indeed the cause of the problem by looking at the output of an instrumented version of Git that indicates what bits are being set during the bitmap generation phase. With replace refs (incorrectly) enabled, we get: [2, 4, 6, 8, 13, 3, 6, 7, 3, 4, 6, 8] and doing the same after calling disable_replace_refs(), we instead get: [2, 5, 6, 13, 3, 6, 7, 3, 4, 6, 8] Single pack bitmaps are unaffected by this issue because we generate them from within pack-objects, which does call disable_replace_refs(). It is tempting to instead do something like: --- 8< --- diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c index c6c8f94cc5..cbc543caad 100644 --- a/pack-bitmap-write.c +++ b/pack-bitmap-write.c @@ -17,6 +17,7 @@ #include "trace2.h" #include "tree.h" #include "tree-walk.h" +#include "replace-object.h" struct bitmapped_commit { struct commit *commit; @@ -223,6 +224,8 @@ static void bitmap_builder_init(struct bitmap_builder *bb, memset(bb, 0, sizeof(*bb)); init_bb_data(&bb->data); + parsed_object_pool_clear(the_repository->parsed_objects); + reset_revision_walk(); repo_init_revisions(writer->to_pack->repo, &revs, NULL); revs.topo_order = 1; --- >8 --- But by then it is too late, because the replace refs have already been taken into account for parsed objects. An alternative is to clear the parsed_object_pool before (or after) calling disable_replace_refs(), but I think that approach that feels sub-optimal for a couple of reasons: - We're wasting time re-parsing objects that we've already seen - We're banking on the fact that the MIDX generation does not lookup objects with the OBJECT_INFO_LOOKUP_REPLACE flag set, which would cause the MIDX to be broken in the same way. So I think that disabling replace refs at the outset within the multi-pack-index builtin is the right way to go. In addition to the test fixes I suggested earlier, I would instead demonstrate the bug by showing a clone (which fails with MIDXs, but doesn't without MIDXs) like so: --- 8< --- diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh index 5e4cdef6a8..1fb3b0f9d7 100755 --- a/t/t5326-multi-pack-bitmaps.sh +++ b/t/t5326-multi-pack-bitmaps.sh @@ -442,19 +442,16 @@ test_expect_success 'do not follow replace objects for MIDX bitmap' ' cd repo && test_commit A && - A=$(git rev-parse HEAD) && test_commit B && - B=$(git rev-parse HEAD) && - git checkout --orphan=orphan $A && + git checkout --orphan=orphan A && test_commit orphan && - C=$(git rev-parse HEAD) && - git rev-list --objects --no-object-names $B |sort >expected && - git replace $A $C && - git repack -ad && - git multi-pack-index write --bitmap && - git rev-list --objects --no-object-names --use-bitmap-index $B |sort >actual && - test_cmp expected actual + git replace A HEAD && + git repack -ad --write-midx --write-bitmap-index && + + # generating reachability bitmaps with replace refs + # enabled will result in broken clones + git clone --no-local --bare . clone.git ) ' --- >8 --- With the change in your patch to call disable_replace_refs() in builtin/multi-pack-index.c, this test passes as expected. With that change compiled out, we instead get: [...] + git clone --no-local --bare . clone.git Cloning into bare repository 'clone.git'... remote: Enumerating objects: 8, done. remote: Total 8 (delta 0), reused 0 (delta 0), pack-reused 8 (from 1) Receiving objects: 100% (8/8), done. fatal: did not receive expected object da5497437fd67ca928333aab79c4b4b55036ea66 fatal: fetch-pack: invalid index-pack output error: last command exited with $?=128 not ok 352 - do not follow replace objects for MIDX bitmap as expected. Thanks, Taylor
On Sun, Apr 07, 2024 at 02:02:17PM -0400, Taylor Blau wrote: > It is tempting to instead do something like: > > --- 8< --- > diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c > index c6c8f94cc5..cbc543caad 100644 > --- a/pack-bitmap-write.c > +++ b/pack-bitmap-write.c > @@ -17,6 +17,7 @@ > #include "trace2.h" > #include "tree.h" > #include "tree-walk.h" > +#include "replace-object.h" > > struct bitmapped_commit { > struct commit *commit; > @@ -223,6 +224,8 @@ static void bitmap_builder_init(struct bitmap_builder *bb, > memset(bb, 0, sizeof(*bb)); > init_bb_data(&bb->data); > > + parsed_object_pool_clear(the_repository->parsed_objects); > + > reset_revision_walk(); > repo_init_revisions(writer->to_pack->repo, &revs, NULL); > revs.topo_order = 1; > --- >8 --- Oops. I meant to attach a diff here that calls disable_replace_refs(), not parsed_object_pool_clear(). > But by then it is too late, because the replace refs have already been > taken into account for parsed objects. Thanks, Taylor
> I had a some more time to look into this, and I think that your original > fix is correct. > > The issue is, as you suggest, due to the following (from your original > patch): > > > After some investigation we found that all repositories experiencing > > failures contain replace references, which seem to be improperly > > acknowledged by the MIDX bitmap generation logic. > > Indeed, the pack-bitmap-write machinery does not itself call > disable_replace_refs(). So when it generates a reachability bitmap, it > is doing so with the replace refs in mind. You can see that this is > indeed the cause of the problem by looking at the output of an > instrumented version of Git that indicates what bits are being set > during the bitmap generation phase. > > With replace refs (incorrectly) enabled, we get: > > [2, 4, 6, 8, 13, 3, 6, 7, 3, 4, 6, 8] > > and doing the same after calling disable_replace_refs(), we instead get: > > [2, 5, 6, 13, 3, 6, 7, 3, 4, 6, 8] > > Single pack bitmaps are unaffected by this issue because we generate > them from within pack-objects, which does call disable_replace_refs(). Thank you for the comprehensive investigation. I have quoted them in the commit message to provide a clearer explanation of the patch. > In addition to the test fixes I suggested earlier, I would instead demonstrate > the bug by showing a clone (which fails with MIDXs, but doesn't without > MIDXs) like so: > > --- 8< --- > diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh > index 5e4cdef6a8..1fb3b0f9d7 100755 > --- a/t/t5326-multi-pack-bitmaps.sh > +++ b/t/t5326-multi-pack-bitmaps.sh > @@ -442,19 +442,16 @@ test_expect_success 'do not follow replace objects for MIDX bitmap' ' > cd repo && > > test_commit A && > - A=$(git rev-parse HEAD) && > test_commit B && > - B=$(git rev-parse HEAD) && > - git checkout --orphan=orphan $A && > + git checkout --orphan=orphan A && > test_commit orphan && > - C=$(git rev-parse HEAD) && > - git rev-list --objects --no-object-names $B |sort >expected && > > - git replace $A $C && > - git repack -ad && > - git multi-pack-index write --bitmap && > - git rev-list --objects --no-object-names --use-bitmap-index $B |sort >actual && > - test_cmp expected actual > + git replace A HEAD && > + git repack -ad --write-midx --write-bitmap-index && > + > + # generating reachability bitmaps with replace refs > + # enabled will result in broken clones > + git clone --no-local --bare . clone.git > ) > ' > --- >8 --- > > With the change in your patch to call disable_replace_refs() in > builtin/multi-pack-index.c, this test passes as expected. With that > change compiled out, we instead get: > > [...] > + git clone --no-local --bare . clone.git > Cloning into bare repository 'clone.git'... > remote: Enumerating objects: 8, done. > remote: Total 8 (delta 0), reused 0 (delta 0), pack-reused 8 (from 1) > Receiving objects: 100% (8/8), done. > fatal: did not receive expected object da5497437fd67ca928333aab79c4b4b55036ea66 > fatal: fetch-pack: invalid index-pack output > error: last command exited with $?=128 > not ok 352 - do not follow replace objects for MIDX bitmap > > as expected. > Applied! The test looks much clearer now, thanks! Xing Xin
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c index a72aebecaa2..8360932d2e7 100644 --- a/builtin/multi-pack-index.c +++ b/builtin/multi-pack-index.c @@ -8,6 +8,7 @@ #include "strbuf.h" #include "trace2.h" #include "object-store-ll.h" +#include "replace-object.h" #define BUILTIN_MIDX_WRITE_USAGE \ N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]" \ @@ -273,6 +274,8 @@ int cmd_multi_pack_index(int argc, const char **argv, }; struct option *options = parse_options_concat(builtin_multi_pack_index_options, common_opts); + disable_replace_refs(); + git_config(git_default_config, NULL); if (the_repository && diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh index 70d1b58709a..5e4cdef6a8b 100755 --- a/t/t5326-multi-pack-bitmaps.sh +++ b/t/t5326-multi-pack-bitmaps.sh @@ -434,6 +434,30 @@ test_expect_success 'tagged commits are selected for bitmapping' ' ) ' +test_expect_success 'do not follow replace objects for MIDX bitmap' ' + rm -fr repo && + git init repo && + test_when_finished "rm -fr repo" && + ( + cd repo && + + test_commit A && + A=$(git rev-parse HEAD) && + test_commit B && + B=$(git rev-parse HEAD) && + git checkout --orphan=orphan $A && + test_commit orphan && + C=$(git rev-parse HEAD) && + git rev-list --objects --no-object-names $B |sort >expected && + + git replace $A $C && + git repack -ad && + git multi-pack-index write --bitmap && + git rev-list --objects --no-object-names --use-bitmap-index $B |sort >actual && + test_cmp expected actual + ) +' + corrupt_file () { chmod a+w "$1" && printf "bogus" | dd of="$1" bs=1 seek="12" conv=notrunc