Toon ClaesThe changes in commit c06793a4ed (allow git-bundle to create bottomless bundle, 2007-08-08) ensure annotated tags are properly preserved when creating a bundle using a revision range operation.
At the time the range notation would peel the ends to their corresponding commit, meaning ref v2.0 would point to the v2.0^0 commit. So the above workaround was introduced. This code looks up the ref before it's written to the bundle, and if the ref doesn't point to the object we expect (for tags this would be a tag object), we skip the ref from the bundle. Instead, when the ref is a tag that's the positive end of the range (e.g. v2.0 from the range "v1.0..v2.0"), then that ref is written to the bundle instead.
Later, in 895c5ba3c1 (revision: do not peel tags used in range notation, 2013-09-19), the behavior of parsing ranges was changed and the problem was fixed at the cause. But the workaround in bundle.c was not reverted.
Now it seems this workaround can cause a race condition. git-bundle(1) uses setup_revisions() to parse the input into `struct rev_info`. Later, in write_bundle_refs(), it uses this info to write refs to the bundle. As mentioned at this point each ref is looked up again and checked whether it points to the object we expect. If not, the ref is not written to the bundle. But, when creating a bundle in a heavy traffic repository (a repo with many references, and frequent ref updates) it's possible a branch ref was updated between setup_revisions() and write_bundle_refs() and thus the extra check causes the ref to be skipped.
The workaround was originally added to deal with tags, but the code path also gets hit by non-tag refs, causing this race condition. Because it's no longer needed, remove it and fix the possible race condition.
Signed-off-by: Toon Claes <toon@iotcl.com> --- Earlier I reported[1] a race condition bug could occur if a ref is updated while a git-bundle(1) creation process is running. I've demonstrated it's possible to reproduce this by using a debugger.
Show 20 quoted lines
> To reproduce, I've been running git-bundle(1) with
> `create my.bndl --all --ignore-missing` in a debugger. I've set a
> breakpoint at bundle.c:515[1] where setup_revisions() is called. After
> stepping over this line I see in the debugger `revs.pending` is
> populated.
>
> (gdb) p *revs.pending.objects
> $6 = {item = 0x7a2fb0, name = 0x78d7e0 "refs/heads/master", path = 0x0, mode = 12288}
> (gdb) p *revs.pending.objects.item
> $7 = {parsed = 1, type = 1, flags = 0, oid = {hash = "R\026\370\365\304\b\236\302\234\344\232\372\024t4\302>\017\001c\000\000\000\000sS\344\367\377\177\000", algo = 1}}
>
> The hash value is the binary representation of
> `5216f8f5c4089ec29ce49afa147434c23e0f0163`, the current HEAD of
> `master`. At this point I've updated `master` in another terminal
> window:
>
> git commit --allow-empty -m"dummy"
>
> Then in the debugger I continue the process to create the bundle. The
> resulting bundle seems to be missing `refs/heads/master`.I started digging into this bug again. And I've discovered the changes made by c06793a4ed (allow git-bundle to create bottomless bundle, 2007-08-08) cause some trouble. Those changes look up the refs for a second time, and if they mismatch, they might not get written to the bundle output file.
It seems the workaround is not needed anymore since 895c5ba3c1 (revision: do not peel tags used in range notation, 2013-09-19) and thus I'm removing it while adding some tests. Unfortunately, I was not able to implement a test that hits the race condition bug.
[1]: https://lore.kernel.org/git/87eddlpx5k.fsf@iotcl.com/ --- Changes in v2:
- No longer user the term "bottomless bundle" and use "full bundle" and "incremental bundle" instead, because they are more commonly used.
- To verify the full bundle created upto tag actually contains the tag object, add a test to clone from the created bundle.
- Explicitly mention in the test names that annotated tags are used, because it's not obvious to see the function test_commit_setvar() used with option `--tag` calls git-tag(1) with `-m` which implies `-a`.
- Link to v1: https://lore.kernel.org/r/20241209-fix-bundle-create-race-v1-1-e6513bdcbf8a@iotcl.com
Range-diff against v1:
1: 2816666488 ! 1: a7974c232f bundle: remove unneeded code
@@ t/t6020-bundle-misc.sh: test_expect_success 'unfiltered bundle with --objects' '
test_cmp expect actual
' -+test_expect_success 'bottomless bundle upto tag' '
++test_expect_success 'full bundle upto annotated tag' '
+ git bundle create v2.bdl \
+ v2 &&
+
@@ t/t6020-bundle-misc.sh: test_expect_success 'unfiltered bundle with --objects' '
+ test_cmp expect actual
+'
+
-+test_expect_success 'bundle between two tags' '
++test_expect_success 'clone from full bundle upto annotated tag' '
++ git clone --mirror v2.bdl tag-clone.git &&
++ git -C tag-clone.git show-ref |
++ make_user_friendly_and_stable_output >actual &&
++ cat >expect <<-\EOF &&
++ <TAG-2> refs/tags/v2
++ EOF
++ false &&
++ test_cmp expect actual
++'
++
++test_expect_success 'incremental bundle between two annotated tags' '
+ git bundle create v1-v2.bdl \
+ v1..v2 &&
+
---
bundle.c | 30 ------------------------------
t/t6020-bundle-misc.sh | 45 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 45 insertions(+), 30 deletions(-)Show changes to 2 files +45 −30
bundle.c, t/t6020-bundle-misc.sh
diff --git a/bundle.c b/bundle.c index 4773b51eb1df8057466c87f48445c49bc1f594ee..dfb5b7a5ec6b98e00078359afe991bac55cae739 100644 --- a/bundle.c +++ b/bundle.c @@ -420,36 +420,6 @@ static int write_bundle_refs(int bundle_fd, struct rev_info *revs) e->name); goto skip_write_ref; } - /* - * If you run "git bundle create bndl v1.0..v2.0", the - * name of the positive ref is "v2.0" but that is the - * commit that is referenced by the tag, and not the tag - * itself. - */ - if (!oideq(&oid, &e->item->oid)) { - /* - * Is this the positive end of a range expressed - * in terms of a tag (e.g. v2.0 from the range - * "v1.0..v2.0")? - */ - struct commit *one = lookup_commit_reference(revs->repo, &oid); - struct object *obj; - - if (e->item == &(one->object)) { - /* - * Need to include e->name as an - * independent ref to the pack-objects - * input, so that the tag is included - * in the output; otherwise we would - * end up triggering "empty bundle" - * error. - */ - obj = parse_object_or_die(&oid, e->name); - obj->flags |= SHOWN; - add_pending_object(revs, obj, e->name); - } - goto skip_write_ref; - } ref_count++; write_or_die(bundle_fd, oid_to_hex(&e->item->oid), the_hash_algo->hexsz); diff --git a/t/t6020-bundle-misc.sh b/t/t6020-bundle-misc.sh index 5d444bfe201a330527e86dde7229721fc386fc93..e7e8de92cd904f2f9a2dd6b4be21b288b6ea6154 100755 --- a/t/t6020-bundle-misc.sh +++ b/t/t6020-bundle-misc.sh @@ -504,6 +504,51 @@ test_expect_success 'unfiltered bundle with --objects' ' test_cmp expect actual ' +test_expect_success 'full bundle upto annotated tag' ' + git bundle create v2.bdl \ + v2 && + + git bundle verify v2.bdl | + make_user_friendly_and_stable_output >actual && + + format_and_save_expect <<-EOF && + The bundle contains this ref: + <TAG-2> refs/tags/v2 + The bundle records a complete history. + $HASH_MESSAGE + EOF + test_cmp expect actual +' + +test_expect_success 'clone from full bundle upto annotated tag' ' + git clone --mirror v2.bdl tag-clone.git && + git -C tag-clone.git show-ref | + make_user_friendly_and_stable_output >actual && + cat >expect <<-\EOF && + <TAG-2> refs/tags/v2 + EOF + false && + test_cmp expect actual +' + +test_expect_success 'incremental bundle between two annotated tags' ' + git bundle create v1-v2.bdl \ + v1..v2 && + + git bundle verify v1-v2.bdl | + make_user_friendly_and_stable_output >actual && + + format_and_save_expect <<-EOF && + The bundle contains this ref: + <TAG-2> refs/tags/v2 + The bundle requires these 2 refs: + <COMMIT-E> Z + <COMMIT-B> Z + $HASH_MESSAGE + EOF + test_cmp expect actual +' + for filter in "blob:none" "tree:0" "tree:1" "blob:limit=100" do test_expect_success "filtered bundle: $filter" ' --- base-commit: e66fd72e972df760a53c3d6da023c17adfc426d6 change-id: 20241204-fix-bundle-create-race-d0bdca1bf999 Best regards,
-- Toon Claes <toon@iotcl.com>
