On 3/3/2022 11:00 AM, Derrick Stolee wrote: > On 3/3/2022 6:19 AM, Patrick Steinhardt wrote: >> On Wed, Mar 02, 2022 at 09:57:17AM -0500, Derrick Stolee wrote: >>> On 3/2/2022 8:57 AM, Patrick Steinhardt wrote: >>>> On Tue, Mar 01, 2022 at 10:25:46AM -0500, Derrick Stolee wrote: >>>>> On 3/1/2022 9:53 AM, Patrick Steinhardt wrote: >>> >>>>>> Hum. I have re-verified, and this indeed seems to play out. So I must've >>>>>> accidentally ran all my testing with the state generated without the >>>>>> final patch which fixes the corruption. I do see lots of the following >>>>>> warnings, but overall I can verify and write the commit-graph just fine: >>>>>> >>>>>> commit-graph generation for commit c80a42de8803e2d77818d0c82f88e748d7f9425f is 1623362063 < 1623362139 >>>>> >>>>> But I'm not able to generate these warnings from either version. I >>>>> tried generating different levels of a split commit-graph, but >>>>> could not reproduce it. If you have reproduction steps using current >>>>> 'master' (or any released Git version) and the four patches here, >>>>> then I would love to get a full understanding of your errors. >>>>> >>>>> Thanks, >>>>> -Stolee >>>> >>>> I haven't yet been able to reproduce it with publicly available data, >>>> but with the internal references I'm able to evoke the warnings >>>> reliably. It only works when I have two repositories connected via >>>> alternates, when generating the commit-graph in the linked-to repo >>>> first, and then generating the commit-graph in the linking repo. >>>> >>>> The following recipe allows me to reproduce, but rely on private data: >>>> >>>> $ git --version >>>> git version 2.35.1 >>>> >>>> # The pool repository is the one we're linked to from the fork. >>>> $ cd "$pool" >>>> $ rm -rf objects/info/commit-graph objects/info/commit-graph >>>> $ git commit-graph write --split >>>> >>>> $ cd "$fork" >>>> $ rm -rf objects/info/commit-graph objects/info/commit-graph >>>> $ git commit-graph write --split >>>> >>>> $ git commit-graph verify --no-progress >>>> $ echo $? >>>> 0 >>>> >>>> # This is 715d08a9e51251ad8290b181b6ac3b9e1f9719d7 with your full v2 >>>> # applied on top. >>>> $ ~/Development/git/bin-wrappers/git --version >>>> git version 2.35.1.358.g7ede1bea24 >>>> >>>> $ ~/Development/git/bin-wrappers/git commit-graph verify --no-progress >>>> commit-graph generation for commit 06a91bac00ed11128becd48d5ae77eacd8f24c97 is 1623273624 < 1623273710 >>>> commit-graph generation for commit 0ae91029f27238e8f8e109c6bb3907f864dda14f is 1622151146 < 1622151220 >>>> commit-graph generation for commit 0d4582a33d8c8e3eb01adbf564f5e1deeb3b56a2 is 1631045222 < 1631045225 >>>> commit-graph generation for commit 0daf8976439d7e0bb9710c5ee63b570580e0dc03 is 1620347739 < 1620347789 >>>> commit-graph generation for commit 0e0ee8ffb3fa22cee7d28e21cbd6df26454932cf is 1623783297 < 1623783380 >>>> commit-graph generation for commit 0f08ab3de6ec115ea8a956a1996cb9759e640e74 is 1621543278 < 1621543339 >>>> commit-graph generation for commit 133ed0319b5a66ae0c2be76e5a887b880452b111 is 1620949864 < 1620949915 >>>> commit-graph generation for commit 1341b3e6c63343ae94a8a473fa057126ddd4669a is 1637344364 < 1637344384 >>>> commit-graph generation for commit 15bdfc501c2c9f23e9353bf6e6a5facd9c32a07a is 1623348103 < 1623348133 >>>> ... >>>> $ echo $? >>>> 1 >>>> >>>> When generating commit-graphs with your patches applied the `verify` >>>> step works alright. >>>> >>>> I've also by accident stumbled over the original error again: >>>> >>>> fatal: commit-graph requires overflow generation data but has none >>>> >>>> This time it's definitely not caused by generating commit-graphs with an >>>> in-between state of your patch series because the data comes straight >>>> from production with no changes to the commit-graphs performed by >>>> myself. There we're running Git v2.33.1 with a couple of backported >>>> patches (see [1]). While those patches cause us to make more use of the >>>> commit-graph, none modify the way we generate them. >>>> >>>> Of note is that the commit-graph contains references to commits which >>>> don't exist in the ODB anymore. >>>> >>>> Patrick >>>> >>>> [1]: https://gitlab.com/gitlab-org/gitlab-git/-/commits/pks-v2.33.1.gl3 >>> >>> Thank you for your diligence here, Patrick. I really appreciate the >>> work you're putting in to verify the situation. >>> >>> Since our repro relies on private information, but is consistent, I >>> wonder if we should take the patch below, which starts to ignore the >>> older generation number v2 data and only writes freshly-computed >>> numbers. >>> >>> Thanks, >>> -Stolee >> >> Thanks. With your patch below the `fatal:` error is gone, but I'm still >> seeing the same errors with regards to the commit-graph generations. > > This is disappointing and unexpected. Thanks for verifying. > >> So to summarize my findings: >> >> - This bug occurs when writing commit-graphs with v2.35.1, but >> reading them with your patches. >> >> - This bug occurs when I have two repositories connected via an >> alternates file. I haven't yet been able to reproduce it in a >> single repository that is not connected to a separate ODB. > > This is an interesting distinction. One that I didn't think would > matter, but I'll look into the code to see how that could affect > things. > >> - This bug only occurs when I first generate the commit-graph in the >> repository I'm borrowing objects from. >> >> - This bug only occurs when I write commit-graphs with `--split` in >> both repositories. "Normal" commit-graphs don't have this issue, >> and neither can I see it with `--split=replace` or mixed-type >> commit-graphs. >> >> Beware, the following explanation is based on my very basic >> understanding of the commit-graph code and thus more likely to be wrong >> than right: >> >> With the old Git version, we've been mis-parsing the generation because >> `read_generation_data` wasn't ever set. As a result it can happen that >> the second split commit-graph we're generating computes its own >> generation numbers from the wrong starting point because it uses the >> mis-parsed generation numbers from the parent commit-graph. >> >> With your patches, we start to correctly account for overflows and would >> thus end up with a different value for the generation depending on where >> we parse the commit from: if we parse it from the first commit-graph it >> would be correct because it's contains the "root" of the generation >> numbers. But if we parse a commit from the second commit-graph we may >> have a mismatch because the generation numbers in there may have been >> derived from generation numbers mis-parsed from the first commit-graph. >> And because these would be wrong in case there was an overflow it is >> clear that the new corrected generation number may be wrong, as well. > > Hm. My expectation was that the older layers of the split commit-graph > would have read_generation_data disabled (because the new Git version > cannot read the GDAT chunk) and then the validate_mixed_generation_chain() > method would remove read_generation_data from all of the graphs in the > list. > > Combining this with your thoughts on cross-alternate split commit-graphs, > this makes me think we should try this: > > --- >8 --- > > diff --git a/commit-graph.c b/commit-graph.c > index fb2ced0bd6..74c6534f56 100644 > --- a/commit-graph.c > +++ b/commit-graph.c > @@ -609,8 +609,6 @@ struct commit_graph *read_commit_graph_one(struct repository *r, > if (!g) > g = load_commit_graph_chain(r, odb); > > - validate_mixed_generation_chain(g); > - > return g; > } > > @@ -668,7 +666,13 @@ static int prepare_commit_graph(struct repository *r) > !r->objects->commit_graph && odb; > odb = odb->next) > prepare_commit_graph_one(r, odb); > - return !!r->objects->commit_graph; > + > + if (r->objects->commit_graph) { > + validate_mixed_generation_chain(r->objects->commit_graph); > + return 1; > + } > + > + return 0; > } > > int generation_numbers_enabled(struct repository *r) > > > --- >8 --- > > Notice that I'm moving the validate_mixed_generation_chain() call > out of read_commit_graph_one() and into prepare_commit_graph(). To > my understanding, this _should_ have an equivalent end state as the > old code, but might be worth trying just as a quick check. > > I will continue investigating and try to reproduce with this > additional constraint of working across an alternate. My attempts to reproduce this across an alternate have failed. I tried running the following test against Git without these patches, then verify with the newer version of Git. (I also have generated a few new layers on top with these patches, and they correctly drop the GDA2 and GDO2 chunks when the lower layers "don't have gen v2".) test_description='commit-graph with offsets across alternates' . ./test-lib.sh if ! test_have_prereq TIME_IS_64BIT || ! test_have_prereq TIME_T_IS_64BIT then skip_all='skipping 64-bit timestamp tests' test_done fi UNIX_EPOCH_ZERO="@0 +0000" FUTURE_DATE="@4147483646 +0000" GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=0 test_expect_success 'generate alternate split commit-graph' ' git init alternate && ( cd alternate && test_commit --date "$UNIX_EPOCH_ZERO" 1 && test_commit --date "$FUTURE_DATE" 2 && git commit-graph write --reachable && test_commit --date "$UNIX_EPOCH_ZERO" 3 && test_commit --date "$FUTURE_DATE" 4 && git commit-graph write --reachable --split=no-merge ) && git clone --shared alternate fork && ( cd fork && test_commit --date "$UNIX_EPOCH_ZERO" 5 && test_commit --date "$FUTURE_DATE" 6 && git commit-graph write --reachable --split=no-merge && test_commit --date "$UNIX_EPOCH_ZERO" 7 && test_commit --date "$FUTURE_DATE" 8 && git commit-graph write --reachable --split=no-merge ) ' test_done My testing after running this with -d allows me to reliably see these layers being created with GDAT and GDOV chunks. Running the 'git commit-graph verify' command with the new code does not show those errors, even after adding commits and another layer to the split commit-graph. I look forward to any additional insights you might have here. Thanks, -Stolee
