Is the date there?

Posted Dec 14, 2024 23:55 UTC (Sat) by skissane (subscriber, #38675)
In reply to: Is the date there? by randomguy3
Parent article: Facing the Git commit-ID collision catastrophe

> the obvious immediate issue you'd have to solve is what counts as "older" in a distributed system? you can't use the date - even if you could trust the date on commits, there could easily be an older commit with the same hash in another branch that you can't currently see from your local clone (but will be able to when you - in the future - pull from someone else)

Git is not the kind of distributed system which needs a highly accurate clock. If someone is talking about a commit with some ID, and there are two commits with that ID – one from this year and one from a decade ago – most likely they mean the one from this year. So even if my clock is 5 minutes off, so long as I know what year it is, I'm fine.

Since there are a lot more commits from years ago than from this year, odds are high that when a collision finally happens, it will be between a recent commit and one much older, so it will be obvious from the timestamps which one is relevant. Yes, it is possible that an ID collision could occur between two recent commits, but that is significantly less likely because historical commits greatly outnumber recent ones.

Also there are other ways to disambiguate commits – if we are talking about the networking subsystem, and one commit with that ID is a networking commit, and the other is touching some unrelated kernel subsystem, we know which one is likely meant.

Is the date there?

Posted Dec 16, 2024 10:22 UTC (Mon) by taladar (subscriber, #68407) [Link] (9 responses)

That is all nice and well for a human reader but how is automated tooling supposed to make decisions like that?

Automated tools should fail on ambiguity

Posted Dec 16, 2024 11:42 UTC (Mon) by farnz (subscriber, #17727) [Link] (8 responses)

Automated tools, when presented with an ambiguous truncated hash, should simply fail and require a human to disambiguate. The extra information in context should allow a human to look up the truncated hash, see the (likely single-digit) possibilities, and ask the automated tool to retry with the correct full hash.

And tools generating truncated hashes should verify that the truncated hash is locally unambiguous; for example, if I ask git for a 6 character hash, and it determines that 123abc is both the first 6 characters and ambiguous locally, it should give me a longer hash that isn't ambiguous (e.g. 123abc45, to distinguish from 123abcde as the other local option).

Automated tools should fail on ambiguity

Posted Dec 16, 2024 11:55 UTC (Mon) by intelfx (subscriber, #130118) [Link] (6 responses)

> Automated tools, when presented with an ambiguous truncated hash, should simply fail and require a human to disambiguate. The extra information in context should allow a human to look up the truncated hash, see the (likely single-digit) possibilities, and ask the automated tool to retry with the correct full hash.
>
> And tools generating truncated hashes should verify that the truncated hash is locally unambiguous; for example, if I ask git for a 6 character hash, and it determines that 123abc is both the first 6 characters and ambiguous locally, it should give me a longer hash that isn't ambiguous (e.g. 123abc45, to distinguish from 123abcde as the other local option).

I mean, that's how it (Git's own CLI, aka porcelain) works right now. However, there clearly are people who feel it's not adequate.

Automated tools should fail on ambiguity

Posted Dec 16, 2024 11:59 UTC (Mon) by farnz (subscriber, #17727) [Link] (4 responses)

Git's porcelain is not the only tool out there that uses or generates truncated hashes; CI systems consume them, and forges generate them in their Web UIs. Hopefully, they all get things right, but the nature of code is that some of these systems will have bugs.

Automated tools should fail on ambiguity

Posted Dec 17, 2024 18:24 UTC (Tue) by mathstuf (subscriber, #69389) [Link] (3 responses)

AFAIK, CI nor forges communicates solely on truncated hashes. Either they use full hashes or hidden refs (e.g., `refs/pipelines/N` on GitLab-CI). The UI might *render* only truncated hashes, but interacting should get full hashes (cf. this GitLab bugfix to do so: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/146052).

Automated tools should fail on ambiguity

Posted Dec 17, 2024 18:28 UTC (Tue) by farnz (subscriber, #17727) [Link] (2 responses)

You do, however, have to cope with users doing things like highlighting the truncated hash and copying it (rather than using the affordances in the UI to get a full hash), taking a screenshot and attaching that to a mail (where the recipient then has to reconstruct the hash from the picture), and pasting a truncated hash into the CI's "request a build of an arbitrary commit" field.

I would hope that all of these cases are at least considered by forges and CIs, but once you have a human in the loop, they will find ways to do weird things that you did not expect, and did not allow for - and the best a forge or CI can do is fail when I ask it to interact with an ambiguous commit hash.

Automated tools should fail on ambiguity

Posted Dec 18, 2024 14:31 UTC (Wed) by mathstuf (subscriber, #69389) [Link] (1 responses)

> I would hope that all of these cases are at least considered by forges and CIs, but once you have a human in the loop, they will find ways to do weird things that you did not expect, and did not allow for - and the best a forge or CI can do is fail when I ask it to interact with an ambiguous commit hash.

Yes, humans always make things messy ;) . AFAIK, all APIs would stop something ambiguous at the API surface rather than after figuring out what work needs to be done. And, AFAIK, most APIs want names of refs or forge entities (like PRs) rather than commit IDs. Not that the latter don't exist, but I don't usually find interest in CI results of a specific commit rather than a ref or current PR state (without naming it by one of those ways).

Automated tools should fail on ambiguity

Posted Dec 18, 2024 14:40 UTC (Wed) by farnz (subscriber, #17727) [Link]

Note that a commit ID in git qualifies as a refspec, and that's what most APIs actually ask for - something that git can resolve to a commit. And the trouble kicks in when you accept a refspec - because main, refs/branches/main, aef25be35d23 and aef25be35d23ec768eed08bfcf7ca3cf9685bc28 are all valid refspecs in git, but the truncated hash can be ambiguous with other commits, and only the full hash is unambiguous over time (since the others are mutable).

And IME, if you don't run CI on every single commit, people do ask for CI for a specific commit by hash, rather than mutable ref - when something worked 10 commits ago, but fails now, asking CI to fill in the gaps is useful.

Automated tools should fail on ambiguity

Posted Dec 16, 2024 19:21 UTC (Mon) by geert (subscriber, #98403) [Link]

Yes, git knows about all commits (in your repository), and will print a longer hash automatically when needed (for your repository).

That is how people end up with patches with Fixes:-tags of 13 characters, even with core.abbrev = 12 in their .gitconfig. Next, a strict maintainer will reject those patches, because scripts/checkpatch.pl complains about a hash that is not exactly 12 characters, despite (some parts of) the documentation stating that _at least 12_ is fine.

So that is what the first patch in the series is supposed to fix, but everybody focuseses on the second patch ;-)

Automated tools should fail on ambiguity

Posted Dec 16, 2024 15:45 UTC (Mon) by taladar (subscriber, #68407) [Link]

When I wrote automated tools I was more thinking of tooling consuming e.g. those Fix annotations in the history of the Linux kernel, i.e. tooling that consumes a large number of the truncated hashes.

Any place where that could become a problem, i.e. long-term storage of commit hashes, should not use truncated hashes for that reason in my opinion.