We discussed the Nix part with @Ralith. He suggested

preConfigure = ''
    NIX_JJ_GIT_HASH=$(git <something> --git-dir=${./.}/jj/repo/store/git)
'';

I might look into that, or I could leave it for some more Nix person to do.

Something like this should work, I think, in flake.nix, but we should change the date format from git log to be YYYYMMDD to be consistent:

gitRev = "${builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}|${self.shortRev or "dirty"}";

Commit cf39ada on branch push-numyszkprmty contains the changes I think you need. Here's an example of what this looks like when using the interactive shell, basically the format <YYYYMMDD>|<HASH>{-dirty}

austin@GANON:~/src/jj$ echo $NIX_JJ_GIT_HASH 
20230811|cf39adafac5ed1f5adc112ede325d6fcbfd7d2b7

austin@GANON:~/src/jj$ echo $NIX_JJ_GIT_HASH 
20230811|17b45d642f39351500d74bc07fe1d70ee0f7bb5c-dirty

The first is a clean commit and the second is dirty. This uses the dirtyRev and dirtyShortRev features from Nix that are only available in Nix as of a month or so ago, so it's very recent; on older versions of Nix, for backwards compatibility the version in a dirty repository will look like this:

austin@GANON:~/src/jj$ echo $NIX_JJ_GIT_HASH 
20230811|dirty

Which isn't useful, but mostly only happens to us during development, so I think it's OK.

If you can unify these two version paths to be the same, that would be great. Feel free to mold this commit to your needs.

self.lastModifiedDate or self.lastModified

Won't this be different than the commit date?

lastModifiedDate is the commit time of the rev in question. (I think it would have to be, otherwise it would break determinism.)

The point of NIX_JJ_GIT_HASH was that nix cleans up the .git folder and build.rs is not run in the jj/git repo, so it couldn't load the hash.

I now realised that a better approach would probably be to set NIX_GIT_REPO="${./.}/jj/repo/store/git" or something similar, and have --git-dir set to that in build.rs - that way there is no two separate ways (in nix and in rust) to calculate this same bleeding version value

@thoughtpolice Thanks! I don't feel like adding that to this PR right now, but it looks like it'd be easy to add. Either the PR can wait or it can go into a separate one.

I think adding "-dirty" should be a separate change, as it'd require adjusting a test, and ideally it'd go together with supporting it in the regular Rust code.

I now realised that a better approach would probably be to set NIX_GIT_REPO="${./.}/jj/repo/store/git" or something similar,

This sounds fine to me, but I don't grok Nix philosophy or implementation enough to really know whether this is a good idea. I think it should be a separate discussion/PR in any case.

Also, I'll leave it up to (the plural) you to decide which Nix expression for the date is exactly right. It would be nice to be consistent, but we can always fix it later.

I changed the date format to YYYYMMDD and incorporated some of @thoughtpolice's changes. Thank you, they were very helpful!

I rewrote them a bit and removed the portion having to do with dirty revisions for now. We should add it back when the rest of build.rs understands dirty revisions.

There's a mild inconsistency in dates. I'm guessing one is in UTC and one is in the local timezone. I think that's fine, resolution of plus/minus a day should be enough here.

However, if somebody wants to ensure UTC everywhere it'd certainly be nicer! The UTC timestamp feature suggested in https://discord.com/channels/968932220549103686/969829516539228222/1139656152926404649 would be helpful here.

FWIW, if you can set TZ=UTC0 in the environment on the invocation to jj, it might handle that fine on both Windows and Unix? It's POSIX specified, at least. This works on my Windows machine, for example:

PS C:\code\thingy> git show --quiet --date=local --format="%cd"
Tue Apr 16 02:17:33 2019
PS C:\code\thingy> $env:TZ = "UTC0"; git show --quiet --date=local --format="%cd"
Tue Apr 16 07:17:33 2019
PS C:\code\thingy> $env:TZ = "UTC0"; git show --quiet --date='format-local:%Y%m%dT%H%M%SZ' --format="%cd"
20190416T071733Z

FWIW, if you can set TZ=UTC0 in the environment on the invocation to jj, it might handle that fine on both Windows and Unix? It's POSIX specified, at least.

Thanks, that sounds good. I don't think I'll use it in this commit, though. It's blocked on the jj side by #2080, I think.