Re: --reset-author does not reset author date when used in post-commit hook

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Apr 1, 2025, 00:43 UTC
Message-ID: <Z-s2p-ak-KpU6uaX@tapette.crustytoothpaste.net>
In-Reply-To: <CANM0SV2huiGtWMcP+eEkNBHoieeoXmy_fM7vAOwyoW4=FXw1pg@mail.gmail.com>

On 2025-03-31 at 20:19:30, Devste Devste wrote:

Show 28 quoted lines

> device time zone is not UTC0
> 
> post-commit hook:
> if [[ -n "${SKIP_POST_COMMIT+x}" ]] && [[ "$SKIP_POST_COMMIT" -eq 1 ]]
> then
> exit
> fi
> 
> TZ=UTC0 SKIP_POST_COMMIT=1 git commit --amend --reset-author --no-edit
> --no-verify --no-post-rewrite --allow-empty --quiet
> 
> ---
> 
> Run:
> git commit -m "foo"
> git show -s --format="Commit: %h%nAuthor: %an%nAuthor Date:
> %ad%nCommitter: %cn%nCommitter Date: %cd%nMessage: %s%n" HEAD
> 
> Expected that both Author Date: and Committer Date: show +0000 as
> timezone. Only committer date shows +0000, while author date shows
> time zone of device.
> 
> When manually running the post-commit code (TZ=UTC0 SKIP_POST_COMMIT=1
> ...) it correctly resets the author date to +0000 too
> 
> When manually setting the date using --date="$(date +%s) +0000"
> instead of  --reset-author it also works correctly in post-commit
> hook.

I can reproduce this, except that I had to use `TZ=America/Toronto` since my system is in UTC. A quick look at the code led me to a guess that ended up correct.

The `git commit` code sets `GIT_AUTHOR_NAME`, `GIT_AUTHOR_EMAIL`, and `GIT_AUTHOR_DATE`. The latter contains a timezone identifier. As a consequence, when the hook is invoked, those values are set in the environment, and the subsequent `git commit` in the hook uses them. You can see this by placing something like `env >/tmp/foo` in your hook and then reading the output.

I will say that this is definitely not the intended use of the post-commit hook and I am very much not surprised that this doesn't work as you intended. You could certainly fix it by unsetting those environment variables, but I'd encourage you to adopt a different approach, such as by using an alias that sets `TZ=UTC0` to commit. That will be more likely to work in the future and will definitely be less brittle.

For instance, your call to `git commit` will still invoke the `prepare-commit-msg` (and possibly `commit-msg`) hook, which might result in unexpected changes to your message or even a failure of the second commit.

-- 
brian m. carlson (they/them)
Toronto, Ontario, CA