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

["brian m. carlson" <sandals@crustytoothpaste.net>]

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

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