All hooks already redirect stdout to stderr with the exception of
pre-push which has a known user who depends on the separate stdout
versus stderr outputs (the git-lfs project).
The pre-push behavior was a surprise which we found out about after
causing a regression for git-lfs. Notably, it might not be the only
exception (it's the one we know about). There might be more.
This presents a challenge because stdout_to_stderr is required for
hook parallelization, so run-command can buffer and de-interleave
the hook outputs using ungroup=0, when hook.jobs > 1.
Introduce an extension to enforce consistency: all hooks merge stdout
into stderr and can be safely parallelized. This provides a clean
separation and avoids breaking existing stdout vs stderr behavior.
When this extension is disabled, the `hook.jobs` config has no
effect for pre-push, to prevent garbled (interleaved) parallel
output, so it runs sequentially like before.
Alternatives I've considered to this extension include:
1. Allowing pre-push to run in parallel with interleaved output.
2. Always running pre-push sequentially (no parallel jobs for it).
3. Making users (only git-lfs? maybe more?) fix their hooks to read
stderr not stdout.
Out of all these alternatives, I think this extension is the most
reasonable compromise, to not break existing users, allow pre-push
parallel jobs for those who need it (with correct outputs) and also
future-proofing in case there are any more exceptions to be added.
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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