Re: [PATCH v4 1/5] setup: add an escape hatch for "no more default hash algorithm" change

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> Partially revert c8aed5e8 (repository: stop setting SHA1 as the
> default object hash, 2024-05-07), to keep end-user systems still
> broken when we have gap in our test coverage but yet give them an
> escape hatch to set the GIT_DEFAULT_HASH environment variable to
> "sha1" in order to revert to the previous behaviour.  This variable
> has been in use for using SHA-256 hash by default, and it should be
> a better fit than inventing a new and test-only knob.

Having done all of this, I actually am very tempted to add the
"always default to SHA-1" back as a fallback position to the
set_default_hash_algo() function.  We know we are going to get the
right hash algorithm when working in the repository, so the only
case the default matters in practice is when working outside the
repository.

We already have such a custom code for "git diff --no-index", and we
are adding a few more back in here, but they can disappear if we had
code to set the fallback default when GIT_DEFAULT_HASH does not
exist here.  The "always use SHA-1 regardless of the hash used by
the repository" code like "patch-id" should not depend on such a
fallback default but should have its own code to explicitly set it.

As the user can tweak what algorithm they want if the wanted to, it
does not sound too bad to have a fallback default when the user did
not choose.