Subject: [PATCH 00/11] Stop relying on SHA1 fallback for `the_hash_algo`

From: Patrick Steinhardt <[email-masked]>
Date: Fri, 19 Apr 2024 11:51:03 +0200

Hi,

when starting up, Git will initialize `the_repository` by calling
`initialize_the_repository()`. Part of this is also that we set the hash
algo of `the_repository` to SHA1, which implicitly sets `the_hash_algo`
because it is a macro expanding to `the_repository->hash_algo`.

Usually, we eventually set up the correct hash algorithm here once we
have properly set up `the_repository` via `setup_git_directory()`. But
in some commands we actually don't require a Git repository, and thus we
will leave the SHA1 hash algorithm in place.

This has led to some subtle bugs when the context really asks for a
SHA256 repository, which this patch series corrects for most of the
part. Some commands need further work, like for example git-diff(1),
where the user might want to have the ability to pick a hash function
when run outside of a repository.

Ultimately, the last patch then drops the setup of the fallback hash
algorithm completely. This will cause `the_hash_algo` to be a `NULL`
pointer unless explicitly configured, and thus we now start to crash
when it gets accessed without doing so beforehand. This is a rather
risky thing to do, but it does catch bugs where we might otherwise
accidentally do the wrong thing. And even though I think it is the right
thing to do even conceptually, I'd be okay to drop it if folks think
that the risk is not worth it.

Patrick

Patrick Steinhardt (11):
  path: harden validation of HEAD with non-standard hashes
  parse-options-cb: only abbreviate hashes when hash algo is known
  attr: don't recompute default attribute source
  attr: fix BUG() when parsing attrs outside of repo
  remote-curl: fix parsing of detached SHA256 heads
  builtin/rev-parse: allow shortening to more than 40 hex characters
  builtin/blame: don't access potentially unitialized `the_hash_algo`
  builtin/bundle: abort "verify" early when there is no repository
  builtin/diff: explicitly set hash algo when there is no repo
  builtin/shortlog: don't set up revisions without repo
  repository: stop setting SHA1 as the default object hash

 attr.c                     | 31 ++++++++++++++++++++++---------
 builtin/blame.c            |  5 ++---
 builtin/bundle.c           |  5 +++++
 builtin/diff.c             |  9 +++++++++
 builtin/rev-parse.c        |  5 ++---
 builtin/shortlog.c         |  2 +-
 parse-options-cb.c         |  3 ++-
 path.c                     |  2 +-
 remote-curl.c              | 19 ++++++++++++++++++-
 repository.c               |  2 --
 t/t0003-attributes.sh      | 15 +++++++++++++++
 t/t0040-parse-options.sh   | 17 +++++++++++++++++
 t/t1500-rev-parse.sh       |  6 ++++++
 t/t5550-http-fetch-dumb.sh | 15 +++++++++++++++
 14 files changed, 115 insertions(+), 21 deletions(-)

-- 
2.44.GIT