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

> On 2024-04-19 at 09:51:03, Patrick Steinhardt wrote:
> > 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.
> 
> I've taken a look, and other than the minor typo I noted, this seems
> fine.  I'm in favour of getting rid of the SHA-1 default, even though my
> gut tells me we might find a bug or two along the way where things
> aren't initialized properly.  I still think that'll be okay and it's
> worth doing, since it'll help us prepare for the case in the future
> where we want to switch the default and also for libification, where we
> won't want to make those kinds of assumptions.