Subject: Re: [ANNOUNCE] Git v2.44.0

From: Patrick Steinhardt <[email-masked]>
Date: Sat, 24 Feb 2024 07:36:21 +0100

On Sat, Feb 24, 2024 at 02:10:40PM +0900, Mike Hommey wrote:
> Hi,
> 
> On Fri, Feb 23, 2024 at 09:17:07AM -0800, Junio C Hamano wrote:
> > Patrick Steinhardt (139):
> >       builtin/clone: create the refdb with the correct object format
> 
> I haven't analyzed how/why exactly yet, but I've bisected a regression
> in the behavior of is_git_directory() during a clone to originate from
> this change.
> 
> Here's a way to reproduce the problem:
> 
> ```
> $ cat > git-remote-foo <<EOF
> #!/bin/sh
> git config --local -l >&2
> exit 1
> EOF
> $ chmod +x git-remote-foo
> $ PATH=$PWD:$PATH git clone foo::bar
> ```
> 
> With versions < 2.44.0, it displays the local configuration, e.g.:
> ```
> core.repositoryformatversion=0
> core.filemode=true
> core.bare=false
> core.logallrefupdates=true
> remote.origin.url=foo::bar
> ```
> 
> but with 2.44.0, it fails with:
> ```
> fatal: --local can only be used inside a git repository
> ```

Thanks for your report!

This has to be because we now initialize the refdb at a later point. The
problem here was that before my change, we initialized the refdb at a
point when it wasn't clear what the remote actually used as the object
format. The consequence was twofold:

  - Cloning a repository with bundles was broken in case the remote uses
    the SHA256 object format.

  - Cloning into a repository that uses reftables when the remote uses
    the SHA256 object format was broken, too.

Both of these have the same root cause: because we didn't connect to the
remote yet we had no idea what object format the remote uses. And as we
initialized the refdb early, it was then initialized with the default
object format, which is SHA1.

The change was to move initialization of the refdb to a later point in
time where we know what object format the remote uses. By necessity,
this has to be _after_ we have connected to the remote, because there is
no way to learn about it without connecting to it.

One consequence of initializing the refdb at a later point in time is
that we have no "HEAD" yet, and a repo without the "HEAD" file is not
considered to be a repo. Thus, git-config(1) would now rightfully fail.

I assume that you discovered it via a remote helper that does something
more interesting than git-config(1). I have to wonder whether we ever
really specified what the environment of a remote helper should look
like when used during cloning. Conceptually it doesn't feel _wrong_ to
have a not-yet-initialized repo during clone.

But on the other hand, regressing functionality like this is of course
bad. I was wondering whether we can get around this issue by setting
e.g. GIT_DIR explicitly when spawning the remote helper, but I don't
think it's as easy as that.

Another idea would be to simply pre-create HEAD regardless of the ref
format, pointing to an invalid ref "refs/heads/.invalid". This is the
same trick we use for the reftable backend, and should likely address
your issue.

I will have a deeper look on Tuesday and send a patch.

Patrick