Re: Strange behaviour when pushing a commit object to remote's refs/HEAD

From: Karthik Nayak <karthik.188@gmail.com>
Date: Jan 16, 2024, 09:54 UTC
Message-ID: <CAOLa=ZS8YBhzaYx=9016KxErsMsazsF09rcuPs=-WpEGjV+ruw@mail.gmail.com>
In-Reply-To: <mafs0fryypg82.fsf@yadavpratyush.com>

Pratyush Yadav <me@yadavpratyush.com> writes:

> Hi,
>

Hello,

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> I ran into a strange Magit bug, where when I ran magit-show-refs on a
> particular repo it threw an error. The details of the Magit bug are not
> very interesting, but when attempting to reproduce it, I also saw git
> misbehaving for such repos.
>
> The strange behaviour happens when you push a commit object to remote's
> refs/HEAD instead of pushing a symbolic ref. Such a repository can be
> found at https://github.com/prati0100/magit-reproducer. I roughly used
> the below steps to create such a repo:
>
>     $ git init
>     $ echo 1 > foo && git add foo && git commit
>     $ echo 2 > bar && git add bar && git commit
>     $ git push
>     $ git checkout 79264c3
>     $ echo 2.1 > bar && git add bar && git commit
>     $ git push origin 707a3d5:refs/heads/HEAD
>

Just to note here that pushing to "refs/heads/HEAD" is not actually updating the remote repositories $GIT_DIR/HEAD file, rather it creates a new reference $GIT_DIR/refs/heads/HEAD.

With this understanding you'll see that this is not a bug, because the remote HEAD was never updated, but only a new branch called HEAD was created [0].

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> Now with such a repo, if you do `git log --all --oneline` it would look
> something like:
>
>     707a3d5 (origin/HEAD) 2.1
>     86e1c97 (HEAD -> main, origin/main) 2
>     79264c3 1
>
> And running `git for-each-ref --format='%(symref:short),%(refname:short),%(refname),%(subject)' refs/remotes/origin` gives:
>
>     ,origin,refs/remotes/origin/HEAD,2.1
>     ,origin/main,refs/remotes/origin/main,2
>
> All well and good so far. Now delete the repo and attempt to clone it.
> This time `git log --all --oneline` gives:
>
>     86e1c97 (HEAD -> main, origin/main, origin/HEAD) 2
>     79264c3 1
>

This is expected since you cloned the repository and you got the default branch 'main'.

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> And running `git for-each-ref --format='%(symref:short),%(refname:short),%(refname),%(subject)' refs/remotes/origin` gives:
>
>     origin/main,origin,refs/remotes/origin/HEAD,2
>     ,origin/main,refs/remotes/origin/main,2
>
> So suddenly the remote's HEAD becomes origin/main (symbolic ref) and the
> commit (707a3d5, "2.1") is nowhere to be found. It neither shows up in
> `git rev-list --all` nor in `git log --all`. The files and trees
> associated with it also do not show up in `git rev-list --all --object`.

Because rev-list's `--all`, iterates over all refs. Since you only cloned, the HEAD branch is not pulled.

Everything else is a consequence of the subtle but important difference between updating $GIT_DIR/HEAD vs creating $GIT_DIR/refs/heads/HEAD.

[0]: https://github.com/prati0100/magit-reproducer/branches/all

Thanks, Karthik