Re: Bug: All git operations fail when .git contains a non-existent gitdir

> On 22-Jul-2021, at 18:43, Tom Cook <tom.k.cook@gmail.com> wrote:
>> On Wed, Jul 21, 2021 at 11:59 PM brian m. carlson
>> <sandals@crustytoothpaste.net> wrote:
>>> 
>>> On 2021-07-21 at 09:17:36, Tom Cook wrote:
>>>> What did you do before the bug happened? (Steps to reproduce your issue)
>>>> 
>>>> Add a git submodule to a git repository.
>>>> Overlay-mount that submodule to another place in the filesystem.
>>>> Attempt any git operation in the overlay-mounted path.
>>> 
>>> I'm not sure about what you mean by an overlay-mount operation.  Can you
>>> provide some specific commands that we can run at a shell that reproduce
>>> the issue?
>>> --
>>> brian m. carlson (he/him or they/them)
>>> Toronto, Ontario, CA
>> 
>> The easiest way to reproduce it is this:
>> 
>> $ mkdir test
>> $ cd test
>> $ echo "gitdir: /foo/bar" > .git
>> $ git ls-remote https://github.com/torvalds/linux
>> 
>> We happen to use overlay mounts in our build system in a way that maps
>> a git submodule from one place to another so that its "gitdir" is
>> invalid and then attempt a `git ls-remote` from that location which
>> unexpectedly fails.  But the above reproduces the problem well enough.
>
> 'ls-remote' needs a valid git directory for the case where the URL is not
> explicitly supplied (to read the git config and learn the default remote).
>
> Making a special case for when an explicit URL is not given is not as
> straightforward as it seems, because by the time 'ls-remote' even knows about
> its arguments, it already takes a worktree prefix and sets up the environment,
> for which a valid Git repository path is required.
>
> I am not too familiar with this area, and I don't know how feasible it is to
> delay setting up the environment until after looking at the 'ls-remote'
> arguments. At a cursory glance, it looks difficult to do without large
> structural changes to the code.
>
> This might have been less of a problem with old-form submodules, where '.git'
> was an actual directory, rather than a text file pointer [1], but newer
> versions of Git discourage their usage.
>
> [1] https://git-scm.com/docs/gitsubmodules#_forms
>
> PS: we prefer bottom posting or inline replies :)
>
> ---
> Pointers for others who might be interested in looking into this:
>
> The immediate cause of this seems to be 'setup.c:setup_gitdir_gently()' [2]
> which calls 'setup_gitdir_gently_1()' with the 'die_on_error' argument set
> to true. This function then calls 'read_gitfile_gently()' with the same flag,
> which errors out when it runs 'is_git_directory()' [3], because the path in
> the gitfile is not a valid repository.
>
> [2] https://github.com/git/git/blob/eb27b338a3e71c7c4079fbac8aeae3f8fbb5c687/setup.c#L1234
> [3] https://github.com/git/git/blob/eb27b338a3e71c7c4079fbac8aeae3f8fbb5c687/setup.c#L784-L799