Re: git submodules implementation question

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 31, 2016, 16:42 UTC
Message-ID: <xmqq7faw3n5w.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <CAN5XQfuoq6MV4e98RzUCG02KvZO6VZAbs1oxAzpdg5zswqpHGw@mail.gmail.com>

Uma Srinivasan <usrinivasan@twitter.com> writes:

Show 11 quoted lines

>> I might suggest to update prepare_submodule_repo_env() so that the
>> spawned process will *NOT* have to guess where the working tree and
>> repository by exporting GIT_DIR (set to "git_dir" we discover above)
>> and GIT_WORK_TREE (set to "." as cp.dir is set to the path to the
>> working tree of the submodule).  That would stop the "git status" to
>> guess (and fooled by a corrupted dir/.git that is not a git
>> repository).
>
> Here's where I am struggling with my lack of knowledge of git
> internals and the implementation particularly in the context of how
> environment variables are passed from the parent to the child process.

Ah, I was primarily addressing Jacob in the latter part of my message, as he's looked at similar things in his recent topic.

> Are you suggesting that we set up the child process environment array
> (the "out" argument) in prepare_submodule_repo_env() to include
> GIT_DIR_ENVIRONMENT and GIT_WORK_TREE_ENVIRONMENT in addition to
> CONFIG_DATA_ENVIRONMENT that is there now?

I was wondering if we should unconditionally stuff GIT_DIR=<the repository location for the submodule> in the cp.env_array passed to the function prepare_submodule_repo_env(). As cp.dir will be set to the submodule's working tree, there is no need to set GIT_WORK_TREE and export it, I think, although it would not hurt.

After all, we _are_ going into a separate and different project's repository (that is what a submodule is), and we _know_ where its repository data (i.e. GIT_DIR) and the location of its working tree (i.e. GIT_WORK_TREE). There is no reason for the process that will work in the submodule to go through the usual "do we have .git in our $cwd that is a repository? if not how about the parent directory of $cwd? go up until we find one and that directory is the top of the working tree" discovery.

More importantly, this matters when your GIT_DIR for the submodule is somehow corrupt. The discovery process would say "there is .git in $cwd but that is not a repository" and continue upwards, which likely would find the repository for the top-level superproject, which definitely is _not_ want you want to happen. And one way to avoid it is to tell the setup.c code that it should not do the discovery by being explicit.