Re: bug: setting GIT_DIR to $(git rev-parse --git-dir) changes behavior

Martin Nicolay [off-list ref] writes:

What did you do before the bug happened? (Steps to reproduce your issue)

Thanks for concise and clear descriptions for us to learn everything
you did.  I wish all the reports were written like this one.

$ env | grep GIT
$ git --version
git version 2.30.1

Showed there is no funny environment variable involved and the
version.  Good.

$ git init t
Initialized empty Git repository in /tmp/t/.git/
$ mkdir t/foo
$ cd t/foo
$ git rev-parse --show-toplevel
/tmp/t

The root of the working tree is at /tmp/t/, the repository at
/tmp/t/.git/, and you are in the foo/ subdirectory.  When you ask
"where is the top level of the working tree in this state, because
you do not have GIT_DIR or GIT_WORK_TREE environment variables, you
are asking Git to "discover" both the .git/ directory and the top of
the working tree, by starting at the current directory,
i.e. /tmp/t/foo, which is where you are.

First Git looks to see /tmp/t/foo/.git/ exists and is a repository;
it is not.  So it goes one level up and does so for /tmp/t/.git/ and
it finds that it is the repository.  The directory in which the
repository was discovered is the top of the working tree, hence you
get /tmp/t back.

$ GIT_DIR=$(git rev-parse --git-dir) git rev-parse --show-toplevel
/tmp/t/foo

When you give the GIT_DIR environment variable, you are telling Git
not to perform the repository discovery, AND also you are telling
Git that you are at the top-level of the working tree.  So this is
quite expected (it is a feature that is used by folks who want to
have their repository data in a distant and unrelated directory from
their working tree).

In other words, this is 100% expected behaviour.

If you want to also tell Git where the top-level of your working
tree is, you can export GIT_WORK_TREE at the same time.

    Side note: the latter environment variable was invented for this
    exact reason.  Back when only GIT_DIR existed as a way to tell
    Git where the distant repository is, those who wanted to use the
    "split" layout had to stay at the top-level of their working
    tree, and they got tired of not being able to work from a
    subdirectory.  GIT_WORK_TREE was introduced to tell where the
    top-level is separately from GIT_DIR when GIT_DIR environment
    variable is in use.

$ git rev-parse --git-dir
/tmp/t/.git