Re: [PATCH] make git barf when an alias changes environment variables

[Matthias Lederhofer <matled@gmx.net>]

Junio C Hamano <gitster@pobox.com> wrote:
> What does this patch do when you do this?
> 
> 	: Because I usually work with this repository...
>       $ GIT_DIR=$some_repository
>       $ export GIT_DIR
> 
>       : Then much later, I happen to visit another repository
>       : to take a peek...
> 	$ cd $somewhere_else
>       $ git --git-dir .git some-command
> 

This one has no alias involved as far as I see, so there is no change.

The patch makes git barf when an alias changes the environment (i.e.
an alias uses --git-dir/--work-tree/--bare).  When git has read the
configuration file to find out about aliases setup_git_env has been
called and a pointer returned by getenv("GIT_DIR") is already stored
in a static pointer.  If the environment variable is changed
afterwards git might or might not use the new value depending on the
implementation.

That is what my example shows: FreeBSD libc reuses the old memory
location if the new value of an environment variable fits in the old
place.  Because setup_git_env uses xstrdup on the object dir and some
other directories the old value is used for those but the new one is
used for git_dir (which is just a pointer returned by getenv).  This
way HEAD is read from the new repository but objects are searched in
the old repository.