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

Junio C Hamano [off-list ref] 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.