Subject: Re: [PATCH 2/2] Remember and use GIT_EXEC_PATH on exec()'s

From: Andreas Ericsson <[email-masked]>
Date: Tue, 10 Jan 2006 20:13:34 +0100

Michal Ostrowski wrote:
> On Tue, 2006-01-10 at 16:01 +0100, Andreas Ericsson wrote:
> 
> 
>>This is the case in the git potty already. git.c must prepend 
>>--exec-path to $PATH, or the whole idea of being able to move scripts 
>>out of the $PATH fails (at least it fails without changing quite a few 
>>of the scripts).
> 
> 
> One could make all the scripts depend on GIT_EXEC_PATH instead of PATH.
> At build time one could generate wrapper functions in git-sh-setup:
> 
> function git-foo () {
> 	$(GIT_EXEC_PATH)/git-foo $*;
> }
> 
> Presuming that all scripts include git-sh-setup, no other shell script
> changes would be needed.
> 

Yuck, for two reasons.

* Not all scripts include git-sh-setup, and for good reasons. If this is 
what you intend please make sure you don't break anything in the process.

* This will spawn a sub-shell for each git-foo process called. Shells 
are way more expensive than the git potty, so the performance hit in 
iterations might be considerable. Think StGit and Cogito as well.

On a side-note, $* will break quoting (you should use "$@" instead, with 
double-quotes attached), and

	$(GIT_EXEC_PATH)/git-foo $*

will try to execute GIT_EXEC_PATH and prepend its output to the rest of 
the command, which is quite obviously wrong.

> 
>>Since it's already in place in the potty and that's required to be in 
>>the $PATH, I think Junio's suggestion of running execlp("git", "git", 
>>...) is a good one. It will add one extra fork() and execve() for each 
>>clone/pull/push, but that isn't much of an issue, really.
>>
> 
> 
> The patch I posted most recently does something comparable; all exec's
> by C git programs go through exec_git_cmd, which actually implements the
> "git potty" logic (and git.c itself uses exec_git_cmd).  If there is to
> be a consistent rule for how to exec a git program from a git C program,
> I think that it's reasonable that there be an API to enforce it.  
> 

True. Perhaps I misread your patch or your reasoning.

> Note that the creation and use of such a function simply means that we
> hide the logic that handles PATH/GIT_EXEC_PATH; how git_exec_cmd()
> actually calls execve() and how PATH and GIT_EXEC_PATH are used is a
> separate issue.  When it comes to the former, I think it is best to have
> all exec's of git programs go through an interface that imposes the same
> PATH/GIT_EXEC_PATH logics.  As to the latter, my only concern is that we
> should never do 'setenv("PATH",....)'.
> 

setenv("PATH", ..) is way preferrable over the git-setup.sh hackery 
suggested above, so long as it's only ever done in the git potty. That's 
what the potty is there for, after all.

> 
>>>An approach that I think is better is to require all exec's of git
>>>programs from within git programs to use a specific git interface,
>>>rather than letting each one set up it's own exec parameters.
>>>
>>
>>A better idea would be to teach {send,upload}-pack about $GIT_EXEX_PATH 
>>and export it from your shells rc-file.
>>
> 
> 
> My shell's rc-file doesn't get invoked when using ssh as a transport;
> that's part of the problem.
> 

It does for me and everybody else. $HOME/.bashrc is read even for 
non-interactive shells. $HOME/.bash_profile isn't. If you're using 
git-shell you're in to a whole different situation, but you didn't say 
so so I don't think you are.

>>
>>If a user invokes "/home/user/bin/git-foo" rather than 
>>"/home/user/bin/git foo" he/she will have to have the rest of the 
>>git-suite in the $PATH. Prepending whatever directory any git-* program 
>>happens to reside in to $PATH is not a good idea. 
> 
> 
> Isn't this exactly what git.c is doing currently via prepend_to_path()?
> 

No. git prepends whatever was passed in --exec-path=/some/path, what's 
found in $GIT_EXEC_PATH or the GIT_EXEC_PATH pre-processor macro set at 
compile-time, in that order of preference. There's a very big difference.

> git programs exec other git programs, but they also exec non-git
> programs.  I think it is not appropriate to change PATH (via
> prepend_to_path) because this may result in unexpected behavior when
> exec'ing non-git programs:
> 
> Suppose git is installed in /usr/bin, where a "diff" resided.
> I've got my own version of "diff" in /home/user/bin.
> PATH=/home/user/bin:/usr/bin.
> 
> If git now tries to execute "diff", after having run
> prepend_to_path(), /usr/bin/diff gets executed, not /home/user/bin/diff.
> The user has set up PATH to ensure that /home/user/bin/diff is the diff,
> but by mucking with PATH we subvert their intentions.
> 

Good point. Perhaps we should only prepend to path when the directory 
isn't already in $PATH, or append rather than prepend.

-- 
Andreas Ericsson                   [email-masked]
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231