Re: [PATCH] _git: Also complete FETCH_HEAD, ORIG_HEAD and MERGE_HEAD.

["Benjamin R. Haskell" <zsh@benizi.com>]

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On Sun, 2 May 2010, Frank Terbeck wrote:

> Simon Ruderich wrote:
> > On Sat, May 01, 2010 at 02:17:19PM +0200, Frank Terbeck wrote:
> >>> I would prefer mine because of the speed penalty of Benjamin's 
> >>> patch. Checking if the file exists in .git would be another 
> >>> possibility.
> >>
> >> And that would be the one from workers-27813¹?
> >>
> >> Regards, Frank
> >>
> >> ¹ <http://www.zsh.org/mla/workers/2010/msg00265.html>
> >
> > Ah sorry. Yes, that's the one.
> 
> Committed this one. Actually checking whether the head symbols are 
> available at any time probably requires something like this:
> 
>     gdir="$(git rev-parse --git-dir)"
>     [[ -f ${gdir}/ORIG_HEAD ]] && ...
>     [[ -f ${gdir}/FETCH_HEAD ]] && ...
>     ...
> 
> I don't know if it's worth the additional fork.

That's essentially what my 'compromise' patch does.  The one from 
workers-27817 [links below].

==== this portion: ====
+  symbolic_heads=()
+  gitdir=$(_call_program gitdir git rev-parse --git-dir 2>/dev/null)
+  __git_command_successful && symbolic_heads=( $gitdir/{,{ORIG,FETCH,MERGE}_}HEAD(N:t) )
=======================

As stated before, I think the fork is worth it.  The three options were:

[27813]. always-complete:  usually-incorrect, 0 extra forks
[27817]. --git-dir + filesystem tests: possibly-incorrect, 1 extra fork
[27814]. rev-parse each name: always correct, 4 extra forks

Personally, I think the 4 extra forks are worth it (The particular 
execution path that this alters already does two _call_program git 
forks).  But, I'm not stuck on a system where forking is expensive (100 
git forks takes ~1sec)[2], so I see the need for the compromise [27817].

[27813] is usually incorrect (depending on your workflow) because 
FETCH_HEAD, ORIG_HEAD, and MERGE_HEAD come and go depending on what 
actions you take on your repository.

The reason [27817] is possibly-incorrect is that the .git/*HEAD files 
can exist even if the branch hasn't been born.  (e.g. try 'git rev-parse 
HEAD' in a newly-init'ed repo, even though .git/HEAD exists)
But, it's rare enough that the correctness vs. speed tradeoff seems 
reasonable.

-- 
Best,
Ben

[27813] http://www.zsh.org/mla/workers/2010/msg00265.html
[27814] http://www.zsh.org/mla/workers/2010/msg00266.html
[27817] http://www.zsh.org/mla/workers/2010/msg00269.html

[2]
$ date ; r=( HEAD ORIG_HEAD --git-dir ) ; for l in {1..100} ; git rev-parse $r[$(( 1 + $RANDOM % $#r ))] 2>/dev/null >/dev/null ; date
Mon May  3 10:50:06 EDT 2010
Mon May  3 10:50:07 EDT 2010