Re: [PATCH 1/3] git-merge: Honor pre-merge hook

From: Michael J Gruber <git@drmicha.warpmail.net>
Date: Sep 6, 2012, 08:16 UTC
Message-ID: <50485BD3.5020802@drmicha.warpmail.net>
In-Reply-To: <50476FFE.5070602@alum.mit.edu>

Michael Haggerty venit, vidit, dixit 05.09.2012 17:30:

Show 16 quoted lines

> On 09/05/2012 03:39 PM, Michael J Gruber wrote:
>> git-merge does not honor the pre-commit hook when doing automatic merge
>> commits, and for compatibility reasons this is going to stay.
>>
>> Introduce a pre-merge hook which is called for an automatic merge commit
>> just like pre-commit is called for a non-automatic merge commit (or any
>> other commit).
> 
> What exactly is an "automatic merge commit"?  Is it any merge that
> doesn't have a conflict?  A merge that doesn't invoke the editor?  A
> merge done as part of another operation (e.g., pull)?  I don't see the
> term mentioned in the git-merge or githooks man pages.
> 
> I think it would be good if you would define this term in the
> documentation files that your patch touched, and perhaps in the githooks
> section about "pre-commit" as well.

"git merge" can go three ways:

F: fast forward: no commit is created, only a ref is changed
A: automatic: true merge (non-ff) without conflicts (i.e. chosen
strategy can perform the merge); a new commit is created
C: merge with conflicts: no commit is created but the index is prepared
(partially) for a merge commit

In case F, no commit hook is run (talking only about pre-commit/pre-merge).

In case A, no commit is run so far but my patch proposes pre-merge to be run.

In case C, pre-commit (!) is run so far and after my patch.

Show 19 quoted lines

> Secondly, though it is impossible (for backwards compatibility reasons)
> for the pre-commit hook to be invoked for automatic merges, no such
> considerations prohibit the pre-merge commit from being invoked for
> non-automatic merges.  In other words, both hooks, pre-commit *and*
> pre-merge, could be invoked for non-automatic merges.  Would this be
> preferable?
>
> It depends on what pre-merge scripts are likely to be used for.  If they
> will tend to be used for merge-specific actions, then it might be more
> convenient for *all* merges to be vetted by them.  On the other hand, if
> they tend to do the same actions as pre-commit hooks, then having
> non-automatic merge commits go through both hooks would tend to be more
> annoying than helpful.  Specifically, one of the scripts would probably
> have to check whether the merge is a non-automatic merge, and if so do
> nothing (i.e., letting the other script take care of it).  This would
> also require an easy way for a script to determine whether a commit is a
> non-automatic merge commit.
> 
> Have you considered this?

Your second paragraph explains why I did it the way I did. One can easily have pre-merge call pre-commit, or have them be different. One can not easily have only pre-merge called for a non-automatic merge commit, but that is because of backward compatibility. The way *I* would like it is:

- call pre-merge for any non-ff merge commit (automatic or not)
- call pre-commit for any non-merge commit (#parents <=1)

But that would break compatibility.

So I hope my patch is the best approximation to the above which keeps compatibility and is simple to handle in most situations.

Cheers Michael