Re: How to pre-empt git pull merge error?

From: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Date: Nov 27, 2013, 15:54 UTC
Message-ID: <vpq8uw9q1r4.fsf@anie.imag.fr>
In-Reply-To: <20131127194240.2abaff5575961b3d73e1970f@domain007.com>

Konstantin Khomoutov <flatworm@users.sourceforge.net> writes:

Show 30 quoted lines

> On Wed, 27 Nov 2013 15:17:27 +0000
> Pete Forman <petef4+usenet@gmail.com> wrote:
>
>> I am looking for a way of detecting up front whether a git pull or git
>> merge would fail. The sort of script I want to perform is to update a
>> server.
>> 
>>     git fetch
>>     git okay
>>     stop server
>>     backup data
>>     git merge
>>     start server
>> 
>> Here git okay is a place holder for the command I am asking for.
>> 
>> If a file has been changed outside of a commit then git pull fails
>> with the following error.
>> 
>> error: Your local changes to '...' would be overwritten by merge.
>> Aborting. Please, commit your changes or stash them before you can
>> merge.
>
> What's wrong with "git okay" being
>
> if git merge whatever 2>/dev/null; then
>   ... OK path
> else
>   ... "merge failed" path
> fi

The idea seems to be to stop the server before actually doing the merge (and avoid doing so if the merge is bound to fail).

I don't know a simple way to do the pre-merge check without actually doing the merge (other than patching git merge to add a --dry-run option), but you can do a pessimistic check by using the require_work_tree_exists shell function defined in git-sh-setup (copied below, but you can call it from a shell script after doing . "$(git --exec-path)/git-sh-setup"):

require_clean_work_tree () {
	git rev-parse --verify HEAD >/dev/null || exit 1
	git update-index -q --ignore-submodules --refresh
	err=0

	if ! git diff-files --quiet --ignore-submodules
	then
		echo >&2 "Cannot $1: You have unstaged changes."
		err=1
	fi

	if ! git diff-index --cached --quiet --ignore-submodules HEAD --
	then
		if [ $err = 0 ]
		then
		    echo >&2 "Cannot $1: Your index contains uncommitted changes."
		else
		    echo >&2 "Additionally, your index contains uncommitted changes."
		fi
		err=1
	fi

	if [ $err = 1 ]
	then
		test -n "$2" && echo >&2 "$2"
		exit 1
	fi
}

Additionally, you may want to check that the merge is a fast-forward (hence can't result in merge conflict), e.g. by checking that the current commit is the merge base between itself and the commit to merge (git merge-base HEAD $commit).

-- 
Matthieu Moy
http://www-verimag.imag.fr/~moy/