Our team at work is using git submodules to track re-usable code across projects, and it’s been pretty good so far, but we have hit minor snags along the way (such as the absence of a ‘git submodule rm’ command!). Another one is that using submodules adds a step to the sequence of things you have to do to publish changes: pushing submodule commits. It’s an easy thing to forget, but it’s a headache for anyone on the other end of a pull when git-checkout-index fails. This pre-commit hook script will cause the commit to fail if the commit contains new submodule moments and those moments are not present in the corresponding submodule origin.

#!/bin/sh

function array_has
{
    for item in $1
    do
	if [ "$item" = "$2" ]; then
	    return 1;
	fi
    done

    return 0;
}

diffs=`git diff --cached --name-only`

IFS=`echo -en "\n\b"`
for smstat in `git submodule 2>/dev/null`
do
    if [[ "$smstat" =~ '^\+(.*)' ]]; then
	smstat=${BASH_REMATCH[1]}
    fi

    head=$(echo $smstat | awk '{print $1}')
    path=$(echo $smstat | awk '{print $2}')
    moment=$(git ls-files -s $path | awk '{print $2}')

    array_has $diffs $path
    if [ $? ]; then
	pushd >/dev/null $path
	for rhead in $(git ls-remote -h origin
	               | awk '{print $1}')
	do
	    if [ "$(git rev-list $moment ^$rhead)"
		    != "" ]
	    then
		unpub=1;
	    fi
	done
    fi

    if [[ $unpub -gt 0 ]]; then
	echo -n "ERROR: you are trying to commit "
	echo -n "unpublished changes to the $path "
	echo    "submodule."
	exit 1;
    fi

done

exit 0;

Seems that new features and concepts appear in Git at such a steady pace that it’s difficult for the jargon to keep up. I don’t follow the git mailing list as closely as I should: it’s too high-traffic and I already don’t have enough time to do the work I have on my plate.

Right now what’s getting tongue-tied is referring to the commit-id of a submodule stored in the HEAD of a containing repository. As far as I know, there’s not a good, succinct and unambiguous term for this commit-id.

I have decided to call it the ‘submodule moment‘, because moment captures the ideas nicely:

• a submodule at a particular point in its history (which we typically think of as linear in time)
• a property of the submodule as it relates to it’s parent repository (a moment in mathematics is a statistical property of random variables)

More importantly, though, this term disambiguates itself from the other verbal references to commit identifiers.