[BUG] Recursive submodules fails to track branch correctly if . is used

[Krzysztof Opasiak <k.opasiak@samsung.com>]

Dear git community,

I'd like to report you a potential bug in git submodule implementation.

* Problem statement

According to doc a special value of "." can be used as a branch name to 
track. This means that git should trace the same branch as is currently 
in use in superproject. This works perfectly fine as long as you don't 
try to add superproject (SUP1) with such configuration as a submodule to 
another superproject (SUP2). The nesting structure would look like this:

SUP2
     - SUP1
         - SUB_PROJECT

If you try to do this you won't be able to execute any of the following 
commands:

$ git clone --recursive SUP2

$ git clone SUP2 & git submodule update --init --recursive --remote

$ git submodule update --init --recursive --remote

as every time you will get an error message similar to this one:

fatal: Submodule (SUB_PROJECT) branch configured to inherit branch from 
superproject, but the superproject is not on any branch
fatal: Needed a single revision
Unable to find current origin/ revision in submodule path 'SUB_PROJECT'

* Initial investigation results

The error message comes from remote_submodule_branch():1938 in 
submodule--helper.c and it is generated called from SUP1 dir (due to 
--recursive flag). Actually this function behaves correctly. If you go 
to SUP1 dir and check the branch info you will notice that the repo is 
in detached head state.

* Probable root cause

The root cause is that the traced branch information is never propagated 
to the submodules. All the submodules are always initialized in a 
detached head state and there is no way to change this behavior nor to 
propagate the traced branch information to submodules.

* Workaround script

I managed to workaround this bug by dropping the recursive option and 
after the top level project has been cloned I go through every 
submodule, set local branch to point to current head and then just do 
git checkout and I do this recursively in all modules. After this I'm 
able to successfully execute:

$ git submodule update --rebase --recursive --remote

The script (to be executed in SUP2 right after clone):

#!/bin/bash

function prepare_submodules {
	git submodule update --init
	local SUBMODULE_LIST=`git submodule--helper status | awk '{print $2}'`
	while read -r submod; do
		local PARENT_DIR=`pwd`
		local BRANCH=`git submodule--helper remote-branch ${submod}`

		cd ${submod}
		git branch -f ${BRANCH} HEAD;
		git checkout ${BRANCH}
		if [ ! -z "`git submodule status`" ]; then
			prepare_submodules;
		fi
		cd ${PARENT_DIR}
	done <<< "$SUBMODULE_LIST"
}

prepare_submodules

Best regards,
-- 
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics