Re: RFC: Making submodules "track" branches

[Marc Branchaud <marcnarc@xiplink.com>]

On 10-06-08 03:12 AM, Johan Herland wrote:
> 
> There are probably more issues that escape me now...

Thanks for bringing this up!  I'm also very interested in this topic.

The main issue I see is that I don't always want my submodules to track (or
not track) a branch.  What I want changes depending on the circumstances.

One aspect I really like about submodules is the ease of tagging.  I can tag
the super-repo, and know that whenever I checkout that tag I'll always get
the corresponding versions of the submodules as they were when the tag was
made.  It would actually be disastrous, at least in my case, if the
submodules were at the latest HEAD of some branch instead.

This goes for almost any commit in the super-repo's history, not just the
tagged ones:  Whenever I checkout a historical committish, I want to get the
submodules as they were when that commit was made.  Even if I'm working at
the HEAD of some branch, often that branch is based on a historical commit
and I want to use the submodules as they were when that historical commit was
made.

All that said, I do think submodule branch tracking is useful.  Quite often a
development topic will change the super-repo and one or more submodules.  It
would be extremely helpful to do that work in a branch that spans the
super-repo and (a subset of) the submodules.  (In my mind this capability is
one of the main benefits Google's "repo" tool has over submodules.)

So, back to the issue at hand: Sometimes I want static (non-tracking)
submodules, and sometimes I want dynamic (tracking) submodules.  IMO, this
makes Ævar's proposed configuration-based approach impractical.  (Of course,
I'm not looking to replicate svn's externals...)

I'm not sure what the right approach is, but I have some thoughts:

 - Maybe "git branch" should be able to create submodule-spanning branches.

 - If so, then checkout, merge, pull and other branch-related commands should
honor submodule-spanning branches.  "checkout" in particular needs to
distinguish between when it's checking out an actual branch vs. some other
committish, and if the branch being checked out is submodule-spanning it
should checkout the latest HEAD of that branch for the submodules as well.

 - It *may* be good enough to assume that matching branch names in the
super-repo and the submodules are in fact submodule-spanning branches.

 - Automating all this is tricky.  In my super-repo I almost never want to
checkout the master HEAD of all my submodules.  In fact, many of my
submodules are big (e.g. they're different Linux kernels), and are only
needed when building particular things, so that checking all of them out at
once is almost always a huge waste of time.

All this is probably not the kind of feedback you were hoping for!  :)

		M.