Re: A design for subrepositories

["Lauri Alanko" <la@iki.fi>]

Quoting "Junio C Hamano" <gitster@pobox.com>:
> Now
> the subdirectory repositories are bound as submodules of the top
> level directory just fine.

This is indeed possible, but with some serious caveats.

Firstly, if you simply do "git submodule add ./foo" (the obligatory  
"./" being quite an unobvious pitfall), you get something quite  
fragile, since now we have submodule.foo.url = ./foo. If the  
submodules ever get reorganized and foo is moved to ./bar, then it is  
impossible to check out older versions or alternate branches, since  
the submodule is no longer where it is expected to be at the origin.

A more robust solution is to use submodule.foo.url =  
./.git/modules/foo, since logical name of a module doesn't change.  
This seems quite kludgy, though, and this cannot be how git-submodule  
is supposed to be used.

But still, "git submodule update" only looks at the modules in the  
currently checked-out tree. If we have other branches or old tags that  
refer to other submodules, there's no simple way to fetch those, too.  
And there is not even such a concept as a bare repository with modules.

So git-submodule is fundamentally a tool to attach repositories into a  
tree, not to attach repositories into a repository. That's why it's  
not really fit for my purposes.

The core problem is that to clone an entire repository and all its  
submodules, there needs to be a way to list them all remotely. But the  
git protocol doesn't just allow us to list the subdirectories under  
.git/modules. Still, there are several ways to do this:

* Just read .gitmodules in every ref and find by brute force every  
submodule referred to even by a single ref. This doesn't really scale.

* Maintain a list of all the submodules in a repository. This would  
have to be in a separate metadata branch, and would get rather hairy  
when we need to merge from a remote that has added other submodules.

* Represent the submodules as refs instead of independent  
repositories. This is my proposal for subrepositories.

However, I feel that all of these are too drastic changes to make in  
git-submodule, given that it is already well-established.

The minor problems, like lack of active branch tracking and multiple  
mount points of a module, could in principle be fixed in  
git-submodule. But again, I have no fondness for complex shell  
programming. Perhaps it was justified when the only interface to git's  
functionality were the command-line tools, but nowadays there are  
various ways to manipulate git repositories from real programming  
languages through real libraries (libgit2, dulwich, etc), and I prefer  
to use those, so I don't really have any motivation to touch  
git-submodule.


Lauri