Re: A design for subrepositories

Quoting "Junio C Hamano" <gitster@pobox.com>:

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