Re: submodules, was Re: RFC: display dirty submodule working directory in git gui and gitk

On Mon, Jan 4, 2010 at 6:46 AM, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:

I suffered the current implementation, which is why I wrote git-subtree :) I'm still suffering, though; git-subtree works much better for my own use cases, but after some experience with it, I'm still not totally happy.

For me one big problem comes down to producing accurate output for 'git log'. git submodules assume that the history inside the module is entirely separate (you need to run multiple 'git log' instances to see the full history); git-subtree assumes that it's entirely integrated. In that sense, git-subtree is somewhat more in line with the core principle of git (we track the history of "the content", not any particular file or subdir). Unfortunately, it also exposes a problem with that core principle: taken to its extreme, "the content" includes all data in the universe. And while git could branch and merge the universe very efficiently in about O(log n) time, 'git log' output gets less useful about O(n) with the size of the tree.

Neither git-subtree nor git submodules seem to help with this "log pollution" problem very much - but I don't know what to do that would be better.

Outside of this, my major problem with submodules is they use separate work trees and repositories, and thus require lots of extra housekeeping to get anything done. I'd be much happier if submodules would share the same objects/packs/.gitdir/refs/indexfile as the superproject, and the *only* thing special about them would be that the superproject's tree points at a commit object instead of a tree object. In other words, I think the actual repo format is correct as-is, but the tools surrounding it cause a lot of confusion.

Imagine if cloning a superproject also checked out the subproject transparently, and committing dirty data inside the subproject's tree created a new commit object for the subproject, then tacked that commit object into the superproject's index for a later commit (exactly as changing a subdir creates a new tree object that the parent directory can refer to).

This doesn't solve some use cases, however, such as ones where people really don't want to check out (or even fetch) the contents of some submodules, even when they check out the superproject. The current implementation *does* handle that situation. I'm not sure how many people rely on that behaviour, though. (And maybe the correct solution to *that* is proper support for sparse clone/checkout regardless of submodules.)

Have fun,

Avery