git submodule update --init --remote --recursive Considered Harmful

03 Jul 2014

I use a repository (gh.el) that has a submodule to itself. The .gitmodules looks like:

[submodule "docs/build/html"]
	path = docs/build/html
	url = git://github.com/sigma/gh.el.git

That’s right. We’re adding a submodule located at docs/build/html that points to… ourselves.

The purpose (I believe) is to make it more convenient to edit the documentation for the project, since GitHub’s convention is to host project documentation from a specially-named branch within your project that has no common ancestor with your project itself. It’s kind of a neat trick.

The problem is when I try to do a git submodule update --remote --recursive. I get stuck in an infinite recursion of submodule checkouts. You can reproduce it yourself with:

$ git clone https://github.com/sigma/gh.el.git
$ cd gh.el
$ git submodule update --init

All fine and dandy up to this point… But now try this:

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

You’ll see docs/build/html being cloned recursively forever like so:

Submodule path 'docs/build/html': checked out 'a1b24e13d368e0595d147e9b068e6904c1514c19'
Submodule 'docs/build/html' (git://github.com/sigma/gh.el.git) registered for path 'docs/build/html'
Cloning into 'docs/build/html'...
remote: Reusing existing pack: 849, done.
remote: Total 849 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (849/849), 249.11 KiB | 375.00 KiB/s, done.
Resolving deltas: 100% (438/438), done.
Checking connectivity... done.
Submodule path 'docs/build/html/docs/build/html': checked out 'a1b24e13d368e0595d147e9b068e6904c1514c19'
Submodule 'docs/build/html' (git://github.com/sigma/gh.el.git) registered for path 'docs/build/html'
Cloning into 'docs/build/html'...
remote: Reusing existing pack: 849, done.
remote: Total 849 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (849/849), 249.11 KiB | 0 bytes/s, done.
Resolving deltas: 100% (438/438), done.
Checking connectivity... done.
Submodule path 'docs/build/html/docs/build/html/docs/build/html': checked out 'a1b24e13d368e0595d147e9b068e6904c1514c19'
...

The Lesson

Don’t use git submodule update --init --remote --recursive. I generally stick to a two step process:

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

and let the submodules handle their own submodules, or something like that. Recursion is fun.

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