https://phabricator.wikimedia.org/T130703 added support for submodules:

1. $somebody files a code review for module X
2. code review gets approved and merged in the submodule's repository
3. $somebody updates the related sha signature in the main puppet repo and files another code review
4. $somebody can finally use the puppet compiler to check diffs

It would be great to get 4) directly after 1), namely figuring out a way to instruct the puppet compiler that a code review filed for a repository that is not the main puppet one should trigger a fetch/merge to the appropriate submodule folder.

This change should target the fetch_change [1] function. Caveat:

1. link gerrit repo name with submodule name/path (git config --get-regexp submodule ?)
2. fetch/merge the code in the appropriate submodule folder (git submodule foreach $magic ?)

[1]: https://github.com/wikimedia/operations-software-puppet-compiler/blob/master/puppet_compiler/prepare.py#L117