Re: [PATCH] FYI / RFC: submodules: introduce repo-like workflow

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Sep 27, 2018, 22:27 UTC
Message-ID: <20180927222728.GD193497@aiede.svl.corp.google.com>
In-Reply-To: <20180927221603.148025-1-sbeller@google.com>

(dropping cc-s to my internal address that I don't use on this list
 and to git-core@google.com which bounces)
Hi,

Stefan Beller wrote:

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> Internally we have rolled out this as an experiment for
> "submodules replacing the repo tool[1]". The repo tool is described as:
>
>     Repo unifies Git repositories when necessary, performs uploads to the
>     Gerrit revision control system, and automates parts of the Android
>     development workflow. Repo is not meant to replace Git, only to make
>     it easier to work with Git in the context of Android. The repo command
>     is an executable Python script that you can put anywhere in your path.

[...]

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> Submodules can also be understood as unifying Git repositories, even more,
> in the superproject the submodules have relationships between their
> versions, which the repo tool only provides in releases.
>
> The repo tool does not provide these relationships between versions, but
> all the repositories (in case of Android think of ~1000 git repositories)
> are put in their place without depending on each other.
>
> This comes with a couple of advantages and disadvantages:

Thanks for describing this background.

[...]

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> This changes the following commands in the superproject:
>
>   checkout -B/-b create branches in subs, too
>   checkout (-f): update branch in submodule (create if needed) and check
>                  it out; Pass the argument down literally if it is a branch
>                  name (e.g. "checkout -f master" will run a
>                             "checkout -f master" in the submodule as well)
>   clone: see checkout
>   reset --hard: see checkout

As you mentioned, I've been using this submodule.repoLike=true mode for my own use for a while. You did a nice job of explaining on how it fits into a Gerrit-driven workflow; I'd like to add that I find it pleasant in non-Gerrit-driven contexts as well.

The primary difference from repoLike=false is that this makes the normal state to have branches checked out in submodules. For example, if I run

	git checkout --recurse-submodules -B master origin/master

then this will create and check out a "master" branch in all submodules instead of only in the superproject. This helps avoid some issues in Git's submodule handling where submodule commits can be pruned if they have not been checked out in a while because there is no ref pointing to them.

Some next steps:

- now that we have a repository object, some of the implementation can
  be simplified and made more robust.  I expect that will also make
  these patches easier to review

- also in the direction of reviewability, at that point we may want to
  split this into multiple patches

- gitsubmodules.txt and config.txt should describe the new option, to
  help new users understand what this new repoLike workflow does

- there are some edge cases in the UX that get... messy that I should
  describe in another message

All that said, thanks for sending this out, and I'd be happy to hear from any interested people --- feedback from anyone adventurous enough to try this out would be very welcome.

Happy hacking, Jonathan