Re: [RFD/PATCH] submodule doc: describe where we can configure them

From: Heiko Voigt <hvoigt@hvoigt.net>
Date: May 4, 2016, 15:01 UTC
Message-ID: <20160504150143.GA8575@sandbox>
In-Reply-To: <CAGZ79kb4GNZi93JpsPKUb7ScmePqDC1Xp+WZTGD7hfPUn9tWaQ@mail.gmail.com>

On Tue, May 03, 2016 at 05:59:58PM -0700, Stefan Beller wrote:

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> On Tue, May 3, 2016 at 4:56 PM, Jonathan Nieder <jrnieder@gmail.com> wrote:
> > Stefan Beller wrote:
> >
> >> This is similar to the gitignore document, but doesn't mirror
> >> the current situation. It is rather meant to start a discussion for
> >> the right approach for mirroring repositories with submodules.
> >
> > Ooh.
> 
> Thanks for writing such a detailed answer. :)

BTW, here is a pointer to the discussion (and what I wrote down) about this from back in 2014:

https://github.com/jlehmann/git-submod-enhancements/wiki/Ideas#special-ref-overriding-gitmodules-values

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> > To fix this, we could allow additional .gitmodules settings to be put
> > in another ref (perhaps something like "refs/repository/config" to allow
> > sharing additional repository-specific configuration in other files
> > within the same tree --- e.g., branch descriptions).  The semantics:
> >
> > * If there is a gitmodules file in refs/repository/config in the
> >   repository I clone, then the submodule settings from it are stored
> >   locally somewhere that overrides .gitmodules.  Perhaps
> >   .git/info/<remotename>/gitmodules?
> >
> > * Later fetches from the remote would also update this gitmodules
> >   file.
> >
> > * Settings from this gitmodules file can be overridden locally
> >   using 'git config' until an explicit "git submodule sync" to
> >   override the local configuration.
> >
> > What do you think?
> >
> > If two different remotes provide conflicting values for a setting
> > in their gitmodules files, git would error out and ask the user
> > to intervene with a tie-breaking "git config" setting.
> 
> Let's look at an example with C mirroring from B, who mirrors from A.
> 
> The user who clones the superproject from C may want to obtain submodules
> from either C or B or A. All this can be configured in
> the refs/repository/config value of C, but in case it is not configured in C,
> it may fall back to the same branch from B. When and how would B get
> that branch?

I think B has to setup that branch on its own when it starts to mirror A and uses different submodule urls or other configs.

Jonathan you suggested to copy the content from a remote to .git/info/<remotename>/gitmodules locally. How would one get it to the remote side? It seems to me as if we would need to implement additional infrastructure to do this. Would it not be simpler if we just kept it on a ref on the local side as well? We already have the infrastructure to read those values from a ref. We only would need to add something to write them. Then a simple push, which could be aliased in form of a git-submodule subcommand, suffices to get the values to the remote.

That also solves issues when people clone from their working copy.

I would like to think a little bit further about the conflict situation when two remotes are providing values. Configuring this looks to me like a nightmare for users. Maybe there is some sort of elegant solution? E.g. like we use the values from remote A during a fetch from A, the ones from B during a fetch from B and no values from a special ref in case there is no remote operation involved. Since the main goal is to support forking of submodules isn't there always a remote operation involved?

My suggested scheme above does not solve the currently quite typical use case where you might 'git fetch' without submodules first and then do the submodule fetches during a 'git submodule update'. On the other hand in the 'ideal future world' where submodules behave like "normal files" the fetch will be done during the superproject fetch so in that case we could solve such conflicts.

The main thing which we could keep in mind is that we only allow certain values in such special refs. E.g. only the ones needed to support the fork workflow. BTW, do we actually need to change other values than the URL? Addtionally we ignore other values that are more related to the overall project structure. E.g. like submodule.<name>.ignore.

Ok after writing this it really feels like special casing a lot. I would not really call it elegant. At the same time limiting these special refs to one special use case (forking) might help us to keep the user interface[1] simpler and conflict free in the long run. Not sure. What do you think?

Cheers Heiko

[1] Which is not the simplest already.