Re: [RFC/PATCH 0/7] Rework git core for native submodules

[John Keeping <john@keeping.me.uk>]

On Sun, Apr 07, 2013 at 11:37:02PM +0530, Ramkumar Ramachandra wrote:
> John Keeping wrote:
> > On Sun, Apr 07, 2013 at 10:52:50PM +0530, Ramkumar Ramachandra wrote:
> >> Sure, I'll write it out for you from an end-user perspective:
> >
> > To play Devil's Advocate for a bit...
> 
> Yes!
> 
> >> 0. Great UI/UX.  No more cd-to-toplevel, and a beautiful set of native
> >> commands that are consistent with the overall design of git-core.
> >> Which means: clone (to put something in an unstaged place), add (to
> >> stage), and commit (to commit the change).  There's now exactly one
> >> place in your worktree (which is represented as one file in git; think
> >> of it a sort of symlink)  to look in for all the information.  git
> >> cat-link <link> to figure out its parameters, git edit-link to edit
> >> its parameters: no more "find the matching pwd in .gitmodules in
> >> toplevel".  To remove a submodule, just git rm.  And git mv works!
> >
> > Presumably now without .git/config support, so I can't override the
> > checked-in settings without my own custom branch.  Even carrying a dirty
> > working tree seems problematic here since a checked-out link object is a
> > directory, which can't have information like the remote URL in it.
> 
> Sure you can have a dirty worktree.  It's just like .gitmodules:
> there's zero difference but for the fact that .gitmodules is
> accessible directly via your filesystem, while links are not.

I can't see how this gets me a dirty working tree.  Since the link needs
to be stored somewhere, I assume it's in the index; so I can have staged
changes, but not unstaged changes.

> >> 1. True floating submodules.  You can have a submodule checked out at
> >> `master` or `v3.1`: no more detached HEADs in submodules unless you
> >> want fixed submodules.  No additional cruft required to do the
> >> floating: the information is native, in a link object.
> >
> > Can't I do that now with "submodule.<name>.branch" and "git submodule
> > update --remote --rebase" and friends?
> 
> Yes, but that is not true floating: you shouldn't have to be sorry and
> rebase.  In new-style submodules, they're first class citizens (ie.
> true): you can just replace the SHA-1 with a ref in the link.

But what happens if I make any changes on top?  With --rebase and
--merge I can specify exactly what I want to happen (and obviously if I
don't have any changes then whichever I choose simply sets my branch to
the upstream ref).

> >> 2. Initializing a nested submodule without having to initialize the
> >> outer one: no more repo XML nonsense.  And it's composable: you don't
> >> need to put the information about all submodules in one central place.
> >
> > How does this interact when there is the following structure:
> >
> >     super
> >     `-- sub
> >         `-- subsub   (specified by sub)
> >
> > and subsub is specified as a submodule in *both* super and sub but with
> > different settings.  Do I get different behaviour depending on $PWD?
> 
> This is a very fringe case that I haven't thought about.  I don't know
> how it will behave: I haven't built it yet (and don't have the entire
> implementation in my head yet).
> 
> >> 3. Ability to have very many large submodule repositories without the
> >> performance hit.  It makes sense to block stat() from going through
> >> when you have floating submodules.  This means that many levels of
> >> nesting are very easily possible.
> >
> > Can't I already control this to some degree?  Certainly the following
> > commands take different amounts of time to run:
> >
> >     git status
> >     git -c status.submodulesummary=true status
> 
> You can't control the most fundamental thing, stat(): this is the
> primary killer of performance on a large worktree.  There is currently
> no way to block stat(): new-style submodules offers a way to configure
> which submodules to block the stat() on.

So it would be something like per-submodule --untracked-files and
--ignore-submodules settings?  I can see that being useful.