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

From: Ramkumar Ramachandra <artagnon@gmail.com>
Date: Apr 7, 2013, 18:07 UTC
Message-ID: <CALkWK0n=vtPT7aFn9+T+bRxUpfXG+mYvV29YKC=_OAampQXJSA@mail.gmail.com>
In-Reply-To: <20130407175210.GI2222@serenity.lan>

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!

Show 14 quoted lines

>> 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.

Show 7 quoted lines

>> 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.

Show 12 quoted lines

>> 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).

Show 10 quoted lines

>> 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.