Re: Extending "extended SHA1" syntax to traverse through gitlinks?

From: Jakub Narębski <jnareb@gmail.com>
Date: Aug 24, 2016, 17:05 UTC
Message-ID: <165ac159-29e8-10d1-49ee-e5ff6543d0d2@gmail.com>
In-Reply-To: <20160824142024.xnaehfo2spw26apj@x>

W dniu 24.08.2016 o 16:20, Josh Triplett pisze:

> On Wed, Aug 24, 2016 at 03:16:56PM +0200, Jakub Narębski wrote:

[...]

Show 19 quoted lines

>> Not really.
>>
>> The above means only that the support for new syntax would be not
>> as easy as adding it to 'git rev-parse' (and it's built-in equivalent),
>> except for the case where submodule uses the same object database as
>> supermodule.
>>
>> So it wouldn't be as easy (on conceptual level) as adding support
>> for ':/<text>' or '<commit>^{/<text>}'.  It would be at least as
>> hard, if not harder, as adding support for '@{-1}' and its '-'
>> shortcut.
> 
> Depends on which cases you want to handle.  In the most general case,
> you'd need to find and process the applicable .gitmodules file, which
> would only work if you started from the top-level tree, not a random
> treeish.  On the other hand, in the most general case, you don't
> necessarily even have the module you need, because .git/modules only
> contains the modules the *current* version needed, not every past
> version.

There is an additional problem, namely that directory with submodule can be renamed.

I don't know if there is an existing API, but assuming modern git-submodule (with repository in .git/modules) you would have to do the following steps for <revision>:<path/to/submodule>//<path>:

 * look up <revision>:.gitmodules for module which 'path'
   is <path/to/submodule>; let's say it is named <submodule>
 * check if <revision>:<path/to/submodule> commit object
   is present in .git/modules/<submodule>
 * look up this object

In the case of legacy submodule setup, with submodule repository
in the supermodule working directory, you would need:
   
 * look up <revision>:.gitmodules for module which 'path'
   is <path/to/submodule>; let's say it is named <submodule>
 * look up current .gitmodules for current path of submodule
   named <submodule>; let's say it is <new/path/submodule>
 * check of <revision>:</path/to/submodule> commit object
   is present in :(top)<new/path/submodule>/.git repository
 * look up this object

You could also check if the submodule repository (as stored in config) is a path, and use it if it is... but that might be going to far.

BTW. all that reminds me that gitweb should handle submodules better.

> As an alternate approach (pun intended): treat every module in
> .git/modules as an alternate and just look up the object by hash.

This could be a good fallback, to search through all submodules.

> Or, teach git-submodule to store all the objects for submodules in the
> supermodule's .git/objects (and teach git's reachability algorithm to
> respect refs in .git/modules, or store their refs in
> .git/refs/submodules/ or in a namespace).

And fallback to this fallback could be searching through supermodule object repository.

Storing all objects in single repository is counter to the design
decision of submodules (though I don't remember what it was), but
it might be done.  Still, Git needs to be able to deal with legacy
situations anyway.

Show 9 quoted lines

>> Josh, what was the reason behind proposing this feature? Was it
>> conceived as adding completeness to gitrevisions syntax, a low-hanging
>> fruit?  It isn't (the latter).  Or was it some problem with submodule
>> handling that you would want to use this syntax for?
> 
> This wasn't an abstract/theoretical completeness issue.  I specifically
> wanted this syntax for practical use with actual trees containing
> gitlinks, motivated by having a tool that creates and uses such
> gitlinks. :)

Could you explain what you need in more detail?  Is it a fragment
of history of submodule, a contents of a file at given point of
superproject history, diff between file-in-submodule and something
else, or what?

Show 16 quoted lines

>> As for usefulness: this fills the hole in accessing submodules, one
>> that could be handled by combining plumbing-level commands.  Namely,
>> there are 5 states of submodule (as I understand it)
>>
>>  * recorded in ref / commit in supermodule
>>  * recorded in the index in supermodule
>>  - recorded in ref / commit in submodule
>>  - recorded in the index in submodule
>>  - state of worktree in submodule
>>
>> The last three can be easyly acessed by cd-ing to submodule.  The first
>> two are not easy to get, AFAIUC.
> 
> Right.  I primarily care about those first two cases, especially the
> first one: given a commit containing a gitlink, how can I easily dig
> into the linked commit?

All right.

Though you can cobble it with plumbing... just saying.

-- 
Jakub Narębski