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

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

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

[...]

quoted

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.
 

quoted

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?
 

quoted

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