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

On Wed, Aug 24, 2016 at 07:05:17PM +0200, Jakub Narębski wrote:

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

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

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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

This also assumes your lookup started with a <committish> and not an
intermediate <treeish>, but that'll work in many cases.

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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.

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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.

I'd flip those around: first search registered .gitmodules, then look up
the object in the superproject (since you have it at hand), and then
maybe search every submodule.

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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?

As part of git-series, I have commits, whose trees contain various
gitlinks, such as "series" and "base".  Those gitlinks point to commits
in the same repository.  I'd like to use those gitlinks everywhere I
could use any other committish, such as a branch name.  In particular,
I'd like to write things like some_feature:series:path/to/file ("what
does path/to/file look like in the current version of some_feature"),
some_feature:series^ ("what's the second-to-last commit in
some_feature"), some_feature~5:series:path/to/file ("what did
path/to/file look like in an older version of some_feature"), or
some_feature~5:base..some_feature~5:series~2 ("all but the last two
patches in some_feature~5").  Those should work with show, diff,
format-patch, log, etc.

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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.

Sure, but that makes the expression much more complex.