Subject: Re: RFC: git cat-file --follow-symlinks?

From: David Turner <[email-masked]>
Date: Thu, 30 Apr 2015 12:25:24 -0700

On Wed, 2015-04-29 at 23:37 -0400, Jeff King wrote:
>   3. Return an object with the symlink relative to the original
>      filename (so "../external" in this case). This is kind of weird,
>      though, because we're not just returning a string from the name
>      resolution. It's an actual object.  So we'd be generating a fake
>      object that doesn't actually exist in the object db and
>      returning that. Feeding that sha1 to another program would fail.

> I can't say that I'm excited about any of them. Perhaps you or somebody
> else can think of a more clever solution.
> 
> Note that the complication with (3) does come from my trying to push
> this down into the name-resolution code.

All else being equal, I would prefer the more general solution.  But
here, the generality comes with a price that seems somewhat high. 

When I think about the commands that might use this, cat-file and
ls-tree are at the top of the list (although as noted, I am only likely
to use cat-file, and it's not clear what ls-tree should do in the event
of an out-of-repo link). 

I could imagine someone caring about grep and diff.  Someone who cares
about grep would likely want it to be willing to go out-of-worktree (as
opposed to silently missing things).  I think we all agree that having
git go out-of-worktree is a mistake, so I'm not sure this use-case is
one that is supportable.

The weirdest case is log.  If I say git log HEAD^{resolve} --
foo/bar/baz, does it mean "commits that have touched what is now pointed
to by foo/bar/baz"?  Or does it mean "commits that have touched a thing
that was at that time pointed to by foo/bar/baz"? [1]  The second one is
more useful, since it could not otherwise be achieved.  But I think this
would require additional code in log on top of whatever additional code
is in sha1_name.  In other words, we would not get it for free just by
adjusting sha1_name.

Are there other relevant commands that I'm missing?  

If not, I think we should reconsider the original thought of just
supporting cat-file.  The nice thing about just supporting cat-file is
that for out-of-repo links we can add a special form to the output, that
does not contain a sha (since there is no corresponding sha in the
repo).  In other words, something like your solution 3, quoted above.

[1] See page 11 of http://inform7.com/learn/documents/WhitePaper.pdf
("Has the president ever been ill?") for a similar case.