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

From: David Turner <dturner@twopensource.com>
Date: Apr 30, 2015, 19:25 UTC
Message-ID: <1430421924.22711.57.camel@ubuntu>
In-Reply-To: <20150430033725.GB12361@peff.net>

On Wed, 2015-04-29 at 23:37 -0400, Jeff King wrote:

Show 6 quoted lines

>   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.

Show 5 quoted lines

> 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.