Re: RCS keyword expansion

From: Jan Hudec <bulb@ucw.cz>
Date: Oct 12, 2007, 12:57 UTC
Message-ID: <20071012125751.GB7865@efreet.light.src>
In-Reply-To: <Pine.LNX.4.64.0710121144001.3682@ds9.cixit.se>

On Fri, Oct 12, 2007 at 11:50:51 +0100, Peter Karlsson wrote:

Show 5 quoted lines

> > But don't you see?  When switching branches, this totally breaks down.
> 
> Why would it? If the file is the same on both branches, nothing would
> happen (it is still the same version), and if the file is different, it
> gets changed anyway, and a new keyword expansion would take place.

It does not -- the blob ID is the same. And you can indeed get $Id$ expanded to that (see gitattributes(5)). However, that's the ONLY thing that is the same. The date of last modification or ID of commit that last touched that file might not.

Why? Because git tracks content, not history. The trees and blobs (files) are identified by SHA1 hashes of their content. Only commit objects add the notion of history. Thus if two commits contain file with the same text, it's the same file. Even if the file is the same in those commit purely by accident.

Show 13 quoted lines

> > No, really, IMHO it is enough to show either the commit name or the
> > blob name of the file. After all, you are not interested in the date
> > that this file was last committed, but in the _contents_.
> 
> Yes, but I want it on the lowest addressable unit size, i.e on the file
> level (I could possibly want to have the last commit for a set of files
> when I have something that get generated from several sources, but that
> is rare for a regular website, unless since javascripts and stylesheets
> etc. are delivered separately).
> 
> Already today when you do "git log" on a file, you get the log filtered
> for only changes to that file. So the mechanisms seem already to be
> there, I just need to figure out how to apply them to what I want.

Yes. But you need the (current) commit for that. Now if a file foo is the same on branches A and B, switching between them will not touch that file, but git log foo may well give you completely different results. That's why there's no date or commit that last touched a file.

Show 14 quoted lines

> > So why not go for the contents?  With CVS/SVN you only have the
> > chance to do that by date or version number. With git, we have a more
> > powerful way: we do it by a hash of the contents.
> 
> Yes, but the hash if of "everything". I'm not interested in
> "everything" in this context, and I don't want to have a separate git
> repository for each file...
> 
> > If it's not what you want, I suggest rethinking what you want ;-)
> > Otherwise it is scripting time for you.  It's easy enough with git.
> 
> That's what I'm trying to figure out. I assume it would be possible to
> do with some kind of hook that is run on checkout. But I can't figure
> out how to do that.

If you read the (already mentioned) gitattributes(5) manpage, you'll find description of smudge and clean filters. They will be applied to each file written out to the tree and read back respectively. But be sure to understand why you can't -- for principial, not techical reasons -- have the date or commit ID expanded correctly in all cases.

-- 
						 Jan 'Bulb' Hudec <bulb@ucw.cz>