Re: [PATCH 2/2] Add keyword unexpansion support to convert.c

From: Junio C Hamano <junkio@cox.net>
Date: Apr 17, 2007, 22:40 UTC
Message-ID: <7vy7kqlj5r.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <alpine.LFD.0.98.0704171708360.4504@xanadu.home>

Nicolas Pitre <nico@cam.org> writes:

Show 12 quoted lines

>> I would like to, however this doesn't currently integrate
>> well with git. I've been told in the past that once
>> .gitattributes is in place then the hooks for the crlf stuff
>> can be generalized to allow for calls out to custom code to
>> do this sort of thing.
>
> And I agree that this is a perfectly sensible thing to do.  The facility 
> should be there for you to apply any kind of transformation with 
> external tools on data going in or out from Git.  There are good and bad 
> things you can do with such a facility, but at least it becomes your 
> responsibility to screw^H^H^H^Hfilter your data and not something that 
> is enforced by Git itself.

You have to be careful, though. Depending on what kind of transformation you implement with the external tools, you would end up having to slow down everything we would do.

It boils down to this statement from Andy:

    ..., keywords (in other VCSs, and so why not in git) are
    only updated when a file is checked out.  There is no need
    to touch every file.  It's actually beneficial, because the
    keyword in the file is the state of the file at the time it
    was checked in - which is actually more useful than updating
    it to the latest commit every time.

    That means you're only ever expanding in a file that your
    changing anyway - so it's effectively free.  git-checkout
    would still be immediate and instantaneous.

Back up a bit and think what "when a file is checked out" means. His argument assumes the current behaviour of not checking out when the underlying blob objects before munging are the same.

But with keyword expansion and fancier "external tools" whose semantics are not well defined (iow, defined to be "do whatever they please"), does it still make sense to consider two blobs that appear in totally different context "the same" and omit checking out (and causing the external tools hook not getting run)? I already pointed out to Andy that the branch name the file was taken from, if it were to take part of the keyword expansion, would come out incorrectly in his printed svg drawing.

If you want somebody's earlier example of "giving a file with embedded keyword to somebody, who modifies and sends the result back in full, now you would want to incorporate the change by identifying the origin" to work, you would want "$Source$" (I am looking at CVS documentation, "Keyword substitution/Keyword List") to identify where that file came from (after all, a source tree could have duplicated files) so that you can tell which file the update is about, and this keyword would expand differently depending on where in the project tree the blob appears.

It is not just the checkout codepath. We omit diffs when we know from SHA-1 that the blobs are the same before decoration. We even omit diffs when we know from SHA-1 that two trees are the same without taking possible decorations that can be applied differently to the blobs they contain into account. Earlier, Andy said he wanted to grep for the expanded text if he is grepping in the working tree, and I think that makes sense, but that means git-grep cannot do the same "borrow from working tree when expanding from blob object is more expensive" optimization we have for diff. We also need to disable that optimization from the diff, regardless of what the correct semantics for grepping in working trees should be.

I suspect that you would have to play safe and say "when external tools are involved, we need to disable the existing content SHA-1 based optimization for all paths that ask for them" to keep your sanity.