Re: CAREFUL! No more delta object support!

From: Linus Torvalds <torvalds@osdl.org>
Date: Jun 29, 2005, 18:59 UTC
Message-ID: <Pine.LNX.4.58.0506291142510.14331@ppc970.osdl.org>
In-Reply-To: <Pine.LNX.4.58.0506271910390.19755@ppc970.osdl.org>

On Mon, 27 Jun 2005, Linus Torvalds wrote:

Show 10 quoted lines

> 
> On Mon, 27 Jun 2005, Junio C Hamano wrote:
> > 
> > Shouldn't feeding "git-rev-list --object" output plus
> > handcrafted list of objects in 2.6.11 tree object to
> > git-pack-objects just work???
> 
> You could do that. And yes, we can add support for "tag" objects too 
> (which the packing doesn't do at all right now. So this is not a 
> "fundamental" problem, it's just a practical one right now.

Ok, I've added the logic to "git-rev-list --object" to handle arbitrary object dependencies.

So you can do things like this, if you want to:

	git-rev-list --object HEAD ^v2.6.11-tree

which basically generates the complete list of every object reachable from HEAD, but not reachable from the v2.6.11 tree. It also understands about tags, so if you do

	git-rev-list --object v2.6.12 ^v2.6.11-tree

the end result will have the "v2.6.12" tag in it (along with all the objects reachable from it, but not reachable from v2.6.11-tree).

What does this mean? It means that you can do a "push" from repository "a" to repository "b" by doing

 - in "b", do

	refs_in_b=($(find .git/refs -type f | xargs cat))

 - in "a" do

	refs_in_a=($(find .git/refs -type f | xargs cat))

 - then, in "a", do

	git-rev-list "${refs_in_a[@]}" --not "${refs_in_b[@]}" |
		git-pack-objects --stdout > push.pack

   to generate the objects pack in "push.pack"

 - then, in "b", do

	git-unpack-objects < push.pack

and you now have moved over _all_ the objects that were referenced in "a", but not in "b". Including tags etc. So after that last stage, when you've unpacked the objects, the only thing left to do is to make the refs in "b" point to the new references from "a" (which basically boils down to a "cp", except it would be good to verify that the refs in "b" still have the same values as they did before we did the object push).

Daniel (or anybody else), interested? Please?

Of course, you can do this one branch at a time, too, if you want to, but the above was meant as an example of how you can actually do all the branches in one single pack-file, which is a lot more efficient (if you do it one branch at a time, you'll quite possible end up transferring objects that are reachable in other branches multiple times, while the "all in one go" thing will pack each object just once).

Now, have I actually _tested_ the above? Hell no. But all the heavy lifting should now be done for doing an efficient "git push" that pushes all branches in one go (or one at a time, it's your choice on how you end up using git-rev-list).

		Linus