Re: [PATCH] Do not send "want" lines for complete objects

From: Junio C Hamano <junkio@cox.net>
Date: Oct 19, 2005, 23:09 UTC
Message-ID: <7vzmp5xfwu.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <Pine.LNX.4.63.0510200004290.7689@wbgn013.biozentrum.uni-wuerzburg.de>

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

> Also, git-fetch-pack does not have to ask for descendants of remote refs 
> which are complete (for now, git-rev-list is told to ignore only the first 
> parent).

I should interpret what you said with s/descendant/ancestor/ applied, I think. I would like to make sure I understand what you are doing here.

Show 7 quoted lines

> +		if (has_sha1_file(remote)
> +				&& parse_object(remote)->flags & COMPLETE) {
> +			if (rev_command_len + 44 < sizeof(rev_command)) {
> +				snprintf(rev_command + rev_command_len, 44,
> +					" ^%s^", sha1_to_hex(remote));
> +				rev_command_len += 43;
> +			}

This rev-list command is to generate the list of "have", and we learned that the other side says she has remote -- we choose not to tell her that we have ancestors of it, but we do tell her about the remote head itself.

Let's draw a single strand of pearl case to illustrate. You have a commit chain A->B->C->D, and the other end says she has C. At this point, telling her that you have C is enough, and by not telling her about A and B, you would save her from depreting MAX_HAVE slots. Of course, if the other end has D as another head, then the above logic would give "^D^" to rev-list as well, telling it not to tell her about C, but that is what we want -- because she already knows you have C too when you tell her that you have D. I think I like this optimization.

One thing that might help, when we are telling the other end about what we have, is an output ordering option to get-rev-list that shows not in chronological order, but in the order of distance from the tip. That may give the other end a better chance to find the latest (in commit order) common commit in each branch without running out is MAX_HAS buffer.

> 	NOTE: I'd rather have git-rev-list circumvented, so that 
> 	descendants of ack'ed objects are not sent via "have" lines.

I think since we do the traversal ourselves this may start to make more sense.