From: Jeff King <peff@peff.net>
Date: Fri, 25 Jul 2025 11:02:43 GMT
Subject: Re: `git remote rename` does not work when `refs/remotes/server/HEAD` is unborn (when right after `git remote add -m`)
Message-ID: <20250725110243.GA3014187@coredump.intra.peff.net>
In-Reply-To: <aIIvHxR8wXLTCgMW@pks.im>

On Thu, Jul 24, 2025 at 03:03:27PM +0200, Patrick Steinhardt wrote:

> I've quickly hacked something together now, see the work-in-progress
> patch below. The patch does not yet handle reflogs, but that isn't too
> hard to implement.
> 
> And these changes indeed speed up things by quite a lot: instead of
> hours it now takes 7 seconds :) I'll polish this patch series and will
> likely send it in tomorrow.

Cool. I agree with all of the pain points you outlined, and the general
direction. There was one other sub-optimal thing I noticed, which was...

> -	refs_for_each_ref(get_main_ref_store(the_repository),
> -			  read_remote_branches, &rename);
> [...]
> +	result = refs_for_each_rawref(get_main_ref_store(the_repository),
> +				      queue_one_rename, &rename);

Both before and after your patch, we're iterating over _all_ refs and
skipping ones that aren't in "refs/remotes/<remote>/". If we just ask to
iterate over that subset of refs, then we save the effort of iterating
over the others that we don't care about.

But:

  1. We have refs_for_each_ref_in() and refs_for_each_rawref(), but no
     refs_for_each_rawref_in(). Feels like it should be easy to add it,
     though.

  2. It's an obvious small optimization, but it doesn't help us in a
     big-O way. Iterating the refs is obviously O(n), and in the worst
     case rewriting the packed-refs file is likewise O(n). So I wouldn't
     expect to see the dramatic improvements you found by removing the
     quadratic bits. But I'd bet it's still measurable in a repo with a
     lot of refs (and maybe with reftables it actually would be bigger,
     since the goal there is to amortize the rewrites).

-Peff