From: Jeff King <peff@peff.net>
Date: Mon, 03 Aug 2020 19:40:06 GMT
Subject: Re: Renaming the "master" branch without breaking existing clones
Message-ID: <20200803194006.GA2715275@coredump.intra.peff.net>
In-Reply-To: <20200803160051.GA50799@syl.lan>

On Mon, Aug 03, 2020 at 12:00:51PM -0400, Taylor Blau wrote:

> This is more-or-less what I was proposing in the message that I linked
> above. Maybe a more solidified proposal might look something as follows:
> 
>   - We could introduce a mechanism to mark certain refs as aliases to
>     other refs. For example, a remote might publish its
>     'refs/heads/master' as an alias to 'refs/heads/main', so that any
>     reads or writes to the former get applied to the latter
>     transparently.

I think symrefs do this already. Try this:

  git init parent
  git -C parent checkout -b main
  git -C parent commit --allow-empty -m one
  git -C parent symbolic-ref refs/heads/master refs/heads/main

  git clone parent clone
  cd clone
  GIT_TRACE_PACKET=1 git ls-remote 2>&1 >/dev/null |
  perl -lne '/git</ && /packet: (.*)/ and print $1'

Assuming you have the v2 protocol enabled, you should see:

  git< 3ba9220cd714e9350cb4becd1cb56d0cacf29d9b HEAD symref-target:refs/heads/main
  git< 3ba9220cd714e9350cb4becd1cb56d0cacf29d9b refs/heads/main
  git< 3ba9220cd714e9350cb4becd1cb56d0cacf29d9b refs/heads/master symref-target:refs/heads/main

(if you don't you'll still see them as aliases, but you won't be told
that master is a symref).

And it even works for pushing. Doing:

  git checkout -b master origin/master
  git commit --allow-empty -m two
  git push

updates both "main" and "master".

The real trick is that you can't create or update symbolic refs on the
server side using a client. So this would have to be something that
hosting providers allow (and there might be some security implications;
I'm not sure what happens if you create a loop in the symref
resolution).

>   - A ref alias can be annotated to say "I am a transition ref alias",
>     i.e., that clients should be taught to rename their copy of 'master'
>     to 'main' (and update remote-tracking refs accordingly).

It's not specifically marked as a transition, but a client could act on
the symref advertisement above.

-Peff