Re: gc and repack ignore .git/*HEAD when checking reachability

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Duy Nguyen <pclouds@gmail.com>
Cc: Jeff King <peff@peff.net>, Junio C Hamano <gitster@pobox.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Git Mailing List <git@vger.kernel.org>
Subject: Re: gc and repack ignore .git/*HEAD when checking reachability
Date: Wed, 13 Jul 2016 10:20:43 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.20.1607131004410.6426@virtualbox> (raw)
In-Reply-To: <CACsJy8BfXSvKM3=rRCDYzR=rpLRi+FBYs4r1WGXFisq0Esk00w@mail.gmail.com>

Hi Duy,

On Tue, 12 Jul 2016, Duy Nguyen wrote:

> On Tue, Jul 12, 2016 at 5:51 PM, Jeff King <peff@peff.net> wrote:
> > On Tue, Jul 12, 2016 at 05:46:12PM +0200, Duy Nguyen wrote:
> >
> >> I'm not opposed to letting one worktree see everything, but this move
> >> makes it harder to write new scripts (or new builtin commands, even)
> >> that works with both single and multiple worktrees because you refer
> >> to one ref (in current worktree perspective) differently. If we kill
> >> of the main worktree (i.e. git init always creates a linked worktree)
> >> then it's less of a problem, but still a nuisance to write
> >> refs/worktree/$CURRENT/<something> everywhere.
> >
> > True. I gave a suggestion for the reading side, but the writing side
> > would still remain tedious.
> >
> > I wonder if, in a worktree, we could simply convert requests to read or
> > write names that do not begin with "refs/" as "refs/worktree/$CURRENT/"?
> > That makes it a read/write-time alias conversion, but the actual storage
> > is just vanilla (so the ref storage doesn't need to care, and
> > reachability just works).
> 
> A conversion like that is already happening, but it works at
> git_path() level instead and maps anything outside refs/ to
> worktrees/$CURRENT.

Wouldn't you agree that the entire discussion goes into a direction that
reveals that it might simply be a better idea to require commands that want
to have per-worktree refs to do that explicitly?

I mean, it looks to me that the harder we try to avoid that, the more
problems crop up, some of that as serious as my reported data loss.

I do not see any indication that trying even harder to "protect" commands
from knowing that they are running in one of many worktrees is making
things easier. To the contrary, I expect that direction to hold many more
awful surprises for us.

The same holds true for the config, BTW. I really have no love for the
idea to make the config per-worktree. It just holds too many nasty
opportunities for violate the Law of Least Surprises.

Just to name one: imagine you check out a different branch in worktree A,
then switch worktree B to the branch that A had, and all of a sudden you
may end up with a different upstream!

Ciao,
Dscho