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

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Duy Nguyen <pclouds@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
	Junio C Hamano <gitster@pobox.com>,
	Git Mailing List <git@vger.kernel.org>
Subject: Re: gc and repack ignore .git/*HEAD when checking reachability
Date: Sun, 10 Jul 2016 12:59:20 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.20.1607101255300.6426@virtualbox> (raw)
In-Reply-To: <CACsJy8A6fiPUtNZow_oOEQSi64GMxA2Jy84h4OznaSxBMePtbQ@mail.gmail.com>

Hi Duy,

On Sat, 9 Jul 2016, Duy Nguyen wrote:

> On Sat, Jul 9, 2016 at 4:09 PM, Josh Triplett <josh@joshtriplett.org> wrote:
> > On Sat, Jul 09, 2016 at 09:35:24AM +0200, Johannes Schindelin wrote:
> >> On Fri, 8 Jul 2016, Junio C Hamano wrote:
> >> > Josh Triplett <josh@joshtriplett.org> writes:
> >> >
> >> > > That sounds reasonable.  And if they *do* end up taking any time to
> >> > > traverse, it's because they weren't reachable from other anchoring
> >> > > points, so taking the extra time to traverse them seems fine.
> >> >
> >> > The only thing that is hard is to clearly define _what_ are the new
> >> > anchoring points.
> >> >
> >> > It cannot be "anything directly under .git that has all-caps name
> >> > that ends with _HEAD".  The ones we write we know are going to be
> >> > removed at some point in time (e.g. "git reset", "git bisect reset",
> >> > "git merge --abort", etc.).  We do not have any control on random
> >> > ones that the users and third-party tools leave behind, holding onto
> >> > irrelevant objects forever.
> >>
> >> Please note that bisect already uses the (transient) refs/bisect/
> >> namespace. So I do not think we need to take specific care of the
> >> BISECT_* files.
> >>
> >> If we had thought of it back then, we could have used such a transient
> >> namespace also for FETCH_HEAD, CHERRY_PICK_HEAD and also for detached
> >> HEADs (which we should have called "unnamed branches").
> >>
> >> Now, how about special-casing *just* these legacy files in gc: HEAD,
> >> FETCH_HEAD, MERGE_HEAD and CHERRY_PICK_HEAD? Any new transient refs should
> >> live in the refs/ namespace, which is already handled.
> >
> > That seems workable as well; in that case, we should also document this
> > (in the git-gc manpage at a minimum), and explicitly suggest creating
> > refs in refs/ but outside of refs/heads/ and refs/tags/, rather than
> > directly in .git/.
> 
> Not just outside refs/heads and refs/tags. It has to be in a specified
> namespace like refs/worktree/ or something (we are close to be ready
> for that). We could update the man page about git-gc shortcomings now,
> but I think we should wait until refs/worktree (or something like
> that) becomes true before suggesting more.

We have a precedent for a ref that is directly underneath refs/:
refs/stash.

IMO that is okay: depending on the use case, we would need multiple refs
(like refs/notes/*) or a single ref (like refs/stash).

The important part is that the new refs start with refs/, and if they are
to be transient, start neither with refs/heads/ nor with refs/tags/.

Ciao,
Dscho