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

From: Duy Nguyen <pclouds@gmail.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	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: Sat, 9 Jul 2016 18:45:38 +0200	[thread overview]
Message-ID: <CACsJy8A6fiPUtNZow_oOEQSi64GMxA2Jy84h4OznaSxBMePtbQ@mail.gmail.com> (raw)
In-Reply-To: <20160709140931.GA3185@x>

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.
-- 
Duy