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

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jul 9, 2016, 07:35 UTC
Message-ID: <alpine.DEB.2.20.1607090928500.6426@virtualbox>
In-Reply-To: <xmqqa8hsm4qu.fsf@gitster.mtv.corp.google.com>

Hi Junio,

On Fri, 8 Jul 2016, Junio C Hamano wrote:

Show 15 quoted lines

> 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.

BTW this issue is getting much more problematic when you have a lot of worktrees, some of which operate on detached HEADs. Which I do.

Ciao, Dscho