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

[Junio C Hamano <gitster@pobox.com>]

On Fri, Jul 8, 2016 at 12:25 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> On Fri, Jul 08, 2016 at 10:14:33AM -0700, Junio C Hamano wrote:
>>
>> 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.
>
> What about anything that is all-caps and ends in _HEAD which has a
> mod-time within the last N days?  (Where N is 2-7 days.)  If it's
> older than that, it's almost certainly stale...

I can imagine I'd start hacking on a project that I rarely touch, give up
resolving a "git pull" from an unconfigured place (hence, some stuff is
only reachable from FETCH_HEAD), and after 2*N days, come back
to the repository and find that I cannot continue working on it.

Turning the rule to "*_HEAD we know about, and those we don't that
are young" would not change the situation, as I may be depending on
some third-party tool that uses its OWN_HEAD just like we use
FETCH_HEAD in the above example.

So I dunno if that is a good solution. If we are going to declare that
transient stuff will now be kept, i.e. keeping them alive is no longer
end user's responsibility, then probably we should make it end user's
responsibility to clean things up.