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From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Git Mailing List" <git@vger.kernel.org>
Subject: Re: [PATCH 2/4] peel_ref: do not return a null sha1
Date: Thu, 04 Oct 2012 11:32:13 -0700	[thread overview]
Message-ID: <7vfw5uytoy.fsf@alter.siamese.dyndns.org> (raw)
In-Reply-To: <20121004080019.GB31325@sigill.intra.peff.net> (Jeff King's message of "Thu, 4 Oct 2012 04:00:19 -0400")

Jeff King <peff@peff.net> writes:

> The idea of the peel_ref function is to dereference tag
> objects recursively until we hit a non-tag, and return the
> sha1. Conceptually, it should return 0 if it is successful
> (and fill in the sha1), or -1 if there was nothing to peel.
>
> However, the current behavior is much more confusing. For a
> regular loose ref, the behavior is as described above. But
> there is an optimization to reuse the peeled-ref value for a
> ref that came from a packed-refs file. If we have such a
> ref, we return its peeled value, even if that peeled value
> is null (indicating that we know the ref definitely does
> _not_ peel).
>
> It might seem like such information is useful to the caller,
> who would then know not to bother loading and trying to peel
> the object. Except that they should not bother loading and
> trying to peel the object _anyway_, because that fallback is
> already handled by peel_ref. In other words, the whole point
> of calling this function is that it handles those details
> internally, and you either get a sha1, or you know that it
> is not peel-able.
>
> This patch catches the null sha1 case internally and
> converts it into a -1 return value (i.e., there is nothing
> to peel). This simplifies callers, which do not need to
> bother checking themselves.
>
> Two callers are worth noting:
>
>   - in pack-objects, a comment indicates that there is a
>     difference between non-peelable tags and unannotated
>     tags. But that is not the case (before or after this
>     patch). Whether you get a null sha1 has to do with
>     internal details of how peel_ref operated.

Yeah, this is what I was wondering while reviewing [1/4].

We traditionally said both HEAD^0 and HEAD^0^0 peel to HEAD, but
this at least at the internal API level redefines them to "these do
not peel at all and is a failure".  In other words, peel_ref(ref,
sha1) that returns 0 is a way to see if ref points at a real tag
object, and if the function returns non-zero, the caller cannot tell
if the failure is because it was a valid commit or a corrupt object.

The check !is_null_sha1(peeled) always looked fishy to me.  Good
riddance.

  reply	other threads:[~2012-10-04 22:13 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-10-03 12:36 upload-pack is slow with lots of refs Ævar Arnfjörð Bjarmason
2012-10-03 13:06 ` Nguyen Thai Ngoc Duy
2012-10-03 18:03 ` Jeff King
2012-10-03 18:53   ` Junio C Hamano
2012-10-03 18:55     ` Jeff King
2012-10-03 19:41       ` Shawn Pearce
2012-10-03 20:13         ` Jeff King
2012-10-04 21:52           ` Sascha Cunz
2012-10-05  0:20             ` Jeff King
2012-10-05  6:24         ` Johannes Sixt
2012-10-05 16:57           ` Shawn Pearce
2012-10-08 15:05             ` Johannes Sixt
2012-10-09  6:46               ` Shawn Pearce
2012-10-09 20:30                 ` Johannes Sixt
2012-10-09 20:46                   ` Johannes Sixt
2012-10-03 20:16   ` Ævar Arnfjörð Bjarmason
2012-10-03 21:20     ` Jeff King
2012-10-03 22:15       ` Ævar Arnfjörð Bjarmason
2012-10-03 23:15         ` Jeff King
2012-10-03 23:54           ` Ævar Arnfjörð Bjarmason
2012-10-04  7:56             ` [PATCH 0/4] optimizing upload-pack ref peeling Jeff King
2012-10-04  7:58               ` [PATCH 1/4] peel_ref: use faster deref_tag_noverify Jeff King
2012-10-04 18:24                 ` Junio C Hamano
2012-10-04  8:00               ` [PATCH 2/4] peel_ref: do not return a null sha1 Jeff King
2012-10-04 18:32                 ` Junio C Hamano [this message]
2012-10-04  8:02               ` [PATCH 3/4] peel_ref: check object type before loading Jeff King
2012-10-04 19:06                 ` Junio C Hamano
2012-10-04 19:41                   ` Jeff King
2012-10-04 20:41                     ` Junio C Hamano
2012-10-04 21:59                       ` Jeff King
2012-10-04  8:03               ` [PATCH 4/4] upload-pack: use peel_ref for ref advertisements Jeff King
2012-10-04  8:04               ` [PATCH 0/4] optimizing upload-pack ref peeling Jeff King
2012-10-04  9:01                 ` Ævar Arnfjörð Bjarmason
2012-10-04 12:14                   ` Nazri Ramliy
2012-10-03 22:32   ` upload-pack is slow with lots of refs Ævar Arnfjörð Bjarmason
2012-10-03 23:21     ` Jeff King
2012-10-03 23:47       ` Ævar Arnfjörð Bjarmason
2012-10-03 19:13 ` Junio C Hamano

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