Received: (at 10836) by debbugs.gnu.org; 12 Apr 2012 20:23:46 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Thu Apr 12 16:23:46 2012 Received: from localhost ([127.0.0.1]:54921 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1SIQYU-0004YN-69 for submit <at> debbugs.gnu.org; Thu, 12 Apr 2012 16:23:46 -0400 Received: from xanadu.aquilenet.fr ([88.191.123.111]:39617) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <ludo@HIDDEN>) id 1SIQYS-0004YE-2W for 10836 <at> debbugs.gnu.org; Thu, 12 Apr 2012 16:23:44 -0400 Received: from localhost (xanadu.aquilenet.fr [127.0.0.1]) by xanadu.aquilenet.fr (Postfix) with ESMTP id 53A0B1A64; Thu, 12 Apr 2012 22:22:28 +0200 (CEST) Received: from xanadu.aquilenet.fr ([127.0.0.1]) by localhost (xanadu.aquilenet.fr [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ahXHRoiUwPbn; Thu, 12 Apr 2012 22:22:28 +0200 (CEST) Received: from pluto (reverse-83.fdn.fr [80.67.176.83]) by xanadu.aquilenet.fr (Postfix) with ESMTPSA id B7FC91207; Thu, 12 Apr 2012 22:22:27 +0200 (CEST) From: ludo@HIDDEN (Ludovic =?iso-8859-1?Q?Court=E8s?=) To: Andy Wingo <wingo@HIDDEN> Subject: Re: bug#10836: guardians and weak references References: <87ipj5o6lp.fsf@HIDDEN> <87haym1qpc.fsf@HIDDEN> Date: Thu, 12 Apr 2012 22:22:27 +0200 In-Reply-To: <87haym1qpc.fsf@HIDDEN> (Andy Wingo's message of "Sun, 19 Feb 2012 22:03:11 +0100") Message-ID: <87aa2g1z0c.fsf@HIDDEN> User-Agent: Gnus/5.110018 (No Gnus v0.18) Emacs/24.0.93 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Spam-Score: -0.7 (/) X-Debbugs-Envelope-To: 10836 Cc: 10836 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.13 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Sender: debbugs-submit-bounces <at> debbugs.gnu.org Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org X-Spam-Score: -0.7 (/) Hi! Andy Wingo <wingo@HIDDEN> skribis: > However, using finalizers has the downside that it introduces races with > custom hash/equality predicates. This sounds similar to <http://savannah.gnu.org/bugs/?29616>. Back then, e9bac3be613f549b932d58913307ae18c89b9ffe fixed it by calling the custom =E2=80=98assoc=E2=80=99 with the allocation lock released (this = was with disappearing links, not finalizers.) Ludo=E2=80=99.
bug-guile@HIDDEN:bug#10836; Package guile.
Full text available.Received: (at 10836) by debbugs.gnu.org; 29 Feb 2012 09:27:08 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Wed Feb 29 04:27:08 2012 Received: from localhost ([127.0.0.1]:55139 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1S2foM-00013k-N6 for submit <at> debbugs.gnu.org; Wed, 29 Feb 2012 04:27:08 -0500 Received: from a-pb-sasl-sd.pobox.com ([74.115.168.62]:53642 helo=sasl.smtp.pobox.com) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <wingo@HIDDEN>) id 1S2fnv-000139-Cf for 10836 <at> debbugs.gnu.org; Wed, 29 Feb 2012 04:26:50 -0500 Received: from sasl.smtp.pobox.com (unknown [127.0.0.1]) by a-pb-sasl-sd.pobox.com (Postfix) with ESMTP id EE78084AB; Wed, 29 Feb 2012 04:26:16 -0500 (EST) DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=pobox.com; h=from:to:cc :subject:references:date:in-reply-to:message-id:mime-version :content-type; s=sasl; bh=a3o+GittMiDZ6lxEzodVgAXMuM0=; b=MylV0Y pqg3buJZqyRvtCT/uPAh0iE9zklZbcv9XfEvBaRhON24Aix7f2oD5fCmGg8ACQ3E ahdNHVPjIP2PosOgALo0x0pnQdvUVBKyBEwpbZDtECdkyhGV1s5mQXAK4RQOSfmp NLTz2CSmRwDTNSsjgLZzVtIjzU/YarouE6FRI= DomainKey-Signature: a=rsa-sha1; c=nofws; d=pobox.com; h=from:to:cc :subject:references:date:in-reply-to:message-id:mime-version :content-type; q=dns; s=sasl; b=nmgqJdoztbXLKbyOc/c0SjgSR7gIXPc5 ZpdVEb6ioB96KhS8frYFDkVXMsmj/tYjdXDGRi0G6ZSU42mQc7LmX9zIVo5+uqY5 R/iEZdw3WHLt/p6N7nbNLOCvvmuHSVqZCjI/rjdyHWvcX/nDX7NOgp7+qan95db6 oQ0saGVG20g= Received: from a-pb-sasl-sd.pobox.com (unknown [127.0.0.1]) by a-pb-sasl-sd.pobox.com (Postfix) with ESMTP id DB5AA84A1; Wed, 29 Feb 2012 04:26:16 -0500 (EST) Received: from badger (unknown [85.50.113.138]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by a-pb-sasl-sd.pobox.com (Postfix) with ESMTPSA id 4652784A0; Wed, 29 Feb 2012 04:26:16 -0500 (EST) From: Andy Wingo <wingo@HIDDEN> To: Mark H Weaver <mhw@HIDDEN> Subject: Re: bug#10836: guardians and weak references References: <87ipj5o6lp.fsf@HIDDEN> <87haym1qpc.fsf@HIDDEN> <87399u8mcr.fsf@HIDDEN> Date: Wed, 29 Feb 2012 10:26:12 +0100 In-Reply-To: <87399u8mcr.fsf@HIDDEN> (Mark H. Weaver's message of "Tue, 28 Feb 2012 20:22:28 -0500") Message-ID: <87ehtedm8b.fsf@HIDDEN> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Pobox-Relay-ID: 6E25BA0A-62B7-11E1-8FE8-65B1DE995924-02397024!a-pb-sasl-sd.pobox.com X-Spam-Score: -1.9 (-) X-Debbugs-Envelope-To: 10836 Cc: 10836 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.13 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Sender: debbugs-submit-bounces <at> debbugs.gnu.org Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org X-Spam-Score: -1.9 (-) Hi Mark, Thanks for your thoughts :-) On Wed 29 Feb 2012 02:22, Mark H Weaver <mhw@HIDDEN> writes: > Is there a need for weak-key tables that use something other than 'eq?' > as the predicate? There are two examples in Guile. One is (ice-9 poe), and the other is the symbol table. > It's not clear to me that any other predicate makes > sense for a weak-key table. It wasn't clear to me either, but given those two examples, it makes me hesitant to remove that functionality. > On the other hand, even for 'eq?' weak-key tables, there's another way > to retrieve entries without the key: the table iterators 'hash-fold' et > al. I guess this introduces the same race for _any_ kind of hash table. Yes. I am wondering whether or not to expose this choice to the user -- they could choose to have the associations present after finalization (i.e., after a guardian has resurrected an object), or they could choose to see updates atomically (avoiding this race). There is a relevant thread on the libgc list: http://thread.gmane.org/gmane.comp.programming.garbage-collection.boehmgc/4787/focus=4793 Regards, Andy -- http://wingolog.org/
bug-guile@HIDDEN:bug#10836; Package guile.
Full text available.
Received: (at 10836) by debbugs.gnu.org; 29 Feb 2012 01:25:04 +0000
From debbugs-submit-bounces <at> debbugs.gnu.org Tue Feb 28 20:25:04 2012
Received: from localhost ([127.0.0.1]:54922 helo=debbugs.gnu.org)
by debbugs.gnu.org with esmtp (Exim 4.72)
(envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>)
id 1S2YHv-0005Hn-8E
for submit <at> debbugs.gnu.org; Tue, 28 Feb 2012 20:25:03 -0500
Received: from world.peace.net ([96.39.62.75]:60178)
by debbugs.gnu.org with esmtp (Exim 4.72)
(envelope-from <mhw@HIDDEN>) id 1S2YHZ-0005HG-NN
for 10836 <at> debbugs.gnu.org; Tue, 28 Feb 2012 20:24:51 -0500
Received: from 209-6-91-212.c3-0.smr-ubr1.sbo-smr.ma.cable.rcn.com
([209.6.91.212] helo=yeeloong)
by world.peace.net with esmtpsa (TLS1.0:DHE_RSA_AES_128_CBC_SHA1:16)
(Exim 4.72) (envelope-from <mhw@HIDDEN>)
id 1S2YHC-0006BK-Fp; Tue, 28 Feb 2012 20:24:18 -0500
From: Mark H Weaver <mhw@HIDDEN>
To: Andy Wingo <wingo@HIDDEN>
Subject: Re: bug#10836: guardians and weak references
References: <87ipj5o6lp.fsf@HIDDEN> <87haym1qpc.fsf@HIDDEN>
Date: Tue, 28 Feb 2012 20:22:28 -0500
In-Reply-To: <87haym1qpc.fsf@HIDDEN> (Andy Wingo's message of "Sun, 19 Feb
2012 22:03:11 +0100")
Message-ID: <87399u8mcr.fsf@HIDDEN>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
X-Spam-Score: -1.9 (-)
X-Debbugs-Envelope-To: 10836
Cc: 10836 <at> debbugs.gnu.org
X-BeenThere: debbugs-submit <at> debbugs.gnu.org
X-Mailman-Version: 2.1.13
Precedence: list
List-Id: <debbugs-submit.debbugs.gnu.org>
List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>,
<mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe>
List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit>
List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org>
List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help>
List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>,
<mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe>
Sender: debbugs-submit-bounces <at> debbugs.gnu.org
Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org
X-Spam-Score: -1.9 (-)
Andy Wingo <wingo@HIDDEN> writes:
> I tried reimplementing weak tables using finalizers. (The finalizers
> remove elements from the table). As long as we ensure that the
> guardian's finalizers run first, which is possible for Guile to do, that
> does fix this bug.
>
> However, using finalizers has the downside that it introduces races with
> custom hash/equality predicates. Consider:
>
> * Add an entry X -> Y to a weak-key table.
>
> * X becomes unreachable. Libgc enqueues a finalizer.
>
> * Lookup the handle for (X -> Y), but using a custom predicate that
> relies on some aspect of X that is not its identity. Keep X
> around.
Is there a need for weak-key tables that use something other than 'eq?'
as the predicate? It's not clear to me that any other predicate makes
sense for a weak-key table. To my mind, the idea is that we can safely
delete table entries if and only if we can prove that they could not be
looked up anyway. This fits in very nicely with the concept of garbage
collection.
On the other hand, even for 'eq?' weak-key tables, there's another way
to retrieve entries without the key: the table iterators 'hash-fold' et
al. I guess this introduces the same race for _any_ kind of hash table.
Mark
bug-guile@HIDDEN:bug#10836; Package guile.
Full text available.
Received: (at 10836) by debbugs.gnu.org; 19 Feb 2012 21:05:26 +0000
From debbugs-submit-bounces <at> debbugs.gnu.org Sun Feb 19 16:05:26 2012
Received: from localhost ([127.0.0.1]:46247 helo=debbugs.gnu.org)
by debbugs.gnu.org with esmtp (Exim 4.72)
(envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>)
id 1RzDwi-0007Kb-Hm
for submit <at> debbugs.gnu.org; Sun, 19 Feb 2012 16:05:26 -0500
Received: from a-pb-sasl-sd.pobox.com ([74.115.168.62]:40698
helo=sasl.smtp.pobox.com) by debbugs.gnu.org with esmtp (Exim 4.72)
(envelope-from <wingo@HIDDEN>) id 1RzDwg-0007KR-5Q
for 10836 <at> debbugs.gnu.org; Sun, 19 Feb 2012 16:05:23 -0500
Received: from sasl.smtp.pobox.com (unknown [127.0.0.1])
by a-pb-sasl-sd.pobox.com (Postfix) with ESMTP id 9EAB18E93
for <10836 <at> debbugs.gnu.org>; Sun, 19 Feb 2012 16:03:14 -0500 (EST)
DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=pobox.com; h=from:to
:subject:references:date:in-reply-to:message-id:mime-version
:content-type; s=sasl; bh=XercwvzjVWrtJfrs5If+6nWXlXU=; b=t2PG3v
524RNztQm1RUWwGdankw08wx0pXr29tg70mCDGLJrzKD2T8nZZNlLruHiKnwsjQF
JZbeTHIqZLBiodauPmBdhWiZolYgu917fytZfzqH8bKBcMJX1Ras4gpL814mjsCK
J/6G+f6j+RIh3PNCGv4uGnclxIdfe71G/OAnU=
DomainKey-Signature: a=rsa-sha1; c=nofws; d=pobox.com; h=from:to:subject
:references:date:in-reply-to:message-id:mime-version
:content-type; q=dns; s=sasl; b=k/An8VWjgrlOyUWamJqaCa6B9lM3gR8x
Cyr2PLAFNR7nTMZZ0ehanru3TXml8GU2+Mklggl3r/XpvxFodjcitIzsDat1pGZV
W5oyXY90AZTJ5L4afwR22CembVA9w4iSp4+FN7/oUuU7RCdeaJVGGMU+RNBdjVNW
/fuvFFzt1vE=
Received: from a-pb-sasl-sd.pobox.com (unknown [127.0.0.1])
by a-pb-sasl-sd.pobox.com (Postfix) with ESMTP id 967B48E92
for <10836 <at> debbugs.gnu.org>; Sun, 19 Feb 2012 16:03:14 -0500 (EST)
Received: from badger (unknown [90.164.198.39]) (using TLSv1 with cipher
DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by
a-pb-sasl-sd.pobox.com (Postfix) with ESMTPSA id CB54B8E91 for
<10836 <at> debbugs.gnu.org>; Sun, 19 Feb 2012 16:03:13 -0500 (EST)
From: Andy Wingo <wingo@HIDDEN>
To: 10836 <at> debbugs.gnu.org
Subject: Re: bug#10836: guardians and weak references
References: <87ipj5o6lp.fsf@HIDDEN>
Date: Sun, 19 Feb 2012 22:03:11 +0100
In-Reply-To: <87ipj5o6lp.fsf@HIDDEN> (Andy Wingo's message of "Fri, 17 Feb
2012 15:52:02 +0100")
Message-ID: <87haym1qpc.fsf@HIDDEN>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Pobox-Relay-ID: 2337B5CE-5B3D-11E1-B2F6-65B1DE995924-02397024!a-pb-sasl-sd.pobox.com
X-Spam-Score: -1.9 (-)
X-Debbugs-Envelope-To: 10836
X-BeenThere: debbugs-submit <at> debbugs.gnu.org
X-Mailman-Version: 2.1.13
Precedence: list
List-Id: <debbugs-submit.debbugs.gnu.org>
List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>,
<mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe>
List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit>
List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org>
List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help>
List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>,
<mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe>
Sender: debbugs-submit-bounces <at> debbugs.gnu.org
Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org
X-Spam-Score: -1.9 (-)
On Fri 17 Feb 2012 15:52, Andy Wingo <wingo@HIDDEN> writes:
> (define x (list 1))
> (define g (make-guardian))
> (define p (make-object-property ))
> (g x)
> (set! (p x) 100)
> $1 = 100
> (p x)
> $2 = 100
> (set! x #f)
> (gc) (gc) (gc)
> (g)
> $3 = (1)
> (p $3)
> $4 = #f
>
> The problem: weak references are dropped at finalization time, when
> guardians do their magic. If the guardian resuscitates the value, weak
> references (like those in the object property) are lost. This is in
> contradiction to what the manual says on the subject:
>
> Being an element in a weak vector, a key in a hash table with weak
> keys, or a value in a hash table with weak values does not prevent
> an object from being returned by a guardian. But as long as an
> object can be returned from a guardian it will not be removed from
> such a weak vector or hash table. In other words, a weak link
> does not prevent an object from being considered collectable, but
> being inside a guardian prevents a weak link from being broken.
I have looked at this issue in depth. Disappearing links are indeed
broken at finalization time (when the object is still reachable, from
the finalizer itself).
I tried reimplementing weak tables using finalizers. (The finalizers
remove elements from the table). As long as we ensure that the
guardian's finalizers run first, which is possible for Guile to do, that
does fix this bug.
However, using finalizers has the downside that it introduces races with
custom hash/equality predicates. Consider:
* Add an entry X -> Y to a weak-key table.
* X becomes unreachable. Libgc enqueues a finalizer.
* Lookup the handle for (X -> Y), but using a custom predicate that
relies on some aspect of X that is not its identity. Keep X
around.
* The finalizer runs, removing the entry from the table.
* Add an entry of a somehow equivalent X -> Y to a weak-key table.
* A future lookup of X fails to return the same handle as before.
Disappearing links avoid this race, it seems, because they disappear
synchronously with the observable unreachability of the object, not
asynchronously (via finalizers).
This synchronicity is reflected in the use of the alloc-lock -- you can
only access disappearing links from within the global allocator lock.
OTOH finalizers can take more fine-grained mutexen on the structures
they manipulate, scaling better but at the cost of introducing
noticeable asynchrony.
I actually saw this bug, in which two symbols had the same characters
but different identities, with the race condition outlined above (lookup
vs finalizer).
Not sure what to do here.
Andy
--
http://wingolog.org/
bug-guile@HIDDEN:bug#10836; Package guile.
Full text available.
Received: (at submit) by debbugs.gnu.org; 17 Feb 2012 14:54:28 +0000
From debbugs-submit-bounces <at> debbugs.gnu.org Fri Feb 17 09:54:28 2012
Received: from localhost ([127.0.0.1]:43541 helo=debbugs.gnu.org)
by debbugs.gnu.org with esmtp (Exim 4.72)
(envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>)
id 1RyPCd-0005yK-3c
for submit <at> debbugs.gnu.org; Fri, 17 Feb 2012 09:54:27 -0500
Received: from eggs.gnu.org ([140.186.70.92]:43357)
by debbugs.gnu.org with esmtp (Exim 4.72)
(envelope-from <wingo@HIDDEN>) id 1RyPCZ-0005y3-GU
for submit <at> debbugs.gnu.org; Fri, 17 Feb 2012 09:54:25 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
(envelope-from <wingo@HIDDEN>) id 1RyPAd-00053i-7w
for submit <at> debbugs.gnu.org; Fri, 17 Feb 2012 09:52:24 -0500
X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on eggs.gnu.org
X-Spam-Level:
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_NONE,
T_DKIM_INVALID,
T_RP_MATCHES_RCVD autolearn=unavailable version=3.3.2
Received: from lists.gnu.org ([140.186.70.17]:33737)
by eggs.gnu.org with esmtp (Exim 4.71)
(envelope-from <wingo@HIDDEN>) id 1RyPAc-00053e-Ta
for submit <at> debbugs.gnu.org; Fri, 17 Feb 2012 09:52:23 -0500
Received: from eggs.gnu.org ([140.186.70.92]:40642)
by lists.gnu.org with esmtp (Exim 4.71)
(envelope-from <wingo@HIDDEN>) id 1RyPAX-0005bu-9h
for bug-guile@HIDDEN; Fri, 17 Feb 2012 09:52:22 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
(envelope-from <wingo@HIDDEN>) id 1RyPAS-00052h-L0
for bug-guile@HIDDEN; Fri, 17 Feb 2012 09:52:17 -0500
Received: from a-pb-sasl-sd.pobox.com ([74.115.168.62]:46707
helo=sasl.smtp.pobox.com) by eggs.gnu.org with esmtp (Exim 4.71)
(envelope-from <wingo@HIDDEN>) id 1RyPAS-00052b-Fs
for bug-guile@HIDDEN; Fri, 17 Feb 2012 09:52:12 -0500
Received: from sasl.smtp.pobox.com (unknown [127.0.0.1])
by a-pb-sasl-sd.pobox.com (Postfix) with ESMTP id B0BDF8B95
for <bug-guile@HIDDEN>; Fri, 17 Feb 2012 09:52:11 -0500 (EST)
DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=pobox.com; h=from:to
:subject:date:message-id:mime-version:content-type; s=sasl; bh=z
/Fqy3aGaF/bgNACcxIJ8NGVvUg=; b=ti91FZj45Wvvu5CeOszz9dl55UQY97EYV
/bSbfwn7DftWMLDQcwu2Gh07IOIq9LsFc6XTXIRQK/mKPPIqlMd+dlr4lFkDYMLJ
mFOMTDRsEWUEtOvKNkeyUq8qHDYWg2W20tJvL0/VextG0OCwYZevlR6a1f2VmqNO
bOrklAYuKs=
DomainKey-Signature: a=rsa-sha1; c=nofws; d=pobox.com; h=from:to:subject
:date:message-id:mime-version:content-type; q=dns; s=sasl; b=aya
EoqS2YB5TptUeMWBPG1lpcKib+n+YnKH72y91PwNmzljOIdZkBi/GedgR7qoqddW
Map6G/m/Ej4u/JStEn6J7kx+WPuV9woukxgpHs1e36FuTWyZhe/S0Q39SJF6cser
G/zW49ujHO1wJ4btIYHLrOWIDRKIlWs6hTVo1m+s=
Received: from a-pb-sasl-sd.pobox.com (unknown [127.0.0.1])
by a-pb-sasl-sd.pobox.com (Postfix) with ESMTP id AA7458B94
for <bug-guile@HIDDEN>; Fri, 17 Feb 2012 09:52:11 -0500 (EST)
Received: from badger (unknown [90.164.198.39]) (using TLSv1 with cipher
DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by
a-pb-sasl-sd.pobox.com (Postfix) with ESMTPSA id 0A4978B93 for
<bug-guile@HIDDEN>; Fri, 17 Feb 2012 09:52:10 -0500 (EST)
From: Andy Wingo <wingo@HIDDEN>
To: bug-guile <bug-guile@HIDDEN>
Subject: guardians and weak references
Date: Fri, 17 Feb 2012 15:52:02 +0100
Message-ID: <87ipj5o6lp.fsf@HIDDEN>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Pobox-Relay-ID: F8BF4932-5976-11E1-9CCD-65B1DE995924-02397024!a-pb-sasl-sd.pobox.com
X-detected-operating-system: by eggs.gnu.org: Solaris 10 (beta)
X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.6 (newer, 3)
X-Received-From: 140.186.70.17
X-Spam-Score: -3.4 (---)
X-Debbugs-Envelope-To: submit
X-BeenThere: debbugs-submit <at> debbugs.gnu.org
X-Mailman-Version: 2.1.13
Precedence: list
List-Id: <debbugs-submit.debbugs.gnu.org>
List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>,
<mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe>
List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit>
List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org>
List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help>
List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>,
<mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe>
Sender: debbugs-submit-bounces <at> debbugs.gnu.org
Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org
X-Spam-Score: -3.4 (---)
Hi,
scheme@(guile-user)> (define x (list 1))
scheme@(guile-user)> (define g (make-guardian))
scheme@(guile-user)> (define p (make-object-property ))
scheme@(guile-user)> (g x)
scheme@(guile-user)> (set! (p x) 100)
$1 = 100
scheme@(guile-user)> (p x)
$2 = 100
scheme@(guile-user)> (set! x #f)
scheme@(guile-user)> (gc) (gc) (gc)
scheme@(guile-user)> (g)
$3 = (1)
scheme@(guile-user)> (p $3)
$4 = #f
scheme@(guile-user)>
The problem: weak references are dropped at finalization time, when
guardians do their magic. If the guardian resuscitates the value, weak
references (like those in the object property) are lost. This is in
contradiction to what the manual says on the subject:
Being an element in a weak vector, a key in a hash table with weak
keys, or a value in a hash table with weak values does not prevent
an object from being returned by a guardian. But as long as an
object can be returned from a guardian it will not be removed from
such a weak vector or hash table. In other words, a weak link
does not prevent an object from being considered collectable, but
being inside a guardian prevents a weak link from being broken.
Andy
--
http://wingolog.org/
Andy Wingo <wingo@HIDDEN>:bug-guile@HIDDEN.
Full text available.bug-guile@HIDDEN:bug#10836; Package guile.
Full text available.
GNU bug tracking system
Copyright (C) 1999 Darren O. Benham,
1997 nCipher Corporation Ltd,
1994-97 Ian Jackson.