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Thu, 5 Dec 2019 12:36:42 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id BB0531138606; Thu, 5 Dec 2019 13:36:40 +0100 (CET) From: Markus Armbruster To: Vladimir Sementsov-Ogievskiy Subject: Re: [RFC v5 024/126] error: auto propagated local_err References: <20191011160552.22907-1-vsementsov@virtuozzo.com> <20191011160552.22907-25-vsementsov@virtuozzo.com> <87muc8p24w.fsf@dusky.pond.sub.org> <55393c08-5bda-8042-1a95-f350b3781d99@virtuozzo.com> Date: Thu, 05 Dec 2019 13:36:40 +0100 In-Reply-To: <55393c08-5bda-8042-1a95-f350b3781d99@virtuozzo.com> (Vladimir Sementsov-Ogievskiy's message of "Thu, 5 Dec 2019 09:38:49 +0000") Message-ID: <87d0d3c5k7.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-MC-Unique: In8rOFh3PGWTvk4no7Xklw-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Ronnie Sahlberg , Jeff Cody , Jan Kiszka , Alberto Garcia , Hailiang Zhang , "qemu-block@nongnu.org" , Aleksandar Rikalo , Halil Pasic , =?utf-8?Q?Herv=C3=A9?= Poussineau , Anthony Perard , Samuel Thibault , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Anthony Green , Laurent Vivier , Eduardo Habkost , Xie Changlong , Peter Lieven , "Dr. David Alan Gilbert" , Beniamino Galvani , Eric Auger , Alex Williamson , Stefan Hajnoczi , John Snow , Richard Henderson , Kevin Wolf , Andrew Jeffery , Chris Wulff , Subbaraya Sundeep , Michael Walle , "qemu-ppc@nongnu.org" , Bastian Koppelmann , Igor Mammedov , Fam Zheng , Peter Maydell , "sheepdog@lists.wpkg.org" , Matthew Rosato , David Hildenbrand , Palmer Dabbelt , Eric Farman , Max Filippov , Hannes Reinecke , Stefano Stabellini , "Gonglei \(Arei\)" , Liu Yuan , Artyom Tarasenko , Thomas Huth , Amit Shah , Stefan Weil , Greg Kurz , Yuval Shaia , "qemu-s390x@nongnu.org" , "qemu-arm@nongnu.org" , Peter Chubb , =?utf-8?Q?C=C3=A9dric?= Le Goater , Stafford Horne , "qemu-riscv@nongnu.org" , Cornelia Huck , Aleksandar Markovic , Aurelien Jarno , Paul Burton , Sagar Karandikar , Paul Durrant , Jason Wang , Gerd Hoffmann , "Edgar E. Iglesias" , Guan Xuetao , Ari Sundholm , Juan Quintela , Michael Roth , Christian Borntraeger , Joel Stanley , Jason Dillaman , Antony Pavlov , "xen-devel@lists.xenproject.org" , "integration@gluster.org" , Laszlo Ersek , "Richard W.M. Jones" , Andrew Baumann , Max Reitz , Denis Lunev , "Michael S. Tsirkin" , Mark Cave-Ayland , "qemu-devel@nongnu.org" , Vincenzo Maffione , Marek Vasut , Markus Armbruster , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Alistair Francis , Pavel Dovgalyuk , Giuseppe Lettieri , Luigi Rizzo , David Gibson , Tony Krowiak , "Daniel P. =?utf-8?Q?Berrang=C3=A9?=" , Xiao Guangrong , Pierre Morel , Wen Congyang , Jean-Christophe Dubois , Paolo Bonzini , Stefan Berger Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Archived-At: List-Archive: Vladimir Sementsov-Ogievskiy writes: > 04.12.2019 17:59, Markus Armbruster wrote: >> Vladimir Sementsov-Ogievskiy writes: >>=20 >>> Here is introduced ERRP_AUTO_PROPAGATE macro, to be used at start of >>> functions with errp OUT parameter. >>> >>> It has three goals: >>> >>> 1. Fix issue with error_fatal & error_prepend/error_append_hint: user >>> can't see this additional information, because exit() happens in >>> error_setg earlier than information is added. [Reported by Greg Kurz] >>> >>> 2. Fix issue with error_abort & error_propagate: when we wrap >>> error_abort by local_err+error_propagate, resulting coredump will >>> refer to error_propagate and not to the place where error happened. >>=20 >> I get what you mean, but I have plenty of context. >>=20 >>> (the macro itself doesn't fix the issue, but it allows to [3.] drop all >>> local_err+error_propagate pattern, which will definitely fix the issue) >>=20 >> The parenthesis is not part of the goal. >>=20 >>> [Reported by Kevin Wolf] >>> >>> 3. Drop local_err+error_propagate pattern, which is used to workaround >>> void functions with errp parameter, when caller wants to know resulting >>> status. (Note: actually these functions could be merely updated to >>> return int error code). >>> >>> To achieve these goals, we need to add invocation of the macro at start >>> of functions, which needs error_prepend/error_append_hint (1.); add >>> invocation of the macro at start of functions which do >>> local_err+error_propagate scenario the check errors, drop local errors >>> from them and just use *errp instead (2., 3.). >>=20 >> The paragraph talks about two cases: 1. and 2.+3.=20 > > Hmm, I don't think so.. 1. and 2. are issues. 3. is a refactoring.. We ju= st > fix achieve 2 and 3 by one action. > >> Makes me think we >> want two paragraphs, each illustrated with an example. >>=20 >> What about you provide the examples, and then I try to polish the prose? > > 1: error_fatal problem > > Assume the following code flow: > > int f1(errp) { > ... > ret =3D f2(errp); > if (ret < 0) { > error_append_hint(errp, "very useful hint"); > return ret; > } > ... > } > > Now, if we call f1 with &error_fatal argument and f2 fails, the program > will exit immediately inside f2, when setting the errp. User will not > see the hint. > > So, in this case we should use local_err. How does this example look after the transformation? > 2: error_abort problem > > Now, consider functions without return value. We normally use local_err > variable to catch failures: > > void f1(errp) { > Error *local_err =3D NULL; > ... > f2(&local_err); > if (local_err) { > error_propagate(errp, local_err); > return; > } > ... > } > > Now, if we call f2 with &error_abort and f2 fails, the stack in resulting > crash dump will point to error_propagate, not to the failure point in f2, > which complicates debugging. > > So, we should never wrap error_abort by local_err. Likewise. > > =3D=3D=3D > > Our solution: > > - Fixes [1.], adding invocation of new macro into functions with error_ap= pen_hint/error_prepend, > New macro will wrap error_fatal. > - Fixes [2.], by switching from hand-written local_err to smart macro, wh= ich never > wraps error_abort. > - Handles [3.], by switching to macro, which is less code > - Additionally, macro doesn't wrap normal non-zero errp, to avoid extra p= ropagations > (in fact, error_propagate is called, but returns immediately on first = if (!local_err))