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[209.51.188.17]) by mx.google.com with ESMTPS id t4si9139026ejs.387.2019.10.08.09.06.47 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 08 Oct 2019 09:06:47 -0700 (PDT) Received-SPF: pass (google.com: domain of qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; Authentication-Results: mx.google.com; spf=pass (google.com: domain of qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom="qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org"; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from localhost ([::1]:58098 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iHs0b-0002DD-It for alex.bennee@linaro.org; Tue, 08 Oct 2019 12:06:45 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37378) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iHrxg-000817-1N for qemu-arm@nongnu.org; Tue, 08 Oct 2019 12:03:45 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iHrxd-0008B4-G6 for qemu-arm@nongnu.org; Tue, 08 Oct 2019 12:03:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49902) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iHrxd-0008AF-7M; Tue, 08 Oct 2019 12:03:41 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 250E510C094A; Tue, 8 Oct 2019 16:03:39 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.36.118.123]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DB4D45C1D4; Tue, 8 Oct 2019 16:03:15 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 49B981138648; Tue, 8 Oct 2019 18:03:13 +0200 (CEST) From: Markus Armbruster To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH v4 04/31] error: auto propagated local_err References: <20191001155319.8066-1-vsementsov@virtuozzo.com> <20191001155319.8066-5-vsementsov@virtuozzo.com> Date: Tue, 08 Oct 2019 18:03:13 +0200 In-Reply-To: <20191001155319.8066-5-vsementsov@virtuozzo.com> (Vladimir Sementsov-Ogievskiy's message of "Tue, 1 Oct 2019 18:52:52 +0300") Message-ID: <87k19fgr7i.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.66]); Tue, 08 Oct 2019 16:03:40 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-arm@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: fam@euphon.net, pburton@wavecomp.com, peter.maydell@linaro.org, codyprime@gmail.com, jasowang@redhat.com, mark.cave-ayland@ilande.co.uk, qemu-devel@nongnu.org, mdroth@linux.vnet.ibm.com, kraxel@redhat.com, mreitz@redhat.com, qemu-block@nongnu.org, quintela@redhat.com, arikalo@wavecomp.com, mst@redhat.com, armbru@redhat.com, pasic@linux.ibm.com, borntraeger@de.ibm.com, joel@jms.id.au, marcandre.lureau@redhat.com, david@gibson.dropbear.id.au, farman@linux.ibm.com, ehabkost@redhat.com, sw@weilnetz.de, groug@kaod.org, yuval.shaia@oracle.com, dgilbert@redhat.com, alex.williamson@redhat.com, qemu-arm@nongnu.org, clg@kaod.org, stefanha@redhat.com, david@redhat.com, jsnow@redhat.com, rth@twiddle.net, kwolf@redhat.com, integration@gluster.org, berrange@redhat.com, andrew@aj.id.au, cohuck@redhat.com, qemu-s390x@nongnu.org, sundeep.lkml@gmail.com, qemu-ppc@nongnu.org, pbonzini@redhat.com Errors-To: qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org Sender: "Qemu-arm" X-TUID: 4au7Je7HEslT Vladimir Sementsov-Ogievskiy writes: > 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. > (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) > [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). Starting with stating your goals is an excellent idea. But I'd love to next read a high-level description of how your patch achieves or enables achieving these goals. > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- [...] > diff --git a/include/qapi/error.h b/include/qapi/error.h > index 9376f59c35..02f967ac1d 100644 > --- a/include/qapi/error.h > +++ b/include/qapi/error.h > @@ -322,6 +322,43 @@ void error_set_internal(Error **errp, > ErrorClass err_class, const char *fmt, ...) > GCC_FMT_ATTR(6, 7); > > +typedef struct ErrorPropagator { > + Error *local_err; > + Error **errp; > +} ErrorPropagator; > + > +static inline void error_propagator_cleanup(ErrorPropagator *prop) > +{ > + error_propagate(prop->errp, prop->local_err); > +} > + > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(ErrorPropagator, error_propagator_cleanup); > + > +/* > + * ERRP_AUTO_PROPAGATE > + * > + * This macro is created to be the first line of a function with Error **errp > + * OUT parameter. It's needed only in cases where we want to use error_prepend, > + * error_append_hint or dereference *errp. It's still safe (but useless) in > + * other cases. > + * > + * If errp is NULL or points to error_fatal, it is rewritten to point to a > + * local Error object, which will be automatically propagated to the original > + * errp on function exit (see error_propagator_cleanup). > + * > + * After invocation of this macro it is always safe to dereference errp > + * (as it's not NULL anymore) and to append hints (by error_append_hint) > + * (as, if it was error_fatal, we swapped it with a local_error to be > + * propagated on cleanup). Well, appending hints was always safe, it just didn't work with &error_fatal. Don't worry about that now, I'll probably want to polish this contract comment a bit anyway, but later. > + * > + * Note: we don't wrap the error_abort case, as we want resulting coredump > + * to point to the place where the error happened, not to error_propagate. > + */ > +#define ERRP_AUTO_PROPAGATE() \ > +g_auto(ErrorPropagator) __auto_errp_prop = {.errp = errp}; \ Took me a second to realize: the macro works, because the initializer implicitly initializes .local_error = NULL. __auto_errp_prop is an identifier reserved for any use. I think we could use _auto_errp_prop, which is only reserved for use as identifiers with file scope in both the ordinary and tag name spaces. See ISO/IEC 9899:1999 7.1.3 Reserved identifiers. > +errp = ((errp == NULL || *errp == error_fatal) ? \ > + &__auto_errp_prop.local_err : errp) > + Please indent multi-line macros like elsewhere in this file: #define ERRP_AUTO_PROPAGATE() \ g_auto(ErrorPropagator) __auto_errp_prop = {.errp = errp}; \ errp = ((errp == NULL || *errp == error_fatal) \ ? &__auto_errp_prop.local_err : errp) > /* > * Special error destination to abort on error. > * See error_setg() and error_propagate() for details. To be honest, the cover letter left me a bit skeptical, but now I think you might be up to something. Let's see how the patches putting the macro to use come out.