From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A1321C3A5A6 for ; Thu, 19 Sep 2019 13:58:28 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 772DE20665 for ; Thu, 19 Sep 2019 13:58:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 772DE20665 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:44524 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iAwx1-00076s-BL for qemu-devel@archiver.kernel.org; Thu, 19 Sep 2019 09:58:27 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:49737) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iAwgg-0004W6-Df for qemu-devel@nongnu.org; Thu, 19 Sep 2019 09:41:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iAwgf-0003nz-2D for qemu-devel@nongnu.org; Thu, 19 Sep 2019 09:41:34 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37772) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iAwgS-0003ZD-UB; Thu, 19 Sep 2019 09:41:22 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 413C7109EFC3; Thu, 19 Sep 2019 13:41:18 +0000 (UTC) Received: from [10.3.116.249] (ovpn-116-249.phx2.redhat.com [10.3.116.249]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 41CFD1001281; Thu, 19 Sep 2019 13:40:59 +0000 (UTC) To: Kevin Wolf References: <20190918130244.24257-1-vsementsov@virtuozzo.com> <20190919091720.GB10163@localhost.localdomain> From: Eric Blake Openpgp: preference=signencrypt Autocrypt: addr=eblake@redhat.com; keydata= xsBNBEvHyWwBCACw7DwsQIh0kAbUXyqhfiKAKOTVu6OiMGffw2w90Ggrp4bdVKmCaEXlrVLU xphBM8mb+wsFkU+pq9YR621WXo9REYVIl0FxKeQo9dyQBZ/XvmUMka4NOmHtFg74nvkpJFCD TUNzmqfcjdKhfFV0d7P/ixKQeZr2WP1xMcjmAQY5YvQ2lUoHP43m8TtpB1LkjyYBCodd+LkV GmCx2Bop1LSblbvbrOm2bKpZdBPjncRNob73eTpIXEutvEaHH72LzpzksfcKM+M18cyRH+nP sAd98xIbVjm3Jm4k4d5oQyE2HwOur+trk2EcxTgdp17QapuWPwMfhaNq3runaX7x34zhABEB AAHNHkVyaWMgQmxha2UgPGVibGFrZUByZWRoYXQuY29tPsLAegQTAQgAJAIbAwULCQgHAwUV CgkICwUWAgMBAAIeAQIXgAUCS8fL9QIZAQAKCRCnoWtKJSdDahBHCACbl/5FGkUqJ89GAjeX RjpAeJtdKhujir0iS4CMSIng7fCiGZ0fNJCpL5RpViSo03Q7l37ss+No+dJI8KtAp6ID+PMz wTJe5Egtv/KGUKSDvOLYJ9WIIbftEObekP+GBpWP2+KbpADsc7EsNd70sYxExD3liwVJYqLc Rw7so1PEIFp+Ni9A1DrBR5NaJBnno2PHzHPTS9nmZVYm/4I32qkLXOcdX0XElO8VPDoVobG6 gELf4v/vIImdmxLh/w5WctUpBhWWIfQDvSOW2VZDOihm7pzhQodr3QP/GDLfpK6wI7exeu3P pfPtqwa06s1pae3ad13mZGzkBdNKs1HEm8x6zsBNBEvHyWwBCADGkMFzFjmmyqAEn5D+Mt4P zPdO8NatsDw8Qit3Rmzu+kUygxyYbz52ZO40WUu7EgQ5kDTOeRPnTOd7awWDQcl1gGBXgrkR pAlQ0l0ReO57Q0eglFydLMi5bkwYhfY+TwDPMh3aOP5qBXkm4qIYSsxb8A+i00P72AqFb9Q7 3weG/flxSPApLYQE5qWGSXjOkXJv42NGS6o6gd4RmD6Ap5e8ACo1lSMPfTpGzXlt4aRkBfvb NCfNsQikLZzFYDLbQgKBA33BDeV6vNJ9Cj0SgEGOkYyed4I6AbU0kIy1hHAm1r6+sAnEdIKj cHi3xWH/UPrZW5flM8Kqo14OTDkI9EtlABEBAAHCwF8EGAEIAAkFAkvHyWwCGwwACgkQp6Fr SiUnQ2q03wgAmRFGDeXzc58NX0NrDijUu0zx3Lns/qZ9VrkSWbNZBFjpWKaeL1fdVeE4TDGm I5mRRIsStjQzc2R9b+2VBUhlAqY1nAiBDv0Qnt+9cLiuEICeUwlyl42YdwpmY0ELcy5+u6wz mK/jxrYOpzXKDwLq5k4X+hmGuSNWWAN3gHiJqmJZPkhFPUIozZUCeEc76pS/IUN72NfprZmF Dp6/QDjDFtfS39bHSWXKVZUbqaMPqlj/z6Ugk027/3GUjHHr8WkeL1ezWepYDY7WSoXwfoAL 2UXYsMAr/uUncSKlfjvArhsej0S4zbqim2ZY6S8aRWw94J3bSvJR+Nwbs34GPTD4Pg== Organization: Red Hat, Inc. Message-ID: <57483252-273c-4606-47a8-eddeb840109a@redhat.com> Date: Thu, 19 Sep 2019 08:40:58 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190919091720.GB10163@localhost.localdomain> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.65]); Thu, 19 Sep 2019 13:41:18 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [RFC] error: auto propagated local_err 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: fam@euphon.net, peter.maydell@linaro.org, mst@redhat.com, codyprime@gmail.com, mark.cave-ayland@ilande.co.uk, qemu-devel@nongnu.org, armbru@redhat.com, kraxel@redhat.com, mreitz@redhat.com, qemu-block@nongnu.org, quintela@redhat.com, david@redhat.com, mdroth@linux.vnet.ibm.com, pasic@linux.ibm.com, borntraeger@de.ibm.com, marcandre.lureau@redhat.com, rth@twiddle.net, farman@linux.ibm.com, groug@kaod.org, dgilbert@redhat.com, alex.williamson@redhat.com, qemu-arm@nongnu.org, stefanha@redhat.com, jsnow@redhat.com, david@gibson.dropbear.id.au, Vladimir Sementsov-Ogievskiy , berrange@redhat.com, cohuck@redhat.com, qemu-s390x@nongnu.org, sundeep.lkml@gmail.com, qemu-ppc@nongnu.org, pbonzini@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 9/19/19 4:17 AM, Kevin Wolf wrote: > Am 18.09.2019 um 19:10 hat Eric Blake geschrieben: >> On 9/18/19 8:02 AM, Vladimir Sementsov-Ogievskiy wrote: >>> + */ >>> +#define MAKE_ERRP_SAFE(errp) \ >>> +g_auto(ErrorPropagationStruct) (__auto_errp_prop) = {.errp = (errp)}; \ >>> +if ((errp) == NULL || *(errp) == error_abort || *(errp) == error_fatal) { \ >>> + (errp) = &__auto_errp_prop.local_err; \ >>> +} >> >> Not written to take a trailing semicolon in the caller. >> >> You could even set __auto_errp_prop unconditionally rather than trying >> to reuse incoming errp (the difference being that error_propagate() gets >> called more frequently). > > I think this difference is actually a problem. > > When debugging things, I hate error_propagate(). It means that the Error > (specifically its fields src/func/line) points to the outermost > error_propagate() rather than the place where the error really happened. > It also makes error_abort completely useless because at the point where > the process gets aborted, the interesting information is already lost. Okay, based on that, I see the following desirable semantics: Caller: one of 4 calling paradigms: pass errp=NULL (we don't care about failures) pass errp=&error_abort (we want to abort() as soon as possible as close to the real problem as possible) pass errp=&error_fatal (we want to exit(), but only after collecting as much information as possible) pass errp = anything else (we are collecting an error for other reasons, we may report it or let the caller decide or ...) Callee: we want a SINGLE paradigm: func (Error **errp) { MAKE_ERRP_SAFE(); now we can pass errp to any child function, test '*errp', or do anything else, and we DON'T have to call error_propagate. I think that means we need: #define MAKE_ERRP_SAFE() \ g_auto(...) __auto_errp = { .errp = errp }; \ do { \ if (!errp || errp == &error_fatal) { errp = &__auto_errp.local; } \ } while (0) So back to the caller semantics: if the caller passed NULL, we've redirected errp locally so that we can use *errp at will; the auto-cleanup will free our local error. if the caller passed &error_abort, we keep errp unchanged. *errp tests will never trigger, because we'll have already aborted in the child on the original errp, giving developers the best stack trace. if the caller passed &error_fatal, we redirect errp. auto-cleanup will then error_propagate that back to the caller, producing as much nice information as possible. if the caller passed anything else, we keep errp unchanged, so no extra error_propagate in the mix. > > So I'd really like to restrict the use of error_propagate() to places > where it's absolutely necessary. Unless, of course, you can fix these > practical problems that error_propagate() causes for debugging. > > In fact, in the context of Greg's series, I think we really only need to > support hints for error_fatal, which are cases that users are supposed > to see. We should exclude error_abort in MAKE_ERRP_SAFE() because these > are things that are never supposed to happen. A good stack trace is more > important there than adding a hint to the message. We also want to handle the caller passing NULL, so that we no longer have to introduce 'Error *local_error = NULL' everywhere. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org