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Tue, 28 Apr 2020 05:20:04 +0000 Subject: Re: Questionable aspects of QEMU Error's design To: Markus Armbruster , qemu-devel@nongnu.org References: <87o8sblgto.fsf@dusky.pond.sub.org> <87blo7heag.fsf@dusky.pond.sub.org> <87blndey9c.fsf@dusky.pond.sub.org> From: Vladimir Sementsov-Ogievskiy X-Tagtoolbar-Keys: D20200428082002459 Message-ID: <3b9d53fd-bdd0-c15b-292c-c4952fa24dfd@virtuozzo.com> Date: Tue, 28 Apr 2020 08:20:02 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 In-Reply-To: <87blndey9c.fsf@dusky.pond.sub.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: AM4PR0202CA0003.eurprd02.prod.outlook.com (2603:10a6:200:89::13) To AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.100.2] (185.215.60.155) by AM4PR0202CA0003.eurprd02.prod.outlook.com (2603:10a6:200:89::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2937.13 via Frontend Transport; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR04-HE1-obe.outbound.protection.outlook.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/28 01:20:05 X-ACL-Warn: Detected OS = Windows NT kernel [generic] [fuzzy] X-Received-From: 40.107.7.138 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: Peter Maydell , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 27.04.2020 18:36, Markus Armbruster wrote: > Markus Armbruster writes: > >> Markus Armbruster writes: >> >>> QEMU's Error was patterned after GLib's GError. Differences include: >> [...] >>> * Return value conventions >>> >>> Common: non-void functions return a distinct error value on failure >>> when such a value can be defined. Patterns: >>> >>> - Functions returning non-null pointers on success return null pointer >>> on failure. >>> >>> - Functions returning non-negative integers on success return a >>> negative error code on failure. >>> >>> Different: GLib discourages void functions, because these lead to >>> awkward error checking code. We have tons of them, and tons of >>> awkward error checking code: >>> >>> Error *err = NULL; >>> frobnicate(arg, &err); >>> if (err) { >>> ... recover ... >>> error_propagate(errp, err); >>> } >>> >>> instead of >>> >>> if (!frobnicate(arg, errp)) >>> ... recover ... >>> } >>> >>> Can also lead to pointless creation of Error objects. >>> >>> I consider this a design mistake. Can we still fix it? We have more >>> than 2000 void functions taking an Error ** parameter... >>> >>> Transforming code that receives and checks for errors with Coccinelle >>> shouldn't be hard. Transforming code that returns errors seems more >>> difficult. We need to transform explicit and implicit return to >>> either return true or return false, depending on what we did to the >>> @errp parameter on the way to the return. Hmm. >> [...] >> >> To figure out what functions with an Error ** parameter return, I used >> Coccinelle to find such function definitions and print the return types. >> Summary of results: >> >> 2155 void >> 873 signed integer >> 494 pointer >> 153 bool >> 33 unsigned integer >> 6 enum >> --------------------- >> 3714 total >> >> I then used Coccinelle to find checked calls of void functions (passing >> &error_fatal or &error_abort is not considered "checking" here). These >> calls become simpler if we make the functions return a useful value. I >> found a bit under 600 direct calls, and some 50 indirect calls. >> >> Most frequent direct calls: >> >> 127 object_property_set_bool >> 27 qemu_opts_absorb_qdict >> 16 visit_type_str >> 14 visit_type_int >> 10 visit_type_uint32 >> >> Let's have a closer look at object_property_set() & friends. Out of >> almost 1000 calls, some 150 are checked. While I'm sure many of the >> unchecked calls can't actually fail, I am concerned some unchecked calls >> can. >> >> If we adopt the convention to return a value that indicates success / >> failure, we should consider converting object.h to it sooner rather than >> later. >> >> Please understand these are rough numbers from quick & dirty scripts. > > FYI, I'm working on converting QemuOpts, QAPI visitors and QOM. I keep > running into bugs. So far: > > [PATCH v2 for-5.1 0/9] qemu-option: Fix corner cases and clean up > [PATCH for-5.1 0/5] qobject: Minor spring cleaning > [PATCH v2 00/14] Miscellaneous error handling fixes > [PATCH 0/4] Subject: [PATCH 0/4] smbus: SPD fixes > [PATCH 0/3] fuzz: Probably there is a better way to do this > [PATCH v2 00/15] qapi: Spring cleaning > [PATCH 00/11] More miscellaneous error handling fixes > > I got another one coming for QOM and qdev before I can post the > conversion. > > Vladimir, since the conversion will mess with error_propagate(), I'd > like to get it in before your auto-propagation work. > OK, just let me know when to regenerate the series, it's not hard. -- Best regards, Vladimir