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Fri, 3 Jul 2020 12:21:09 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id B7B1D1138648; Fri, 3 Jul 2020 14:21:07 +0200 (CEST) From: Markus Armbruster To: qemu-devel@nongnu.org Subject: Re: Questionable aspects of QEMU Error's design References: <87o8sblgto.fsf@dusky.pond.sub.org> <87blo7heag.fsf@dusky.pond.sub.org> Date: Fri, 03 Jul 2020 14:21:07 +0200 In-Reply-To: <87blo7heag.fsf@dusky.pond.sub.org> (Markus Armbruster's message of "Sat, 04 Apr 2020 09:59:35 +0200") Message-ID: <874kqooklo.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=205.139.110.61; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/03 03:17:33 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action 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 , Vladimir Sementsov-Ogievskiy , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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 With my "[PATCH v2 00/44] Less clumsy error checking" applied, I now count 1946 void 879 signed integer 484 pointer 301 bool 33 unsigned integer 3 GuestFsfreezeStatus 1 gnutls_x509_crt_t 1 QCryptoCipherAlgorithm 1 COLOMessage 1 BlockdevDetectZeroesOptions --------------------- 3650 total About 7% complete for function definitions. > 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. Different method this time: I count any direct function call that takes &err other than &error_abort, &error_fatal, and whose value, if any, is not used. Current master: 1050 With my "[PATCH v2 00/44] Less clumsy error checking" applied: 649 About 38% complete for function 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 Top scorers master: 151 sysbus_realize() 34 qemu_opts_absorb_qdict() 29 visit_type_int() 24 visit_type_str() 23 cpu_exec_realizefn() 19 visit_type_size() 16 qdev_realize() 14 visit_type_bool() 12 visit_type_uint32() 11 visit_type_int32() 11 object_property_set_bool() 10 object_property_set_uint() 10 object_property_set_int() +420 functions with fewer than 10 calls Top scorers with my patches applied: 23 cpu_exec_realizefn() 15 visit_type_int() 10 visit_type_size() +387 functions with fewer than 10 calls Looks like this is going to be a long slog. With functions into buckets by same name prefix up to the first '_': 63 visit 57 qmp 33 bdrv 29 cpu 26 xen 25 memory +113 buckets with fewer than 25 calls [...] > > Please understand these are rough numbers from quick & dirty scripts. Still are.