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Thu, 17 Sep 2020 15:17:13 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Subject: Re: [PATCH 14/15] spapr: Simplify error handling in spapr_memory_plug() References: <20200914123505.612812-1-groug@kaod.org> <20200914123505.612812-15-groug@kaod.org> <11f0dcb7-7923-0164-df69-4911b3293663@virtuozzo.com> <20200915134340.0cc3f9eb@bahia.lan> <20200917011525.GI5258@yekko.fritz.box> <87lfh8g8dy.fsf@dusky.pond.sub.org> <20200917120437.6ab60ca4@bahia.lan> <87ft7g1uee.fsf@dusky.pond.sub.org> <20200917121851.GA1597829@redhat.com> Date: Thu, 17 Sep 2020 15:17:13 +0200 In-Reply-To: <20200917121851.GA1597829@redhat.com> ("Daniel P. =?utf-8?Q?B?= =?utf-8?Q?errang=C3=A9=22's?= message of "Thu, 17 Sep 2020 13:18:51 +0100") Message-ID: <87y2l835ly.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 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; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/17 02:01:40 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -50 X-Spam_score: -5.1 X-Spam_bar: ----- X-Spam_report: (-5.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.997, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no 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: qemu-devel@nongnu.org, Vladimir Sementsov-Ogievskiy , qemu-ppc@nongnu.org, Greg Kurz , David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 writes: > On Thu, Sep 17, 2020 at 02:04:41PM +0200, Markus Armbruster wrote: >> Greg Kurz writes: >>=20 >> > $ git grep object_property_get_uint -- :^{include,qom/object.c} | wc -= l >> > 60 >> > >> > Manual inspecting the output of >> > >> > $ git grep -W object_property_get_uint -- :^{include,qom/object.c} >> > ... >> > >> > seems to be showing that most users simply ignore errors (ie. pass NUL= L >> > as 3rd argument). Then some users pass &error_abort and only a few >> > pass a &err or errp. >> > >> > Assuming that users know what they're doing, I guess my proposal >> > wouldn't bring much to the code base in the end... I'm not even >> > sure now that it's worth changing the contract. >>=20 >> We'd change >>=20 >> val =3D object_property_get_uint(obj, name, &error_abort); >>=20 >> to >>=20 >> object_property_get_uint(obj, name, &val, &error_abort); >>=20 >> which is not an improvement. >>=20 >> Most of the ones passing NULL should probably pass &error_abort >> instead. Doesn't change the argument. > > I wonder if we actually need to have an Error parameter at all for > the getters. IIUC the only valid runtime error is probably the case > where the property has not been set, and even then, I think properties > usually have a default value that would be returned. All the other > errors look like programmer errors. >From the notes I took when I last hacked a trail through this jungle... Failure modes of object_property_get() @name not found @name permission, i.e. no ->get ->get() fails (how?) object_property_get_{str,bool,int,uint}() object_property_get_qobject() object_property_get() with qobject output visitor which see prop value qobject not a string / bool / int / uint object_property_get_enum() @name not found @typename doesn't match object_property_get() with string output visitor which see qapi_enum_parse() prop value not a value of enum @typename object_property_get_link() object_property_get_str() which see prop value does not resolve I think most of these failures are obviously programming errors most of the time. Many callers treat failures as programming errors by passing &error_abort. Many callers ignore errors by passing NULL. I believe most of them should really pass &error_abort instead. Fixing them is tedious, because you have to check what would happen on error. If the answer is "chaos", fix to pass &error_abort. But the answer can also be "works as intended", or "beats me". > IOW, can we remove the Error parameter and have all the o_p_get* > method impls use error_abort. If we fix the callers that should pass &error_abort to do so, we'll see what remains, and whether we can drop the parameter. > In the rare case where a caller needs to handle a property not > being set, they can use object_property_find() as a test before > invoking the getter. I dislike "if (X() is going to fail) do something else; else X()". I guess it could be okay for the narrow case of "property does not exist". > Of course requires someone motivated to audit all the case that > are not using NULL or error_abort and decide whether the attempt > at passing a local errp was actually justified or not. We got one! Thanks, Greg :)