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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 851F2C2BA16 for ; Sat, 4 Apr 2020 08:00:23 +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 40D93206E9 for ; Sat, 4 Apr 2020 08:00:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Aj7Ph9ja" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 40D93206E9 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]:36378 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jKdj4-0008Gj-7L for qemu-devel@archiver.kernel.org; Sat, 04 Apr 2020 04:00:22 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44470) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jKdiQ-0007ps-UB for qemu-devel@nongnu.org; Sat, 04 Apr 2020 03:59:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jKdiP-00074e-CE for qemu-devel@nongnu.org; Sat, 04 Apr 2020 03:59:42 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:45680 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jKdiP-00073y-41 for qemu-devel@nongnu.org; Sat, 04 Apr 2020 03:59:41 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585987180; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Try7PogN7De/GDH59S6kIaV4GBlN5mN0mJ3dWirpUUY=; b=Aj7Ph9jamUTuRxuV9zFRLuK4LK3vJuovr3sUCLoAqFUwlJu218P0YuWfNTaZM6i4OLubGo 8dH188OYCDbAbK06gkIL+qtVT4LJGWs5OFMM7qFUYax/FjSJgVTZZr/9Q1+3oM7cS8e+hd dlkzlUZNdnUWK06XDZwsp0n77Aad0ts= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-405-eRktZbxvNJGOa6bqgfxV6w-1; Sat, 04 Apr 2020 03:59:38 -0400 X-MC-Unique: eRktZbxvNJGOa6bqgfxV6w-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 53CD21005509; Sat, 4 Apr 2020 07:59:37 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-152.ams2.redhat.com [10.36.112.152]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C67225C1B0; Sat, 4 Apr 2020 07:59:36 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 485581138610; Sat, 4 Apr 2020 09:59:35 +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> Date: Sat, 04 Apr 2020 09:59:35 +0200 In-Reply-To: <87o8sblgto.fsf@dusky.pond.sub.org> (Markus Armbruster's message of "Wed, 01 Apr 2020 11:02:11 +0200") Message-ID: <87blo7heag.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 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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: > 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 =3D 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.