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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id C037DCCD19F for ; Mon, 20 Oct 2025 16:41:23 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vAswL-0006Ig-Nk; Mon, 20 Oct 2025 12:40:57 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vAswJ-0006IK-S5 for qemu-devel@nongnu.org; Mon, 20 Oct 2025 12:40:55 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vAswF-0003xt-F1 for qemu-devel@nongnu.org; Mon, 20 Oct 2025 12:40:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1760978448; 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=aV8zkSvovpCZU88L8xsCQZoMXKpvYWY8pHtjB8U44jw=; b=bH3CexvjmIfGFOTYLBOeWXX/QHpOlGG3IX7BbRGb+9TxkFV5R2LvTnWYSY6E4/Qwbnh7xC t+4lfxUSG0NwXy6qupLJKU1pxrV6Fcx8JFbLrxOr9AuxyXXrNZgsTbPpW3kC024h9N6tib 67aA/jalVlL26tLd03/4Uf4x3+B6lf4= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-538-CNlWnDwLOwuaVaA0Sy3Zzw-1; Mon, 20 Oct 2025 12:40:44 -0400 X-MC-Unique: CNlWnDwLOwuaVaA0Sy3Zzw-1 X-Mimecast-MFC-AGG-ID: CNlWnDwLOwuaVaA0Sy3Zzw_1760978443 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2F6C31956094; Mon, 20 Oct 2025 16:40:43 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.19]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 73BF51956056; Mon, 20 Oct 2025 16:40:42 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id F196F21E6A27; Mon, 20 Oct 2025 18:40:39 +0200 (CEST) From: Markus Armbruster To: Vladimir Sementsov-Ogievskiy Cc: Markus Armbruster , Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9?= , peterx@redhat.com, stefanb@linux.vnet.ibm.com, farosas@suse.de, qemu-devel@nongnu.org Subject: Re: [PATCH] migration: vmsd errp handlers: return bool In-Reply-To: <7d059286-f6a2-4dae-8af1-78a3c1fc5cb4@yandex-team.ru> (Vladimir Sementsov-Ogievskiy's message of "Mon, 20 Oct 2025 18:05:41 +0300") References: <20251020091907.2173711-1-vsementsov@yandex-team.ru> <87347d7s0j.fsf@pond.sub.org> <0ce2f913-36c2-44a2-8141-256ff847529d@yandex-team.ru> <871pmxskug.fsf@pond.sub.org> <7d059286-f6a2-4dae-8af1-78a3c1fc5cb4@yandex-team.ru> Date: Mon, 20 Oct 2025 18:40:39 +0200 Message-ID: <87zf9lplvc.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Vladimir Sementsov-Ogievskiy writes: > On 20.10.25 17:34, Markus Armbruster wrote: >> Daniel P. Berrang=C3=A9 writes: >>=20 >>> On Mon, Oct 20, 2025 at 02:22:22PM +0300, Vladimir Sementsov-Ogievskiy = wrote: >>>> On 20.10.25 14:05, Markus Armbruster wrote: >>>>> Vladimir Sementsov-Ogievskiy writes: >>>>> >>>>>> Recently we moved to returning errp. Why to keep int return value? >>>>>> Generally it doesn't help: you can't use in a logic of handling >>>>>> an error, as you are never sure, that in future the logic in >>>>>> the stack will not change: it may start to return another error >>>>>> code in the same case, or return same error code in another case. >>>>>> >>>>>> Actually, we can only rely on concrete errno code when get it >>>>>> _directly_ from documented library function or syscall. This way we >>>>>> handle for example EINTR. But later in a stack, we can only add >>>>>> this errno to the textual error by strerror(). >>>>> >>>>> It's a matter of the function's contract, actually. >>>>> >>>>> If the contract is "Return negative value on failure", checking for >>>>> failure is all you can do with it. Same information as "Return false= on >>>>> failure". >>>>> >>>>> If the contract is "Return negative errno on failure", the function is >>>>> responsible for returning values that make sense. Ideally, the contr= act >>>>> spells them all out. >>>>> >>>> >>>> Do you know an example in code where we have both errno return value >>>> and errp, and the return value make sense and used by callers? >>> >>> If there are examples of that, I would generally consider them to be >>> bugs. >>> >>> IMHO if a method is using "Error **errp", then it should be considered >>> forbidden to return 'errno' values. >> >> Several subsystems disagree :) > > I'd vote, that in 99% (or more) cases, they don't reasonably disagree, > but blindly follow usual pattern of returning -errno together with > errp, while having no reasonable contract on concrete errno values, > and with this errno finally unused (used only to check, it is it < 0, > like boolean). In other words, the only contract they have is > "< 0 is error, otherwise success". Functions that could just as well return -1 instead of errno exist. Functions that return negative errno with callers that use them also exist. I'm not going to speculate on relative frequency. I much prefer written function contracts. But if a caller relies on negative errno codes, there is an unwritten contract whether we like it or not. >> Quick & dirty search without a claim to accuracy or completeness: >> $ git-ls-files \*.[ch] | xargs awk '/, Error \*\*errp/ { on=3D1 } o= n && /return -E/ { print FILENAME ":" FNR ":" $0 } /^}/ { on=3D0 }' [...]