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 A95D0CCD199 for ; Mon, 20 Oct 2025 14:35:17 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vAqyO-0008Vr-F7; Mon, 20 Oct 2025 10:34:56 -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 1vAqyJ-0008RK-VD for qemu-devel@nongnu.org; Mon, 20 Oct 2025 10:34:52 -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 1vAqyG-0002CP-Nf for qemu-devel@nongnu.org; Mon, 20 Oct 2025 10:34:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1760970882; 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=OiN+B8uqfQZYBsbiXa9pzqFCbbF6xjyDMzUPPSA8U2A=; b=YNLBDqBar4akA3G+1whpw3BxH5Q5GJw2Q0gx50rfpJTizN95iZ8LhFZavX6x24OMFHE+7V XZ8OzObcNQIS1GVL8GKRjCbM6Qf5HgIjCQcCcsxlJkd5baV4VXloYgdUoLYfnstdinlUP8 KAchNFA5HW3HsQiHvn8jvHDigIcY/mM= Received: from mx-prod-mc-03.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-48-1SvGQVIbMQSQfO05OGoRgg-1; Mon, 20 Oct 2025 10:34:36 -0400 X-MC-Unique: 1SvGQVIbMQSQfO05OGoRgg-1 X-Mimecast-MFC-AGG-ID: 1SvGQVIbMQSQfO05OGoRgg_1760970876 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 743641956076; Mon, 20 Oct 2025 14:34:35 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.19]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D9D1118002AD; Mon, 20 Oct 2025 14:34:34 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 38A2021E6A27; Mon, 20 Oct 2025 16:34:31 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: Vladimir Sementsov-Ogievskiy , 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: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Mon, 20 Oct 2025 12:40:27 +0100") References: <20251020091907.2173711-1-vsementsov@yandex-team.ru> <87347d7s0j.fsf@pond.sub.org> <0ce2f913-36c2-44a2-8141-256ff847529d@yandex-team.ru> Date: Mon, 20 Oct 2025 16:34:31 +0200 Message-ID: <871pmxskug.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 3.4.1 on 10.30.177.111 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 Daniel P. Berrang=C3=A9 writes: > On Mon, Oct 20, 2025 at 02:22:22PM +0300, Vladimir Sementsov-Ogievskiy wr= ote: >> On 20.10.25 14:05, Markus Armbruster wrote: >> > Vladimir Sementsov-Ogievskiy writes: >> >=20 >> > > 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. >> > >=20 >> > > 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(). >> >=20 >> > It's a matter of the function's contract, actually. >> >=20 >> > 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". >> >=20 >> > If the contract is "Return negative errno on failure", the function is >> > responsible for returning values that make sense. Ideally, the contra= ct >> > spells them all out. >> >=20 >>=20 >> 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 :) Quick & dirty search without a claim to accuracy or completeness: $ git-ls-files \*.[ch] | xargs awk '/, Error \*\*errp/ { on=3D1 } on &&= /return -E/ { print FILENAME ":" FNR ":" $0 } /^}/ { on=3D0 }' > If there is a need for distinguishing some cases from others, then keep > with int '0/-1' example, but turn it into a multi-value return such as > 1/0/-1, or 0/-1/-2/-3/..., etc with named constants for the unusual > scenarios. An example of that would be QIOChannel were we introduced > "#define QIO_CHANNEL_ERR_BLOCK -2" to replace the need for EAGAIN checks > in callers. Defining your own error codes is fine. Reusing errno codes can also be fine. In both cases, the function contract is a load-bearing component.