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 31598C3DA61 for ; Wed, 24 Jul 2024 08:17:52 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sWXBh-0003i6-6R; Wed, 24 Jul 2024 04:17:29 -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 1sWXBe-0003gc-Cc for qemu-devel@nongnu.org; Wed, 24 Jul 2024 04:17:27 -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 1sWXBc-0000sI-87 for qemu-devel@nongnu.org; Wed, 24 Jul 2024 04:17:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1721809042; 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=k3I+ctKoo5ur6J+tMJVyFK68iS7qDvmMRrmrebHJQSk=; b=ZXyJujFC8k35F2wHsLXTay9/mzsmBr97iefWKRtc2a1o2IRp/l+P/7RFXuk8OmYj/IVyIM 0MOqckKPXu2Ultep2K/pZcKMEYJBp58+nugEaK7xZiID+XxUkoVW9rKwo8zJzn9CSis7Dv cub2KeDO0oOq89aalpWF0+if66nxjfY= Received: from mx-prod-mc-02.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-592-YRKPoi06PxaeO1TcgPZjsg-1; Wed, 24 Jul 2024 04:17:16 -0400 X-MC-Unique: YRKPoi06PxaeO1TcgPZjsg-1 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (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-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id CCAA51955D44; Wed, 24 Jul 2024 08:17:15 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.65]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 67FFF1955D42; Wed, 24 Jul 2024 08:17:15 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 2848021E668F; Wed, 24 Jul 2024 10:17:13 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: Markus Armbruster , qemu-devel@nongnu.org, Michael Roth , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Paolo Bonzini Subject: Re: [PATCH 1/5] qapi: allow for g_autoptr(Error) usage In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Tue, 23 Jul 2024 14:06:14 +0100") References: <20240722131611.2820041-1-berrange@redhat.com> <20240722131611.2820041-2-berrange@redhat.com> <87jzhcuypr.fsf@pond.sub.org> Date: Wed, 24 Jul 2024 10:17:13 +0200 Message-ID: <87frrzqk52.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 10.30.177.15 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: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.133, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=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 Tue, Jul 23, 2024 at 01:36:32PM +0200, Markus Armbruster wrote: >> Daniel P. Berrang=C3=A9 writes: >>=20 >> > While common error propagation practice does not require manually >> > free'ing of local 'Error' objects, there are some cases where this >> > is needed. One example is where the 'Error' object is only used >> > for providing info to a trace event probe. Supporting g_autoptr >> > avoids the need to manually call 'error_free'. >> > >> > Signed-off-by: Daniel P. Berrang=C3=A9 >> > --- >> > include/qapi/error.h | 2 ++ >> > 1 file changed, 2 insertions(+) >> > >> > diff --git a/include/qapi/error.h b/include/qapi/error.h >> > index 71f8fb2c50..6e429809d8 100644 >> > --- a/include/qapi/error.h >> > +++ b/include/qapi/error.h >> > @@ -437,6 +437,8 @@ Error *error_copy(const Error *err); >> > */ >> > void error_free(Error *err); >> >=20=20 >> > +G_DEFINE_AUTOPTR_CLEANUP_FUNC(Error, error_free); >> > + >> > /* >> > * Convenience function to assert that *@errp is set, then silently f= ree it. >> > */ >>=20 >> The Error interface is designed for a certain way of using it: an Error >> object flows from the spot detecting the error to a spot handling it. >> Failure to handle the error is a memory leak. Our tooling can help with >> tracking these down. >>=20 >> The interface tries to make the intended use easy: functions that report >> an error consume the Error object. Explicit error_free() should only >> needed when you handle an error in some other way. >>=20 >> When such an explicit error_free() is needed on all paths to return, >> then replacing it with auto-freeing is nice. But what if it isn't? >>=20 >> Say we add a new error path and use error_report_err(err) there. This >> has always been just fine. No more: if @err is auto-freed, this is a >> double-free. We have to also add err =3D NULL. Feels like a trap for >> developers to me. >>=20 >> Your use of auto-freeing is in the next patch. It's this pattern: >>=20 >> g_autoptr(Error) err =3D NULL; >>=20 >> if (!frobnicate(args, &err)) { >> trace_frobnicate_err(..., error_get_pretty(err)); >> } >>=20 >> You want to report the error to a trace point. That's perfectly >> legitimate. The problem is that this kind of error reporting function >> does not free, unlike the ones provided by qapi/error.h. >>=20 >> We could extend tracing to accept Error values, so that >>=20 >> trace_frobnicate_err(..., err); >>=20 >> does free. Doesn't seem worthwhile unless we find quite a few more uses >> for it. > > That is awkward because the trace calls expand to nothing at all > when tracing is disabled, so we can't rely on them to free any > args. True. Another idea: g_autofree char *errmsg =3D error_to_pretty(err); trace_frobnicate_err(..., errmsg); where error_to_pretty() frees the error except for err->msg, which it returns. >> If we conclude we want to provide auto-free as an option, we at least >> need to point out the trap in a comment. A bit of a pain to write, and >> whether people will read, understand, and remember it is uncertain. >>=20 >> My gut feeling right now: stick to the design, and free manually. If >> you think my gut is wrong, tell me. > > I'll drop this since there's only one place benefitting right now. Sensible. Thanks!