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 B3B7FC3DA63 for ; Tue, 23 Jul 2024 11:37:34 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sWDoz-0001Gp-62; Tue, 23 Jul 2024 07:36:45 -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 1sWDoy-0001E9-82 for qemu-devel@nongnu.org; Tue, 23 Jul 2024 07:36:44 -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 1sWDow-00031B-AF for qemu-devel@nongnu.org; Tue, 23 Jul 2024 07:36:44 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1721734600; 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=qwo9o2hK11YlOWWF7FwRS7b28LoxIw1TvvLmmMzaC8M=; b=JcWyiNWS8/c0odMiDGvpZkAi08HYu/gi982wCTd79lLJKN+/R7O5iZr6y6f3IPvn6rUyMi l213rIjWBlAaAix67dHMC98x4j6BBbMUrAMgTCT7ZYIL454ULzksp96Gw0FT+M/zUUQDNK zzHHQQeinONUX6WIP3wFVYmCIzCy+3A= 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-558-pamfHetuO3qV3qWXsEajOQ-1; Tue, 23 Jul 2024 07:36:37 -0400 X-MC-Unique: pamfHetuO3qV3qWXsEajOQ-1 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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 4DE321955D5A; Tue, 23 Jul 2024 11:36:35 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.65]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6955719560B2; Tue, 23 Jul 2024 11:36:34 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 1BFFC21E668F; Tue, 23 Jul 2024 13:36:32 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: 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: <20240722131611.2820041-2-berrange@redhat.com> ("Daniel P. =?utf-8?Q?Berrang=C3=A9=22's?= message of "Mon, 22 Jul 2024 14:16:07 +0100") References: <20240722131611.2820041-1-berrange@redhat.com> <20240722131611.2820041-2-berrange@redhat.com> Date: Tue, 23 Jul 2024 13:36:32 +0200 Message-ID: <87jzhcuypr.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 10.30.177.12 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: > 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 free= it. > */ 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. 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. 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? 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. Your use of auto-freeing is in the next patch. It's this pattern: g_autoptr(Error) err =3D NULL; if (!frobnicate(args, &err)) { trace_frobnicate_err(..., error_get_pretty(err)); } 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. We could extend tracing to accept Error values, so that trace_frobnicate_err(..., err); does free. Doesn't seem worthwhile unless we find quite a few more uses for it. 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. My gut feeling right now: stick to the design, and free manually. If you think my gut is wrong, tell me.