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 2FEA2C3DA70 for ; Tue, 23 Jul 2024 13:07:22 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sWFDp-0005hx-4s; Tue, 23 Jul 2024 09:06:29 -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 1sWFDo-0005hT-4U for qemu-devel@nongnu.org; Tue, 23 Jul 2024 09:06:28 -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 1sWFDk-0007XU-Gr for qemu-devel@nongnu.org; Tue, 23 Jul 2024 09:06:27 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1721739982; h=from:from:reply-to: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=6HukXOOE5nXxb4X9LQ4TTUCEyGs3VMA4gQ/a9+DQlA8=; b=CRIHzp1ViqGL1hiAAkdSjs2em/dkbb0ZZT1FmAsDFDLrf0OCo1qWwDVil4JgBF1x6Iuu/z 9nZBjmLPNe0rNbdgH9j9gBtHy0CxPqpRnd42UIqdeALps7pQtl80xysBo8kYg9qX59Un+r zuJiHdiVh47Umdxehuj8NiSIsgvAvM0= 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-172-gnHX1opnMM2G4lUvgg8PNg-1; Tue, 23 Jul 2024 09:06:21 -0400 X-MC-Unique: gnHX1opnMM2G4lUvgg8PNg-1 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 A6C941955D4C; Tue, 23 Jul 2024 13:06:20 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.63]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 03AC91955F40; Tue, 23 Jul 2024 13:06:17 +0000 (UTC) Date: Tue, 23 Jul 2024 14:06:14 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster 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 Message-ID: References: <20240722131611.2820041-1-berrange@redhat.com> <20240722131611.2820041-2-berrange@redhat.com> <87jzhcuypr.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87jzhcuypr.fsf@pond.sub.org> User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jul 23, 2024 at 01:36:32PM +0200, Markus Armbruster wrote: > Daniel P. Berrangé 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. Berrangé > > --- > > 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); > > > > +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 = 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 = 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. 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. > 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. I'll drop this since there's only one place benefitting right now. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|