From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.90_1) id 1uN3sl-0002AY-Ij for mharc-qemu-rust@gnu.org; Thu, 05 Jun 2025 02:15:20 -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 1uN3sg-00028T-LN for qemu-rust@nongnu.org; Thu, 05 Jun 2025 02:15:16 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uN3sa-0006nM-QG for qemu-rust@nongnu.org; Thu, 05 Jun 2025 02:15:11 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749104106; 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: in-reply-to:in-reply-to:references:references; bh=tvDqb7b+GASIgWQw0Bv2dNLd5laAlpyt2nE6+HcEUZE=; b=QhJshIUzvQM1T0XAua8X1EhScYSMJBM2odDfky7Xl/k+H16vVHWaJ2NwTK0Mc+VG9k+N11 Q8v2rjREKLjAFjbqu0DA+80Bvyl9kyIw3YR8VnRatVhjW7FEkACKX+zcIacx/dRCwWpfQ7 wzBFv7MgsWDQDf7j3MbfzsjHB+MI0AI= 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-631-fo1DTLRpNmCW-r0ckVIM7A-1; Thu, 05 Jun 2025 02:15:01 -0400 X-MC-Unique: fo1DTLRpNmCW-r0ckVIM7A-1 X-Mimecast-MFC-AGG-ID: fo1DTLRpNmCW-r0ckVIM7A_1749104100 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (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 C8C6C19560B7; Thu, 5 Jun 2025 06:14:59 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.38]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0958B30002C0; Thu, 5 Jun 2025 06:14:59 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 55C6A21E6757; Thu, 05 Jun 2025 08:14:56 +0200 (CEST) From: Markus Armbruster To: Paolo Bonzini Cc: Zhao Liu , qemu-devel@nongnu.org, qemu-rust@nongnu.org Subject: Re: [PATCH 06/14] rust: qemu-api: add bindings to Error In-Reply-To: <97b16175-8d2f-41f9-b305-a532acbad095@redhat.com> (Paolo Bonzini's message of "Wed, 4 Jun 2025 21:19:32 +0200") References: <20250530080307.2055502-1-pbonzini@redhat.com> <20250530080307.2055502-7-pbonzini@redhat.com> <877c1uffj3.fsf@pond.sub.org> <8734ch5d5c.fsf@pond.sub.org> <2b7be73c-d91f-4820-a8ad-6964a8331150@redhat.com> <871ps02j8u.fsf@pond.sub.org> <97b16175-8d2f-41f9-b305-a532acbad095@redhat.com> Date: Thu, 05 Jun 2025 08:14:56 +0200 Message-ID: <875xhaogtb.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; 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=-0.128, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-rust@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: QEMU Rust-related patches and discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 05 Jun 2025 06:15:17 -0000 Paolo Bonzini writes: > On 6/4/25 07:01, Markus Armbruster wrote: >> This is what your FOO_or_propagate() functions are for. >> >> The rule glosses over a subtle detail: the difference between >> error_setg() and error_propagate() isn't just create a new error vs. use >> an existing one, namely error_setg() makes the precondition violation >> mentioned above a programming error, whereas error_propagate() does not, >> it instead *ignores* the error it's supposed to propagate. >> >> I consider this difference a design mistake. Note that GError avoids >> this mistake: g_error_propagate() requieres the destination to NULL or >> point to NULL. We deviated from GError, because we thought we were >> smarter. We weren't. >> >> Mostly harmless in practice, as behavior is identical for callers that >> satisfy the preconditions. >> >> [...] >> >> So here's the bottom line. We want a Rust function to use C Error >> according to its written rules. Due to a design mistake, C functions >> can behave in two different ways when their caller violates a certain >> precondition, depending on how the function transmits the error to the >> caller. For Rust functions, we can >> >> * Always behave the more common way, i.e. like a C function using >> error_setg() to transmit. >> >> * Always behave the less common way, i.e. like a C function using >> error_propagate() to transmit. >> >> * Sometimes one way, sometimes the other way. >> >> This is actually in order of decreasing personal preference. But what >> do *you* think? >> > I agree that there are arguments for both. The reason to use > error_setg() is that, even though these functions "propagate" a > qemu_api::Error into a C Error**, the error is born in the Rust callback > and therefore there is no error_setg() anywhere that could check for > non-NULL abort(). There is a bigger risk of triggering > error_propagate()'s weird behavior. Yes. > The reason to use error_propagate() is that these functions do look a > lot more like error_propagate() than error_setg(). True. > I'm undecided. I > think I'll keep the error_setg() semantics, which is essentially > > assert_eq!(unsafe { *errp }, ptr::null_mut()); > > followed by calling bindings::error_propagate(). Works for me. The error_propagate() then does nothing but call error_handle(). However, error_handle() is static, and making it available for Rust just to cut out a harmless middleman seems hardly worth the bother.