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 1479EC433EF for ; Tue, 10 May 2022 12:54:19 +0000 (UTC) Received: from localhost ([::1]:40854 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1noPNb-0003s0-0q for qemu-devel@archiver.kernel.org; Tue, 10 May 2022 08:54:19 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:45504) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1noPKd-0002MU-0v for qemu-devel@nongnu.org; Tue, 10 May 2022 08:51:15 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:55113) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1noPKa-0003xj-68 for qemu-devel@nongnu.org; Tue, 10 May 2022 08:51:14 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1652187070; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=co/UvIpHS5c4lKktMiObpZUp6Iqqvkhl/eeCFrrFD98=; b=MSQxyUowNuaRCyA5dvJj9dAHQOpJcVxg7i66AbCaY4H4gEZddueoz32EBXvvvJEPTXmTld O2ONzIcHpuyPUqYejNXJc/DrJv+3WgJBr/+hjkjpXpMWnk+rjAV1g5T4OMAfZeNxLL9lAg //3I5DL8nZjXBHm7nmfHOZJSdq9OEjs= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-641-MTceztqgOau0xE-nEKPUsg-1; Tue, 10 May 2022 08:51:09 -0400 X-MC-Unique: MTceztqgOau0xE-nEKPUsg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 1C2C5811E76 for ; Tue, 10 May 2022 12:51:09 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.168]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E48F214C1D4D; Tue, 10 May 2022 12:51:07 +0000 (UTC) Date: Tue, 10 May 2022 13:51:05 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster , Victor Toso , John Snow , Eric Blake , qemu-devel@nongnu.org, =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Subject: Re: [RFC PATCH v1 0/8] qapi: add generator for Golang interface Message-ID: References: <20220401224104.145961-1-victortoso@redhat.com> <87bkwonlkb.fsf@pond.sub.org> <87lev9mw7j.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: User-Agent: Mutt/2.1.5 (2021-12-30) X-Scanned-By: MIMEDefang 2.85 on 10.11.54.7 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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" On Tue, May 10, 2022 at 01:34:03PM +0100, Daniel P. Berrangé wrote: > On Tue, May 10, 2022 at 02:02:56PM +0200, Markus Armbruster wrote: > > > For a minimum viable use case, this doesn't feel all that difficult, as > > > conceptually instead of deleting the field from QAPI, we just need to > > > annotate it to say when it was deleted from the QEMU side. The QAPI > > > generator for internal QEMU usage, can omit any fields annotated as > > > deleted in QAPI schema. The QAPI generator for external app usage, > > > can (optionally) be told to include deleted fields ranging back to > > > a given version number. So apps can chooses what degree of compat > > > they wish to retain. > > > > Consider this evolution of command block_resize > > To help us understand, I'll illustrate some possible interfaces > in both Go and Python, since that covers dynamic and static > languages > > > * Initially, it has a mandatory argument @device[*]. > > Python definition: > > def block_resize(device, size) > > Caller: > > block_resize('dev0', 1*GiB) > > > Golang definition > > type BlockResizeCommand struct { > Device string > Size int > } > > Caller > > cmd := &BlockResizeCommand{ > Device: "dev0", > Size: 1 * GiB, > } > > > * An alternative way to specify the command's object emerges: new > > argument @node-name. Both old @device and new @node-name become > > optional, and exactly one of them must be specified. This is commit > > 3b1dbd11a6 "qmp: Allow block_resize to manipulate bs graph nodes." > > Python definition. Tricky, as non-optional params must be before > optional params, but size is naturally the last arg. One option > is to pointlessly mark 'size' as optional > > def block_resize(device=None, node_name=None, size=None) > > Caller > > block_resize(device="dev0", size=1*GiB) > block_resize(node_name="devnode0", size=1*GiB) > > > In golang definition > > type BlockResizeArguments struct { > Device string > NodeName string > Size int > } > > Caller choice of > > cmd := &BlockResizeCommand{ > Device: "dev0", > Size: 1 * GiB, > } > > cmd := &BlockResizeCommand{ > NodeName: "devnode0", > Size: 1 * GiB, > } > > > Neither case can easily prevent passing Device and NodeName > at same time. > > > * At some future date, the old way gets deprecated: argument @device > > acquires feature @deprecated. > > Ok, no change needed to the APIs in either case. Possibly have > code emit a warning if a deprecated field is set. > > > * Still later, the old way gets removed: @device is deleted, and > > @node-name becomes mandatory. > > Again no change needed to APIs, but QEMU will throw back an > error if the wrong one is used. > > > What is the proper version-spanning interface? > > > > I figure it's both arguments optional, must specify the right one for > > the version of QEMU actually in use. This spans versions, but it fails > > to abstract from them. > > Yep, I think that's inevitable in this scenario. THe plus side > is that apps that want to span versions can do so. The downside > is that apps that don't want smarts to span version, may loose > compile time warnings about use of the now deleted field. Having said that, a different way to approach the problem is to expose the versioning directly in the generated code. Consider a QAPI with versioning info about the fields { 'command': 'block_resize', 'since': '5.0.0', 'data': { 'device': ['type': 'str', 'until': '5.2.0' ], '*device': ['type': 'str', 'since': '5.2.0', 'until': '7.0.0' ], '*node-name': ['type': 'str', 'since': '5.2.0', 'until: '7.0.0' ], 'node-name': ['type': 'str', 'since': '7.0.0' ], 'size': 'int' } } Meaning * Introduced in 5.0.0, with 'device' mandatory * In 5.2.0, 'device' becomes optional, with optional 'node-name' as alternative * In 7.0.0, 'device' is deleted, and 'node-name' becomes mandatory Now consider the Go structs In 5.0.0 we can generate: type BlockResizeArguments struct { V500 *BlockResizeArguments500 } type BlockResizeArgumentsV1 struct { Device string Size int } app can use dev := "dev0" cmd := BlockResizeArguments{ V500: &BlockResizeArguments500{ Device: dev, Size: 1 * GiB } } In 5.2.0 we can now generate type BlockResizeArguments struct { V500 *BlockResizeArgumentsV500 V520 *BlockResizeArgumentsV520 } type BlockResizeArgumentsV500 struct { Device string Size int } type BlockResizeArgumentsV520 struct { Device *string NodeName *string Size int } App can use the same as before, or switch to one of dev := "dev0" cmd := BlockResizeArguments{ V520: &BlockResizeArguments520{ Device: &dev, Size: 1 * GiB } } or node := "nodedev0" cmd := BlockResizeArguments{ V520: &BlockResizeArguments520{ NodeName: &node, Size: 1 * GiB } } In 7.0.0 we can now generate type BlockResizeArguments struct { V500 *BlockResizeArgumentsV500 V520 *BlockResizeArgumentsV520 V700 *BlockResizeArgumentsV700 } type BlockResizeArgumentsV500 struct { Device string Size int } type BlockResizeArgumentsV520 struct { Device *string NodeName *string Size int } type BlockResizeArgumentsV700 struct { NodeName string Size int } App can use the same as before, or switch to node := "nodedev0" cmd := BlockResizeArguments{ V700: &BlockResizeArguments700{ NodeName: node, Size: 1 * GiB } } This kind of per-command/type versioning is not uncommon when defining API protocols/interfaces. 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 :|