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bh=u15rdKUDEHg1bUia8M1CQ049W1gXHOdU/mykuDLHtWE=; b=OUufcIcNmBbqzwbRpiiwrkBhguI4P2X8u70UjJrV16lW6pIL12GxqaqeQ6G9Gv2n6bq4Jp qaFa6Bx41x5FhFizCUxxqWUP4sbPiDJ9DIXiBr8/n84SC4SwKNRWYVGy50aaLx6GJJ9Y+P mKhkQzHgTKUgJcaBBW37uQNv+zuFDDk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-541-MZX-O_SCMjih_iUMDzCqeA-1; Tue, 19 Jan 2021 11:10:28 -0500 X-MC-Unique: MZX-O_SCMjih_iUMDzCqeA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2D86EC745D; Tue, 19 Jan 2021 16:10:06 +0000 (UTC) Received: from localhost (ovpn-118-239.rdu2.redhat.com [10.10.118.239]) by smtp.corp.redhat.com (Postfix) with ESMTP id E71F819C84; Tue, 19 Jan 2021 16:10:05 +0000 (UTC) Date: Tue, 19 Jan 2021 11:10:04 -0500 From: Eduardo Habkost To: Markus Armbruster Subject: Re: [PATCH v2 06/12] qapi/source: Add builtin null-object sentinel Message-ID: <20210119161004.GF1227584@habkost.net> References: <20201217015927.197287-1-jsnow@redhat.com> <20201217015927.197287-7-jsnow@redhat.com> <878s8wyhgc.fsf@dusky.pond.sub.org> <87eeinab8o.fsf@dusky.pond.sub.org> <20210118183608.GB1227584@habkost.net> <87ft2xxm5f.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: <87ft2xxm5f.fsf@dusky.pond.sub.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=ehabkost@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=ehabkost@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.195, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Michael Roth , Cleber Rosa , John Snow , qemu-devel@nongnu.org, =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Jan 19, 2021 at 11:21:16AM +0100, Markus Armbruster wrote: > Eduardo Habkost writes: > > > On Thu, Jan 14, 2021 at 02:39:35PM +0100, Markus Armbruster wrote: > >> John Snow writes: > >> > >> > On 1/13/21 10:39 AM, Markus Armbruster wrote: > >> >> Spelling nitpick: s/builtin/built-in/ in the title. > >> >> > >> > > >> > Sure. > >> > > >> >> John Snow writes: > >> >> > >> >>> We use None to represent an object that has no source information > >> >>> because it's a builtin. This complicates interface typing, since many > >> >>> interfaces expect that there is an info object available to print errors > >> >>> with. > >> >>> > >> >>> Introduce a special QAPISourceInfo that represents these built-ins so > >> >>> that if an error should so happen to occur relating to one of these > >> >>> builtins that we will be able to print its information, and interface > >> >>> typing becomes simpler: you will always have a source info object. > >> >>> > >> >>> This object will evaluate as False, so "if info" remains a valid > >> >>> idiomatic construct. > >> >>> > >> >>> NB: It was intentional to not allow empty constructors or similar to > >> >>> create "empty" source info objects; callers must explicitly invoke > >> >>> 'builtin()' to pro-actively opt into using the sentinel. This should > >> >>> prevent use-by-accident. > >> >>> > >> >>> Signed-off-by: John Snow > >> >> > >> >> As I pointed out in review of v1, this patch has two aspects mixed up: > >> >> > >> >> 1. Represent "no source info" as special QAPISourceInfo instead of > >> >> None > >> >> > >> >> 2. On error with "no source info", don't crash. > >> >> > >> >> The first one is what de-complicates interface typing. It's clearly > >> >> serving this patch series' stated purpose: "static typing conversion". > >> >> > >> >> The second one is not. It sidetracks us into a design discussion that > >> >> isn't related to static typing. Maybe it's something we should discuss. > >> >> Maybe the discussion will make us conclude we want to do this. But > >> >> letting the static typing work get delayed by that discussion would be > >> >> stupid, and I'll do what I can to prevent that. > >> >> > >> > > >> > It's not unrelated. It's about finding the most tactical incision to > >> > make the types as we actually use them correct from a static analysis > >> > context. > >> > > >> > Maybe there's another tactical incision to make that's "smaller", for > >> > some perception of "smaller", but it's not unrelated. > >> > >> We don't have to debate, let alone agree on relatedness. > >> > >> >> The stupidest possible solution that preserves the crash is adding an > >> >> assertion right where it crashes before this patch: in > >> >> QAPISourceInfo.__str__(). Yes, crashing in a __str__() method is not > >> >> nice, but it's no worse than before. Making it better than before is a > >> >> good idea, and you're quite welcome to try, but please not in this > >> >> series. Add a TODO comment asking for "make it better", then sit on > >> >> your hands. > >> > > >> > I'm recently back from a fairly long PTO, so forgive me if I am > >> > forgetting something, but I am not really sure I fundamentally > >> > understand the nature of this critique. > >> > > >> > Making functions not "crash" is a side-effect of making the types > >> > correct. I don't see it as scope-creep, it's a solution to a problem > >> > under active consideration. > >> > >> I disagree. > >> > >> The crash you "fix" is *intentional*. I was too lazy to write something > >> like > >> > >> assert self.info > >> > >> and instead relied in self.info.whatever to crash. I don't care how it > >> crashes, as long as it does crash. > >> > >> I *like* qapi-gen to crash on such internal errors. It's easy, and > >> makes "this is a bug, go report it" perfectly clear. > >> > >> I'd also be fine with reporting "internal error, this is a bug, go > >> report it". Not in this series, unless it's utterly trivial, which I > >> doubt. > >> > >> I'm *not* fine with feeding made-up info objects to the user error > >> reporting machinery without proof that it'll actually produce a useful > >> error message. Definitely not trivial, thus not in this series. > > > > If you really don't want to change the existing behavior of the > > code, I believe we have only two options: > > > > 1) Annotate self.info as QAPISourceInfo (not Optional), > > and add a hack to make the expression `self.info` crash if the > > argument to __init__() was None. > > I figure you mean > > * Represent "no info" as a special QAPISourceInfo (instead of None), so > we can annotate .info as QAPISourceInfo (not Optional). > > * When we report a QAPIError, assert .info is not this special value. Not necessarily. Creating a special QAPISourceInfo would be one solution to let us annotate self.info as non-Optional, but not the only one. Possibly the simplest way to declare self.info as non-Optional is to make it a property that hides an Optional attribute. e.g.: class ...: _info: Optional[QAPISourceInfo] @property def info(self) -> QAPISourceInfo: assert self._info is not None return self._info > > This preserves the existing (and intentional) behavior: we crash when we > dot into QAPISourceInfo, and we do that only when we report a QAPIError > with that info. I'm not sure about the "only when we report a QAPIError" part. We seem to have multiple places in the code where self.info is assumed to never be None, and I'm not sure all of them involve QAPIError. > > The only change in behavior is AssertionError instead of AttributeError. > Minor improvement. > > We could replace the AssertionError crash by a fatal error with suitably > worded error message. I'd prefer not to, because I'd rather keep the > stack backtrace. Admittedly not something I'd fight for. > > > 2) Annotate self.info as Optional[QAPISourceInfo], and adding > > manual asserts everywhere self.info is used. > > > > Which of those two options do you find acceptable, Markus? > > I think John prefers (1), because the typing gets simpler. I'm inclined > to leave the decision to him. -- Eduardo