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Tue, 19 Jan 2021 11:21:16 +0100 (CET) From: Markus Armbruster To: Eduardo Habkost Subject: Re: [PATCH v2 06/12] qapi/source: Add builtin null-object sentinel 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> Date: Tue, 19 Jan 2021 11:21:16 +0100 In-Reply-To: <20210118183608.GB1227584@habkost.net> (Eduardo Habkost's message of "Mon, 18 Jan 2021 13:36:08 -0500") Message-ID: <87ft2xxm5f.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=63.128.21.124; envelope-from=armbru@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=-0.175, 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_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: 2.1.23 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" 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. 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. 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.