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Re: [PATCH v2 06/12] qapi/source: Add builtin null-object sentinel
From: |
Markus Armbruster |
Subject: |
Re: [PATCH v2 06/12] qapi/source: Add builtin null-object sentinel |
Date: |
Tue, 19 Jan 2021 11:21:16 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
Eduardo Habkost <ehabkost@redhat.com> writes:
> On Thu, Jan 14, 2021 at 02:39:35PM +0100, Markus Armbruster wrote:
>> John Snow <jsnow@redhat.com> writes:
>>
>> > On 1/13/21 10:39 AM, Markus Armbruster wrote:
>> >> Spelling nitpick: s/builtin/built-in/ in the title.
>> >>
>> >
>> > Sure.
>> >
>> >> John Snow <jsnow@redhat.com> 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 <jsnow@redhat.com>
>> >>
>> >> 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.