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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.

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