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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: |
Thu, 14 Jan 2021 14:39:35 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
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.
> In my reply to your earlier critique, I (think) I mentioned that I
> didn't understand the difference between:
>
> 1. An exception handler itself crashes because it received a value of
> unexpected type, or
>
> 2. The exception handler printed a message that indicates a problem with
> a built-in source definition.
>
> In either case, QAPI didn't get built and it printed some kind of error
> spaghetti to the screen. In both cases, something much more seriously
> wrong has happened and the error message likely does not prepare the
> human user to really genuinely understand what that seriously wrong
> thing is.
>
> I think this is an on-mission patch that improves circumstances; with
> regards to matters of taste I would see it as a lateral move at worst
> (one weird error for another weird error).
>
> I'm left a little confused by the pushback, so I don't feel equipped to
> try and write code that addresses it.
>
> Let's chat on IRC?
Gladly. If we can make out work days intersect...