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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

    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

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...

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