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Re: [PATCH v2 06/12] qapi/source: Add builtin null-object sentinel

From: John Snow
Subject: Re: [PATCH v2 06/12] qapi/source: Add builtin null-object sentinel
Date: Wed, 13 Jan 2021 17:30:49 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 1/13/21 10:39 AM, Markus Armbruster wrote:
Spelling nitpick: s/builtin/built-in/ in the title.


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

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

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.

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.

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?


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