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

[Markus Armbruster]

John Snow <jsnow@redhat.com> writes:

> On 12/16/20 4:22 AM, Markus Armbruster wrote:
>> 2. On error with "no source info", don't crash.
>>     I have my doubts on this one.
>>     Such an error means the QAPI code generator screwed up, at least
>> in
>>     theory.  Crashing is only proper.  It gets the screwup fixed.
>> 
>
> QAPISemError and friends will also halt the generator and don't
> produce output and will fail tests. They aren't less visible or more
> ignorable somehow.
>
>>     In practice, errors due to interactions between built-in stuff and
>>     user-defined stuff could conceivably escape testing.  I can't
>>     remember such a case offhand.
>>     Will the "no source info" error be more useful than a crash?
>>     Possibly.  Will it get the screwup fixed?  Maybe not.
>
> I don't understand this; if it's an error -- there's no QAPI, there's
> no QEMU. It's definitely getting fixed.
>
> If QAPISourceInfo is primarily used for printing error information, we
> are already in a situation where the generator is hosed and has
> wandered itself into a problem that can't be ignored.
>
> There's no additional value in having python crash twice per every
> crash because we have bad types in our error reporting functions.

Consider the following scenario.  The average developer knows just
enough about QAPI to be dangerous.  That's okay; if you had to be a QAPI
expert to modify the QAPI schema, we would have failed.  Now meet Joe
Average.  He's a good guy.  Today his job happens to require extending
the QAPI schema.  In a hurry, as usual.  So Joe brings his well-honed
voodoo programming skills to bear, and writes something that looks
plausible to him.  His build fails.  He's not surprised; he's voodoo-
programming after all.  However, the error message is less clear than
usual.  Something about a '[builtin]' file.  There is no '[builtin]'
file.  What to do?  Obvious!  If a bit of voodoo doesn't get you over
the finish line, use more: twiddle the schema until it works.

If his build failed with a Python backtrace instead, Joe would
immediately know that he ran into a bug in our tooling he should report.

Again, I don't mean to criticize Joe.  I've walked in his shoes myself.