Re: [Qemu-devel] [PATCH v6 05/12] qapi: Track location that created an implicit type

[Eric Blake]

On 10/02/2015 02:54 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
> 
>> A future patch will enable deferred error detection in the
>> various QAPISchema*.check() methods (rather than the current
>> ad hoc parse checks).
> 
> What's "deferred" about them?

With ad hoc parse checks, we validate the .json before calling
QAPISchemaEntity constructors.  With QAPISchemaEntity.check(), the
constructor is called on various strings, but the strings may not
resolve; we don't know about the problem until check() is called.

> 
> Perhaps simply: A future patch will move error checking into the various
> QAPISchema*.check() methods.
> 
>>                        But that means the user can request
>> a QAPI entity that will only fail validation after it has
>> been initialized.
> 
> I'm not sure I get this sentence.

Trying to point out that while pre-patch, the check() method was only
run on well-formed entities, now post-patch it can raise errors that we
chose not to detect prior to __init__ time.

>> RFC: I used a class-level static flag to track whether we expected
>> 'info is None' when creating a QAPISchemaEntity.  This is gross,
>> because the flag will only be set on the first QAPISchema() instance
>> (it works because none of our client scripts ever instantiate more
>> than one schema).  But the only other thing I could think of would
>> be passing the QAPISchema instance into the constructor for each
>> QAPISchemaEntity, which is a lot of churn.  Any better ideas on how
>> best to do the assertion, or should I just drop it?
>>
>> Signed-off-by: Eric Blake <address@hidden>
> 
> I'd check in QAPISchema._def_entity().

Ah, instead of an assert in QAPISchemaEntity.__init__() (which requires
a leaky abstraction), instead write the assert into QAPISchema (so the
flag can now be instance-local).  Makes sense; I'll play with the idea.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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