Re: [Qemu-devel] qmp-commands.hx and inherited types (Was: Re: [PATCH v6 02/10] qmp: Add block-dirty-bitmap-add and block-dirty-bitmap-remove)

[Eric Blake <eblake@redhat.com>]

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On 11/18/2014 09:44 AM, John Snow wrote:
>> Is it worth using type inheritance, as in:
>>
>> { 'type': 'BlockDirtyBitmapAdd',
>>    'base': 'BlockDirtyBitmap',
>>    'data': { '*granularity': 'int' } }
>>
> 
> Strictly speaking, I would argue against inheritance here because
> "BlockDirtyBitmapAdd" is not "isa" "BlockDirtyBitmap". It's more of a
> "Hasa" relationship.

Fair enough.

> 
> At any rate, I tried to implement this for giggles to see if I could,
> and ran into the following issue with which I'd be curious to get an
> answer for.
> 
> As an example, If you have some type:
> 
> { 'type': 'example',
>   'data': { 'foo': 'int' } }
> 
> And an extension of it:
> 
> { 'type': 'academicExample'
>   'base': 'example',
>   'data': { 'bar': 'str' } }
> 
> How would you write a command that expected both "foo" and "bar"?
> The following doesn't seem appropriate (the generated code SKIPS the
> base fields, which leads to missing arguments in the prototype:
> 
> { 'command': 'academic-command',
>   'data': 'academicExample' }

Ouch.  Sounds like a bug in the code generator.  Obviously, someone will
have to patch that (and add a testsuite entry to make sure it doesn't
regress) before we can rely on it.

> 
> ...
> 
> {
>     .name = "academic-command",
>     .args_type = "foo:i,bar:s",
>     .mhandler.cmd_new = qmp_marshal_input_academic_command,
> },
> 
> 
> The generated prototype appears to skip the "foo" argument, including
> only the arguments associated with the base type, in this case, 'bar'.
> 
> Do we support this kind of use? I didn't see it in-use currently, but I
> only gave it a cursory skimming.

We supposedly document it as working, but as no one is using it
(including no testsuite entry), I'm not surprised that it doesn't work yet.

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


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