Re: [Qemu-devel] Proposal for extensions of block job commands in QEMU 1.2

[Paolo Bonzini <pbonzini@redhat.com>]

Il 21/05/2012 16:40, Anthony Liguori ha scritto:
> On 05/21/2012 09:26 AM, Paolo Bonzini wrote:
>> Il 21/05/2012 16:19, Anthony Liguori ha scritto:
>>>>
>>>
>>> I'm not against it in principle, just in practice.  Today, checking
>>> whether a command exists is:
>>>
>>> commands = qmp.query_commands()
>>>
>>> if 'block-stream' in commands:
>>>      # has block-stream
>>>
>>> I have a hard time envisioning how schema introspection can be
>>> reasonably implemented in a client.
>>
>> schema = qmp.query_command_schema('block-stream')
> 
> What would schema return?
> 
> Did you mean:
> 
> if schema['arguments'].has_key('on_error'):

Yes, something like that.

> What about adding a parameter to a structure?

schema = qmp.query_type('foo')
if schema['data'].has_key('xyz')

> BTW, the other problem with adding arguments like this is that it makes
> a stable C API impossible.

Adding fields to structs is not a problem as long as "libqmp" takes care
of all allocations on part of its client.  Adding parameters to commands
requires some smartness, but there are ways to do it:

1) add the first version number to the schema, generate versioned entry
points

qmp_block_stream_v1_1
qmp_block_stream_v1_2

etc.  Provide multiple headers libqmp-1.1.h, libqmp-1.2.h etc. that take
care of #define'ing qmp_block_stream to one of them

2) Same as (1) but use qmp_block_stream for the oldest version having
the command.

3) have fun with the preprocessor (macro with variable arguments,
sizeof, designated initializers, whatever) and emulate keyword arguments
for the C API.

Paolo