Re: [PATCH v5 1/9] migration: introduced 'MigrateAddress' in QAPI for migration wire protocol.

[Markus Armbruster <armbru@redhat.com>]

Het Gala <het.gala@nutanix.com> writes:

> On 30/05/23 12:28 pm, Markus Armbruster wrote:
>> Het Gala<het.gala@nutanix.com>  writes:
>>
>>> On 25/05/23 11:04 pm, Markus Armbruster wrote:

[...]

>>>> Aside: a more powerful type system would let us extend SocketAddress
>>>> with additional variants instead of wrapping it in a union.
>>>
>>> Markus, what do you mean by additional variants here in context of socket? Can you give a small example.
>>
>> As is, we have a nest of two unions:
>>
>> * The outer union has branches @socket, @exec, @rdma.
>>
>> * Branch @socket is the inner union, it has branches @inet, @unix, ...
>>
>> A more powerful type system would let us extend SocketAddress instead,
>> so MigrateAddress has everything SocketAddress has, plus additional
>> branches @exec, @rdma.  Naturally, the type of the discriminator also
>> needs to be extended, so that it has everything SocketAddress's
>> discriminator type has, plus additional members @exec, @rdma.
>
> Okay, so you mean something like :
>
> +# Since 8.1
> +##
> +{ 'union': 'MigrateAddress',
> +  'base': { 'transport' : 'MigrateTransport'},
> +  'discriminator': 'transport',
> +  'data': {
> +    'inet': 'InetSocketAddress',
> +    'unix': 'UnixSocketAddress',
> +    'vsock': 'VsockSocketAddress',
> +    'fd': 'str',
> +    'exec': 'MigrateExecCommand',
> +    'rdma': 'InetSocketAddress' } }
>
> Even I agree that directly leveraging this is the best option, but then wouldn't it introduce redundancy ? we would not be able to leverage socket union right in that case or am I missing something.

Yes, there's redundancy, due to QAPI's insufficient expressive power.

Is the cleaner external interface worth the (internal) redundancy, and
possibly coding complications that go with it?