Re: [Qemu-devel] [PATCH v10 09/25] qapi: Prefer typesafe upcasts to qapi base classes

[Eric Blake <eblake@redhat.com>]

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On 10/26/2015 01:33 AM, Markus Armbruster wrote:
> Eric Blake <eblake@redhat.com> writes:
> 
>> On 10/23/2015 09:30 AM, Markus Armbruster wrote:
>>> Eric Blake <eblake@redhat.com> writes:
>>>
>>>> A previous patch (commit 1e6c1616) made it possible to
>>>> directly cast from a qapi type to its base type. A future
>>>> patch will do likewise for structs.  However, it requires
>>>> the client code to use a C cast, which turns off compiler
>>>> type-safety checks if the client gets it wrong.  So this
>>>
>>> Who's the client?  Suggest to simply drop "if the client gets it wrong".
>>>
>>>> patch adds inline type-safe wrappers named qapi_FOO_base()
>>>> for any type FOO that has a base, which can be used to
>>>> upcast a qapi type to its base, then uses the new generated
>>>> functions in places where we were already casting.
>>>>

>>
>> Indeed, if I don't port gen_upcast() to types until patch 10/25, then
>> this hunk also has to move to patch 10.  That means no clients of the
>> upcast macros in patch 9/25, _unless_ I add testsuite coverage.  Which I
>> probably ought to do.
>>
>>>>      switch (event) {
>>>>      case QAPI_EVENT_VNC_CONNECTED:
>>>> -        qapi_event_send_vnc_connected(si, vs->info->base, &error_abort);
>>>> +        qapi_event_send_vnc_connected(si, qapi_VncClientInfo_base(vs->info),
>>>> +                                      &error_abort);
>>>>          break;
>>>>      case QAPI_EVENT_VNC_INITIALIZED:
>>>>          qapi_event_send_vnc_initialized(si, vs->info, &error_abort);
>>>
>>> Not a single cast to union base?
>>
>> Not that I could find. So I'll have to create one in the testsuite.
> 
> I guess I'd introduce gen_upcast() only with its first real user,
> i.e. when unboxing struct base.  At that time, its obvious
> implementation should work for both struct and union, even though we
> actually use it only for struct.  If you want to add a test case for
> unions, go ahead.  I'm not sure I'd bother, because once we unify code
> generation for the two, testing them separately won't add much value
> anymore.

It sounds like I have two options for v11:

1. Keep 9/25 introducing gen_upcast(), just for union types, and
including testsuite coverage. In 10/25, make use of the upcast functions
to struct as part of making structs sane.

2. Swap the patch order: do 10/25 to alter struct layout first, using
ugly casts; then implement 9/25 that adds gen_upcast() and fixes the
ugly casts to instead use the new upcast functions.

I can go either way, so do you have any preference?

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


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