Re: [Qemu-devel] [PATCH v3 3/5] QEMUMachine: add events_wait method

[John Snow <jsnow@redhat.com>]

On 5/24/19 8:38 AM, Max Reitz wrote:
> On 23.05.19 20:03, John Snow wrote:
>>
>>
>> On 5/23/19 1:49 PM, Max Reitz wrote:
>>> On 23.05.19 19:06, John Snow wrote:
>>>> Instead of event_wait which looks for a single event, add an events_wait
>>>> which can look for any number of events simultaneously. However, it
>>>> will still only return one at a time, whichever happens first.
>>>>
>>>> Signed-off-by: John Snow <jsnow@redhat.com>
>>>> ---
>>>>  python/qemu/__init__.py | 69 +++++++++++++++++++++++++++++------------
>>>>  1 file changed, 49 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/python/qemu/__init__.py b/python/qemu/__init__.py
>>>> index 81d9657ec0..98ed8a2e28 100644
>>>> --- a/python/qemu/__init__.py
>>>> +++ b/python/qemu/__init__.py
>>>> @@ -402,42 +402,71 @@ class QEMUMachine(object):
>>>>          self._qmp.clear_events()
>>>>          return events
>>>>  
>>>> -    def event_wait(self, name, timeout=60.0, match=None):
>>>> +    @staticmethod
>>>> +    def event_match(event, match=None):
>>>>          """
>>>> -        Wait for specified timeout on named event in QMP; optionally filter
>>>> -        results by match.
>>>> +        Check if an event matches optional match criteria.
>>>>  
>>>> -        The 'match' is checked to be a recursive subset of the 'event'; skips
>>>> -        branch processing on match's value None
>>>> -           {"foo": {"bar": 1}} matches {"foo": None}
>>>> -           {"foo": {"bar": 1}} does not matches {"foo": {"baz": None}}
>>>> +        The match criteria takes the form of a matching subdict. The event is
>>>> +        checked to be a superset of the subdict, recursively, with matching
>>>> +        values whenever those values are not None.
>>>> +
>>>> +        Examples, with the subdict queries on the left:
>>>> +         - None matches any object.
>>>> +         - {"foo": None} matches {"foo": {"bar": 1}}
>>>> +         - {"foo": {"baz": None}} does not match {"foo": {"bar": 1}}
>>>
>>> Pre-existing, but the difference between “bar” and “baz” confused me
>>> quite a bit.
>>>
>>> Also, I wonder...  {"foo": None} would not match {"foo": 1}, right?
>>> Does that make sense?  Shouldn’t None be the wildcard here in general?
>>> (Also pre-existing of course.)
>>>
>>> But this patch doesn’t make things worse, so:
>>>
>>> Reviewed-by: Max Reitz <mreitz@redhat.com>
>>>
>>> (I’d still like your opinion.)
>>>
>>
>> I knew I was inviting trouble by trying to re-document this.
>>
>> The intention I had when writing the docs, which I think are wrong now,
>> was for {"foo": None} to match {"foo": 1}, but I think you're right that
>> it won't because '1' isn't a dict, so it tests for equality instead.
>>
>> So I need to fix this one up a little bit, but I'll take the review as a
>> sign that this approach seems workable.
> 
> I think the comment is technically completely correct.  It’s just that
> (1) it’s hard to discern “bar” from “baz”, and (2) if {"foo": None}
> intentionally does not match {"foo": 1}, we may want to document that,
> because it isn’t intuitively clear from the description.  If it’s a bug,
> the code should be fixed (and it should still be documented).
> 
> Max
> 

Yeah, I see. OK; I have prepared a patch that can be applied
independently after this series, if you'd like to stage this as-is. The
new patch changes behavior of this function a little, but is backwards
compatible with no changes because nobody was using these edge cases.

--js