Do not fire trigger with invalid JSON

[dubee]

Due to the default behavior of Python's json.loads(), invalid JSON can be passed to the OpenWhisk Controller when firing a trigger. This occurs when constants NaN, Infinity, -Infinity are present in the JSON consumed from Kafka. Such an event results in a 400 HTTP error from the trigger invocation request. When a 400 status code is returned to the Kafka provider, the user's corresponding trigger is automatically disabled.

Showing json.loads parsing NaN without a failure.

json.loads('{"a":[NaN,NaN,123]}')
{u'a': [nan, nan, 123]}

Providing a parse_constant function that throws an error allows us to fail when NaN is present.

json.loads('{"a":[NaN]}', parse_constant=parseJSONConstant)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 351, in loads
    return cls(encoding=encoding, **kw).decode(s)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 382, in raw_decode
    obj, end = self.scan_once(s, idx)
  File "<stdin>", line 3, in parseJSONConstant
ValueError: Invalid JSON detected