Argument schema of function stateful_set_update_strategy_tests got type ObjectSchema but expected type StringSchema

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How to reproduce

python3 -m acto --config data/percona-server-mongodb-operator/config.json --learn

It will crash with this log message:

CRITICAL, MainThread, error_handler.py:55, An exception occured: <class 'TypeError'>: Argument `schema` of function <function stateful_set_update_strategy_tests at 0x7f553ed792d0> got type <class 'acto.schema.object.ObjectSchema'> but expected type <class 'acto.schema.string.StringSchema'>.

Causes

1. For this operator, updateStrategy is a string (not an object like in the native schema).

2. At this stage of Acto code:

   acto/acto/input/test_generators/generator.py

   Lines 241 to 246 in 92fb7c3

   	# sort by priority
   	generator_candidates.sort(key=lambda x: x.priority, reverse=True)
   	if len(generator_candidates) > 0:
   	test_cases.append(
   	(schema.path, generator_candidates[0].func(schema)),
   	)

   generator_candidates in fact has two elements: one is the exact match for updateStrategy; the second is for type String. But Acto only uses the first candidate with the highest priority and fails type check later here.

Possible solutions

The quoted code snippet seems to have certain design that allows a few alternative generators (e.g. it sorts the whole candidate list instead of directly picking up the max or min) but ends up with trying only one. Maybe we can add some try-except statement there.

To work around it in my 523 homework, I used the following

        if len(generator_candidates) > 0:
            if (schema.path == ['spec', 'updateStrategy']):
                test_cases.append(
                    (schema.path, generator_candidates[1].func(schema)),
                )
            else:
                test_cases.append(
                    (schema.path, generator_candidates[0].func(schema)),
                )