Feature Request: Conditionally require StartingPosition for AWS::Lambda::EventSourceMapping

[iamed2]

StartingPosition is required for certain source types (Amazon Kinesis, Amazon DynamoDB, and Amazon MSK Streams):

Deploying without specifying StartingPosition results in an error like:

validation error detected: Value null at 'startingPosition' failed to satisfy constraint: Member must not be null. (Service: Lambda, Status Code: 400, Request ID: 7dfa2c0c-8ea6-4d87-a8ef-e9e3d6a50e23, Extended Request ID: null)

It would be helpful to get a better error earlier, from cfn-lint.

[PatMyron]

don't think this fits into any of our existing concise constraint formats, so it would require a new rule in resources/
seems like the service would have to be parsed out of EventSourceArn

...
    def match(self, cfn):
        matches = []
        resources = cfn.get_resources(['AWS::Lambda::EventSourceMapping'])
        ...
        return matches

[kddejong]

Yea, agreed. I think this has come up enough that it would be good to have a generic rule to do this. I think the 3 ways I've seen this.

1. A property is required based on a different properties value (within the same property layer)
2. A value of a property has to be a certain value based on a different properties value (within the same property layer)
3. A property in another resource has values that may depend on another resources property value.

Things get a littler harder and taxing when we have to cross resource or property layers

[iamed2]

Thanks so much!