Serverless-application-model: Make policy template allow logical id of resources

[ljacobsson]

When adding policy templates to functions one has to refer to the cloudformation documentation to find the correct intrinsic function to fetch the reference desired by the policy.

For example:
DynamoDBCrudPolicy requires a TableName that's exposed through Ref:

{
  "DynamoDBCrudPolicy": {
    "TableName": {
      "Ref": "MyTable"
    }
  }
}

...but SNSPublishMessagePolicy expects a TopicName that's exposed via GetAtt:

{
  "SNSPublishMessagePolicy": {
    "TopicName": {
      "Fn::GetAtt": [
        "MyTopic",
        "TopicName"
      ]
    }
  }
}

Would it be possible to somehow just provide the logical ID to the resource and let the policy template processor inject the correct intrinsic function for the resource?

Like this:

{
  "DynamoDBCrudPolicy": {
    "TableName": "MyTable"
  }
}

...or maybe:

{
  "DynamoDBCrudPolicy": {
    "TableLogicalId": "MyTable"
  }
}

Here's a video of what I'm trying to achieve. At the moment I'm defaulting the intrinsic function to Ref.

[praneetap]

@ljacobsson What a cool issue description! Thank you for reporting this. This is a real issue that SAM could help ease the pain with. The biggest hurdle I see is that in CFN every resource has a non standard way of getting the ARN to it, so we would need to build that mapping. We will add this issue to our prioritization list, +1s will help immensely.

[ljacobsson]

@praneetap

Fwiw, I've published this functionality as a standalone CLI here: https://www.npmjs.com/package/@mhlabs/sam-policies-cli

I've made some manual mapping of which intrisic functions to use.