[stepfunctions-tasks] SageMakerCreateEndpointConfig cannot use JSONPath.stringAt for ec2.InstanceType

[ntroutman]

CDK throws an error when passing ec2.InstanceType(JsonPath.stringAt(...)) when defining the instance type used for tasks.SageMakerCreateEndpointConfig.

Reproduction Steps

   new SageMakerCreateEndpointConfig(this, 'SagemakerEndpointConfig', {
             endpointConfigName: sfn.JsonPath.stringAt("$.model.name"),
             productionVariants: [{
                 instanceType:  new ec2.InstanceType(sfn.JsonPath.stringAt("$.sm_endpoint_config.instance_type")), 
                 modelName: sfn.JsonPath.stringAt("$.model.name"),
                 variantName: sfn.JsonPath.stringAt("$.model.name"),
              }]
           });

What did you expect to happen?

That I could refer to the step-functions state using JsonPath to configure the instance type used in an Endpoint Configuration.

What actually happened?

CDK threw the following error:

XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:193
        throw new Error(`Field references must be the entire string, cannot concatenate them (found '${x}')`);
        ^

Error: Field references must be the entire string, cannot concatenate them (found 'ml.${Token[sm_endpoint_config.instance_type.516]}')
    at jsonPathString (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:193:15)
    at Object.renderString [as handleString] (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:142:18)
    at recurseObject (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:86:41)
    at XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:127:24
    at Array.map (<anonymous>)
    at recurseArray (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:120:20)
    at recurseObject (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:92:32)
    at Object.renderObject (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/json-path.js:31:12)
    at Function.renderObject (XXXXX/node_modules/monocdk/lib/aws-stepfunctions/lib/fields.js:212:28)
    at SageMakerCreateEndpointConfig._renderTask (XXXXX/node_modules/monocdk/lib/aws-stepfunctions-tasks/lib/sagemaker/create-endpoint-config.js:32:40)

Environment

• CDK CLI Version : 1.72.0
• Framework Version: @amzn/cdk-build/2.0.0
• Node.js Version: node-v12.19.0
• OS : linux-x64 (AL2012)
• Language (Version): TypeScript

Other

It's pretty clear from the error that the SageMaker Tasks is using TypeScript string interpolation to prefix the instance type with "ml."

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This is 🐛 Bug Report

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