bug(ec2): Endpoint service AZ lookup fails for InterfaceVpcEndpointAwsService

[flemjame-at-amazon]

This bug is a case that wasn't caught in testing of #7521 - if the provided service name has tokens in it, synthesis fails with the error:

software.amazon.jsii.JsiiException: Cannot determine scope for context provider endpoint-service-availability-zones.
This usually happens when one or more of the provider props have unresolved tokens

InterfaceVpcEndpointAwsService objects have tokens in their name which makes them unusable with this functionality.

In this case the serviceName ends up being something like:
com.amazonaws.${Token[AWS::Region.4]}.execute-api

Reproduction Steps

Create an InterfaceVpcEndpoint, with the service being an InterfaceVpcEndpointAwsService, and synthesize the template.

Vpc vpc = new Vpc(this, "VPC", VpcProps.builder().build());
new InterfaceVpcEndpoint(this, "API Gateway VPC Endpoint", InterfaceVpcEndpointProps.builder()
    .service(InterfaceVpcEndpointAwsService.APIGATEWAY)
    .vpc(vpc)
    .lookupSupportedAzs(true)
    .build());

Below is the code that defines the name. InterfaceVpcEndpointAwsService are region-agnostic, which is why they have a token in the name. Unfortunately this breaks AZ lookup for these services.

aws-cdk/packages/@aws-cdk/aws-ec2/lib/vpc-endpoint.ts

Line 317 in a4797b4

this.name = `${prefix || 'com.amazonaws'}.${Aws.REGION}.${name}`;

Error Log

software.amazon.jsii.JsiiException: Cannot determine scope for context provider endpoint-service-availability-zones.
This usually happens when one or more of the provider props have unresolved tokens

Environment

• CLI Version : 1.39
• Framework Version: 1.39
• OS : MacOS Mojave
• Language (Version): all

Other

My proposed solution would be to resolve the AWS.Region token in the InterfaceVpcEndpoint class, before doing the lookup. The AZ lookup already requires the account and region to be set, so we will know the region in advance.

I am not sure how to resolve an AWS.Region token. I've tried using the resolve functionality in the Stack class, but it doesn't return the region, just an object:

process.stdout.write(Stack.of(this).resolve(props.service.name));

[object Object]

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