(go): downcasting support

[MrArnoldPalmer]

Go interface types cannot be downcasted to more specific types using normal go casting functionality. There may be some reflection able to achieve this but its not readily apparent if so and should be documented if it is indeed possible.

Example:

	topic := awssns.NewTopic(stack, jsii.String("MyTopic"), &awssns.TopicProps{
		DisplayName: jsii.String("MyCoolTopic"),
	})

        l1node := topic.Node().DefaultChild()
	l1 := l1node.(awssns.CfnTopic)

error:

panic: interface conversion: *awscdk.jsiiProxy_IConstruct is not awssns.CfnTopic: missing method AddDeletionOverride

goroutine 1 [running]:
main.NewGoDowncastingStack(0x7ff3cf55a0a0, 0xc0000a6420, 0x709ffb, 0x12, 0xc00023ff38, 0xc00007c748, 0x3943d80)
        /home/mvaline/dev/cdk-tests/go-downcasting/go-downcasting.go:31 +0x225
main.main()
        /home/mvaline/dev/cdk-tests/go-downcasting/go-downcasting.go:41 +0xbd
exit status 2
Subprocess exited with error 1

There may be a changes to code generation that would allow the above to work but more investigation is needed.

[xrn]

Hi, am I right that my issue #2798 is related to this?

[chlunde]

A quick chain of hacks :)

I did not want to understand/look at castAndSetToPtr (or any of the other code), so this is just a POC:

func (c *Client) CastAndSetToPtr(ptr interface{}, data interface{}) {
	ptrVal := reflect.ValueOf(ptr).Elem()
	dataVal := reflect.ValueOf(data)

	if data, ok := data.(map[string]interface{}); ok {
		if ref, ok := data["$jsii.byref"]; ok { // uhm
			typRef := strings.Split(ref.(string), "@")[0] // oof
			typ, ok := c.Types().FindType(api.FQN(typRef))

			if ok && typRef == "aws-cdk-lib.aws_ec2.CfnLaunchTemplate" {
				fmt.Println(typ, typRef)
				n := reflect.New(typ)
				c.castAndSetToPtr(n.Elem(), dataVal)
				fmt.Println(n)
				ptrVal.Set(n.Elem())
				return
			}
		}
	}

	c.castAndSetToPtr(ptrVal, dataVal)
}

CDK use case which is now possible:

	launchTemplate := awsec2.NewLaunchTemplate(stack, aws.String("lt"), &awsec2.LaunchTemplateProps{})
	fmt.Println("Calling .DefaultChild")

	cfnlt := launchTemplate.Node().DefaultChild().(awsec2.CfnLaunchTemplate)
	// https://docs.aws.amazon.com/eks/latest/userguide/best-practices-security.html
	cfnlt.AddPropertyOverride(aws.String("LaunchTemplateData.MetadataOptions.HttpTokens"), aws.String("required"))

[gottschalkj-fmr]

I was able to workaround this way for now:

dist := awscloudfront.NewDistribution(...)
...
var cfnDist awscloudfront.CfnDistribution
//lint:ignore SA1019 to use JSII to perform unsafe cast to cloudfront.CfnDistribution for us
jsii.Get(dist.Node(), "defaultChild", &cfnDist)
cfnDist.AddPropertyDeletionOverride(...)

[github-actions]

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[t0mpl]

It's closed but it's still a bug, spent my afternoon trying to override a property, I had to use @krautbax (thanks) even if jsii.Get is deprecated.