CHASM: ComponentPointer support

[alexshtin]

What changed?

CHASM: Add basic ComponentPointer support to CHASM component.

Why?

Part of CHASM workstream. Few things are still pending:

1. RefC and RefD should return error.
2. Implementation of RefC and RefD needs to be revisited. It is not clear if it is possible to implement w/o deserializing entire component starting from root and all the way down, because n.value is needed to compare with passed argument.
3. Dangling pointers protection is not implemented.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

[yycptt]

I think it is the case based on my reading of the code but want to double check: If Component or Data field type, it (T) needs to be pointer to a struct. For Pointer field type, T can be an interface.

[alexshtin]

This is a value of pointer itself. And it must be of []string type. It is set internally, so it is always must be the case.

For other call of the assertStructPointer, it asserts on actual type of the value which is passed to deserialize method, which is always concrete type, even if the field is defined using interface. You can check TestFieldInterface.

[yycptt]

ok I see. The type the T in Field[T] can always be an interface. But when creating a new Field, it needs to be a concrete type (i.e. pointer to struct) when creating the field ().

Then I guess what I want to confirm is: do Component|DataPointerTo() have the same requirement? Looks like the answer is yes because the Ref() impl only check = node.value and node value is always concrete type.

[alexshtin]

Yes, the answer is "yes" :-).

In

func ComponentPointerTo[C Component](
	ctx MutableContext,
	c C,
) (Field[C], error) {

C can be and interface, but c will be always an instance of a concrete type which should match the type of what it points to. If they are mismatch, it can be hard to debug, I guess.

[yycptt]

Got it. Thanks for confirming.

I guess usually the component using Pointer won't actually care or know the concrete type of the component being pointed to. So the use case I have in mind is the component's constructor will just accept an interface and then create a pointer to the passed in interface value, which means c is an interface.
It's easy to support that I think, just need to check for the try to match the underlying value in Ref().