v2 - security - correct the proxy design implementation

[baywet]

The current proxy for OpenAPISchema was implemented using inheritance, probably to save time, which forces all properties to be virtual.

Not only this is not the correct proxy design pattern implementation which requires using an interface instead, but this poses a potential security and reliability issue

[baywet]

This is also causing bugs as we copy the values from the target, but throw away the copy all the time here.

OpenAPI.NET/src/Microsoft.OpenApi/Models/References/OpenApiSchemaReference.cs

Line 29 in f5fb254

OpenApiSchema resolved = new OpenApiSchema(_target);

schema = new OpenApiSchemaReference("foo", null);
// Assume foo is not nullable.
schema.Nullable = true;
// The reference to foo is still not nullable.

The implementation should leverage lazy loading, and KEEP the new OpenAPI schema once created.

[baywet]

Another issue is that the compiler doesn't help people know which properties are writeable for a reference since it currently inherits from OpenAPISchema.

I believe that we should introduce a new interface IOpenAPISchema which ONLY has the getters for the properties.
Then all places that currently return an OpenAPISchema should be updated to return that interface. Forcing consumers to do a type assertion and handle the different cases (any property being or not writeable, or updating the source component).

The target should probably also be a public property, in case the consumers want to walk up the reference.

Lastly, the reference resolving mechanism is cumbersome for write scenarios: you need to manually register the components before they can be resolved.

[baywet]

Additional details from additional internal discussions:

• we don't want to use an interface if we can avoid it, as adding properties would be a breaking change. Maybe introducing an abstract base class is a better middle ground.
• We'll need to work on additional registration methods to make creation of documents from other object models (yoko, asp.net, etc...) easier. I'll start addressing that in an upcoming PR.

[darrelmiller]

The OpenApiSchema should not clone the object.

I'm not convinced about the derived base class thing. Can you add just setters in a derived class?
I'm fine with making target public.

We need to make all the references implement IOpenApiReferenceable because the walker relies on it.