openapi3gen does not generate nullable: true for pointers

[josephbuchma]

This should include "nullable": true
https://github.com/getkin/kin-openapi/blob/master/openapi3gen/openapi3gen_test.go#L115-L117

This issue is not possible to fix using openapi3gen.SchemaCustomizer because reflect.Type passed into openapi3.SchemaCustomizerFn is never a pointer.

[endertunc]

You can still work around it using SchemaCustomizer and custom StructTag.

type StructWithPointer struct {
	A *string `json:"a" nullable`
}
	
customizer := openapi3gen.SchemaCustomizer(func(name string, ft reflect.Type, tag reflect.StructTag, schema *openapi3.Schema) error {
	if _, ok := tag.Lookup("nullable"); ok {
		schema.Nullable = true
	}
	return nil
})

For the solution to the problem, this is where we lose the "pointer" information https://github.com/getkin/kin-openapi/blob/master/openapi3gen/openapi3gen.go#L204-L206

I think something like following should solve the problem:

isNullable := false
for t.Kind() == reflect.Ptr {
	t = t.Elem()
	isNullable = true
}

...

schema := &openapi3.Schema{} // this already exists
schema.Nullable = isNullable // new change

I did run the tests you pointed and it seems to work. I am not sure if there are edge cases for but I am happy to raise a PR if this is acceptable.

On the other hand, openapi3gen seems to be trying to be minimalisti. For example, you also don't get required in anyone your "required" fields. I think that's they idea with SchemaCustomizer which allow you to customize the schema the way you want so the workaround I mentioned might be the suggest way to solve these kind of issues :)

[fenollp]

@endertunc your patch proposal seems very reasonable. Please open a PR :)