How can I mock multiple types when the signature of a concrete method refers to another concrete type, not its interface?

Question

I'm making use of a third party library that doesn't have any interfaces for its classes. I can use them in my structs no problem, but they have side effects that I want to avoid when unit testing.

// Somewhere there are a couple structs, with no interfaces. I don't own the code.
// Each has only one method.
type ThirdPartyEntry struct {}
func (e ThirdPartyEntry) Resolve() string {
    // Do some complex stuff with side effects
    return "I'm me!"
}

// This struct returns an instance of the other one.
type ThirdPartyFetcher struct {}
func (f ThirdPartyFetcher) FetchEntry() ThirdPartyEntry {
    // Do some complex stuff with side effects and return an entry
    return ThirdPartyEntry{}
}

// Now my code.
type AwesomeThing interface {
    BeAwesome() string
}
// I have a class that makes use of the third party.
type Awesome struct {
    F ThirdPartyFetcher
}
func (a Awesome) BeAwesome() string {
    return strings.Repeat(a.F.FetchEntry().Resolve(), 3)
}
func NewAwesome(fetcher ThirdPartyFetcher) Awesome {
    return Awesome{
        F: fetcher,
    }
}

func main() {
    myAwesome := NewAwesome(ThirdPartyFetcher{})
    log.Println(myAwesome.BeAwesome())
}

This works! But I want to write some unit tests, and so I'd like to Mock both the third party structs. To do so I believe I need interfaces for them, but since ThirdPartyFetcher returns ThirdPartyEntrys, I cannot figure out how.

I created a pair of interfaces which match up with the two third party classes. I'd like to then rewrite the Awesome struct and method to use the generic Fetcher interface. In my test, I would call NewAwesome() passing in a testFetcher, a struct which also implements the interface.

type Awesome struct {
    F Fetcher
}
func NewAwesome(fetcher Fetcher) Awesome {
    return Awesome{
        Fetcher: fetcher,
    }
}

type Entry interface {
    Resolve() string
}
// Double check ThirdPartyEntry implements Entry
var _ Entry = (*ThirdPartyEntry)(nil)

type Fetcher interface {
    FetchEntry() Entry
}
// Double check ThirdPartyFetcher implements Fetcher
var _ Fetcher = (*ThirdPartyFetcher)(nil)

I omit the test code because it's not relevant. This fails on the last line shown.

./main.go:49: cannot use (*ThirdPartyFetcher)(nil) (type *ThirdPartyFetcher) as type Fetcher in assignment:
*ThirdPartyFetcher does not implement Fetcher (wrong type for FetchEntry method)
    have FetchEntry() ThirdPartyEntry
    want FetchEntry() Entry

The signatures are different, even though I already showed that ThirdPartyEntry implements Entry. I believe this is disallowed because to would lead to something like slicing (in the polymorphic sense, not the golang sense). Is there any way for me to write a pair of interfaces? It should be the case that the Awesome class doesn't even know ThirdParty exists - it's abstracted behind the interface and injected when main calls NewAwesome.

Answer 1

It's not very pretty, but one way would be to:

type fetcherWrapper struct {
    ThirdPartyFetcher
}

func (fw fetcherWrapper) FetchEntry() Entry {
    return fw.ThirdPartyFetcher.FetchEntry()
}

I'd say mocking things that return structs vs interfaces is a relatively common problem without any great solutions apart from a lot of intermediate wrapping.