Golang interfaces & mocking
IMO this is a super common solution and strikes a good balance between maintainability and testability.
The way I think of this is that your code is programming to an interface, and there just happens to be two implementations:
- "prod" version ie the library that you're testing
- test version, a stub that implements the interface
With this approach, a stub can only verify up to your service and its dependencies boundary along the contract of the interface, it assures very little or nothing about your components collaboration/integration with the library that's being stubbed. In my experiences this approach works awesome and is my personal preference. I have found it to be important though to have 1 or 2 higher level tests making sure that your component can successfully initialize and interact with the "prod" version of the library.
Plug:
I've written about this exact issue https://medium.com/dm03514-tech-blog/you-are-going-to-need-it-using-interfaces-and-dependency-injection-to-future-proof-your-designs-2cf6f58db192
You're doing the right thing here, and this was a conscious decision on the part of the language designers.
The Go philosophy is that your code should "own" those interfaces, not the library. In languages like C# and Java, libraries define their interfaces up front, without really knowing what the consumer actually needs. (Maybe they include too many methods, or too few.) In Go, because the consumer effectively "owns" the interfaces, you're empowered to specify which methods actually need to be present in a minimal interface, and changes in your program requirements mean that you can also change the interface.
Now in this particular case it might seem strange to create an adapter in front of a function for testability, but consider the alternative: if session.Get() were a method of an interface or struct, instead of a function, it would force all library consumers to instantiate a dummy object in order to call the method. Not everyone is going to fake it out---it's easier for them to say that the consumers who want to (like you) are empowered to write adapters, and those that don't can blissfully ignore them.