Support for function mocks in v3

[mattdowdell]

I was struggling to find the rationale for why support for function mocks were removed in v3, so if nothing else I hope this might help others with a similar question.

Previous discussions:

• Generate mock for function type #301
• Generate mocks for function types #307
• unnecessary mock for type definition of func #716
• Add disable-func-mocks parameter #809
• v3: Remove support for function mocks #859
• (I couldn't see where it was removed for v3)

I can totally see that function mocks aren't always useful. Most of the times I end up with such mocks, they go unused. But there are a couple of cases they have been useful, namely when writing interceptors for gRPC and ConnectRPC.

For example, consider the type of grpc.UnaryClientInterceptor:

type UnaryClientInterceptor func(
  ctx context.Context,
  method string,
  req, reply any,
  cc *ClientConn,
  invoker UnaryInvoker,
  opts ...CallOption,
) error

If I want to write my own interceptor, I must create a function of method with this type. When I come to test my interceptor, I will pass in appropriate values for most arguments, but I want to also check my interceptor does or doesn't call the invoker. Until now, I would reach for mockery to generate a mock for grpc.UnaryInvoker which is defined as:

type UnaryInvoker func(ctx context.Context, method string, req, reply any, cc *ClientConn, opts ...CallOption) error

Typically, I want to achieve the following with this:

• Is the invoker called or not called?
• Did the interceptor update the context as expected?
• Does the interceptor handle the result of the invoker, whether that be success or error?

For ConnectRPC, the desire is the same, but I would implement connect.Interceptor and generate a mock for connect.UnaryFunc.

What I found particularly useful was:

• The testing.T.Cleanup function added by mockery to check whether the mock was called.
• The implicit checks on the inputs to UnaryInvoker/UnaryFunc to make sure they haven't been unexpectedly modified.
• The RunAndReturn generated method to perform in-depth checks/assertions on inputs to UnaryInvoker/UnaryFunc.

Obviously, I can replicate what's been removed and the function signatures don't really ever change, so there's little maintenance burden. But nonetheless, I found function mocks reduced the burden of writing such code, as well as keeping tests mostly consistent for interfaces and functions the test didn't need to be super concerned with the implementation of.

Is there a possibility of revisiting the decision to remove function mocks?

[LandonTClipp]

To be honest, it was causing a maintenance headache for me and I found it to not provide enough value for the trouble that it was causing. It's not technically complicated or anything, but during the v3 port, I just decided not to include the logic for it instead of putting effort to port it over. It was more of just an intentional omission rather than an explicit deletion.

I gauge community interest through interactions with issues/discussions so anyone who wants this added back should react to the OP.