mock: allow testing for functional options

[nbaztec]

Summary

This PR fixes #997

In most go libraries nowadays (etcd, kubernets, aws, etc.) the functional options pattern is being used:

c := client.New(client.WithTimeout(), client.WithLabel("bar"))

type opt struct {
  timeout bool
  label string
}

type ClientOption func(*opt)

func WithTimeout() ClientOption {
  return func(o *opt) {
    o.timeout = true
  }
}

func WithLabel(v string) ClientOption {
  return func(o *opt) {
    o.label = v
  }
}

func (c *client) New(opts ...ClientOption) *someClient {
  options := &opt{}
  for _, f := range opts {
    f(options)
  }

  return &someClient{options.timeout, options.label}
}

Changes

Adds a new argument type FunctionalOptions

Motivation

This wasn't possible before.

Related issues

Closes #997 #1006

[alameddinc]

I think It should be inserted into the mock library 👍

[dillonstreator]

@nbaztec thoughts on these changes to simplify the signature?

[dillonstreator]

@boyan-soubachov can you please provide feedback on this PR?

[nbaztec]

@dillonstreator The suggestions look good, I've incorporated them.
I was a bit hesitant at first if there could arise a situation during a refactor/upgrade, where the first parameter could be an incorrect type in the assertion, thereby using an incorrect type - but that seems like a non-issue, since the same can happen when specifying a manual string.

[boyan-soubachov]

Thank you :)

[bhingston-va]

I believe I'm following the test setup nearly the same (options are from another package) and I noticed 2 things.

1. tName is empty for me but reflect.TypeOf(tValue.Index(0).Interface()).String() has the same type as reflect.TypeOf(actual).String()
2. name has the prefix [] so I'm hitting this branch

Curious the choice for tName and reflect.TypeOf(actual).Name() in the log here instead of name and reflect.TypeOf(actual).String() that is used in the conditional.

It was hard for me to debug as the logs showed empty string is not the same type as my option type. When debugging I found that name was really close except for the prefixed []

I might follow up with a PR to fix the logged out line here if it was just an accident.