How to mock http.Head()

Question

I'm studying the outyet example project from https://github.com/golang/example/tree/master/outyet. The test file does not cover the case where http.Head(url) returns an error. I would like to extend the unit tests to cover the if statement where the error is logged (https://github.com/golang/example/blob/master/outyet/main.go#L100). I would like to mock http.Head(), but I'm not sure how to do this. How can this be done?

Answer 1

The http.Head function simply calls the Head method on the default HTTP client (exposed as http.DefaultClient). By replacing the default client within your test, you can change the behaviour of these standard library functions.

In particular, you will want a client that sets a custom transport (any object implementing the http.RoundTripper interface). Something like the following:

type testTransport struct{}

func (t testTransport) RoundTrip(request *http.Request) (*http.Response, error) {
    # Check expectations on request, and return an appropriate response
}

...

savedClient := http.DefaultClient
http.DefaultClient = &http.Client{
    Transport: testTransport{},
}

# perform tests that call http.Head, http.Get, etc

http.DefaultClient = savedClient

You could also use this technique to mock network errors by returning an error from your transport rather than an HTTP response.