RoundTripper returns a non-nil response and error

[bmendric]

The code in question here is within RoundTripper.RoundTrip() which, in the check for url.Error, returns both the response and an error.

https://github.com/hashicorp/go-retryablehttp/blob/master/roundtripper.go#L47

While this behavior is not explicitly disallowed by the interface, the net/http.Client implementation does check for this and will print a corresponding log error. This erroneous log message is particularly annoying when expecting all program output to be JSON, which is how I found this.

From this conversation, that validation is expected.

EDIT: Also of note, this is eluded to in the docs of the net/http.RoundTripper interface with the line "A non-nil err should be reserved for failure to obtain a response", but it could certainly be worded better/more explicitly on that side.