in_process_http_client.go

package client

import (
	"net/http"
	"net/http/httptest"
)

// A *http.Client that talks directly to an http.Handler
// instead of going over the network
// Useful to testing your Go APIs with the same client you provide to your users
func NewTestHTTPClient(server http.Handler) *http.Client {
	tr := &inProcessTransport{
		server: server,
	}

	return &http.Client{Transport: tr}
}

type inProcessTransport struct {
	server http.Handler
}

func (t *inProcessTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	testReq := httptest.NewRequest(req.Method, req.URL.String(), req.Body)
	copyHeader(testReq.Header, req.Header)

	w := httptest.NewRecorder()
	t.server.ServeHTTP(w, testReq)

	return w.Result(), nil
}

func copyHeader(dst, src http.Header) {
	for k, vv := range src {
		for _, v := range vv {
			dst.Add(k, v)
		}
	}
}