Mocking HTTPS responses in Go

Question

I\'m trying to write tests for a package that makes requests to a web service. I\'m running into issues probably due to my lack of understanding of TLS.

Currently my te

Answer 1

Most of the behavior in net/http can be mocked, extended, or altered. Although http.Client is a concrete type that implements HTTP client semantics, all of its fields are exported and may be customized.

The Client.Transport field, in particular, may be replaced to make the Client do anything from using custom protocols (such as ftp:// or file://) to connecting directly to local handlers (without generating HTTP protocol bytes or sending anything over the network).

The client functions, such as http.Get, all utilize the exported http.DefaultClient package variable (which you may modify), so code that utilizes these convenience functions does not, for example, have to be changed to call methods on a custom Client variable. Note that while it would be unreasonable to modify global behavior in a publicly-available library, it's very useful to do so in applications and tests (including library tests).

http://play.golang.org/p/afljO086iB contains a custom http.RoundTripper that rewrites the request URL so that it'll be routed to a locally hosted httptest.Server, and another example that directly passes the request to an http.Handler, along with a custom http.ResponseWriter implementation, in order to create an http.Response. The second approach isn't as diligent as the first (it doesn't fill out as many fields in the Response value) but is more efficient, and should be compatible enough to work with most handlers and client callers.

The above-linked code is included below as well:

package main

import (
    "fmt"
    "io"
    "log"
    "net/http"
    "net/http/httptest"
    "net/url"
    "os"
    "path"
    "strings"
)

func Handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "hello %s\n", path.Base(r.URL.Path))
}

func main() {
    s := httptest.NewServer(http.HandlerFunc(Handler))
    u, err := url.Parse(s.URL)
    if err != nil {
        log.Fatalln("failed to parse httptest.Server URL:", err)
    }
    http.DefaultClient.Transport = RewriteTransport{URL: u}
    resp, err := http.Get("https://google.com/path-one")
    if err != nil {
        log.Fatalln("failed to send first request:", err)
    }
    fmt.Println("[First Response]")
    resp.Write(os.Stdout)

    fmt.Print("\n", strings.Repeat("-", 80), "\n\n")

    http.DefaultClient.Transport = HandlerTransport{http.HandlerFunc(Handler)}
    resp, err = http.Get("https://google.com/path-two")
    if err != nil {
        log.Fatalln("failed to send second request:", err)
    }
    fmt.Println("[Second Response]")
    resp.Write(os.Stdout)
}

// RewriteTransport is an http.RoundTripper that rewrites requests
// using the provided URL's Scheme and Host, and its Path as a prefix.
// The Opaque field is untouched.
// If Transport is nil, http.DefaultTransport is used
type RewriteTransport struct {
    Transport http.RoundTripper
    URL       *url.URL
}

func (t RewriteTransport) RoundTrip(req *http.Request) (*http.Response, error) {
    // note that url.URL.ResolveReference doesn't work here
    // since t.u is an absolute url
    req.URL.Scheme = t.URL.Scheme
    req.URL.Host = t.URL.Host
    req.URL.Path = path.Join(t.URL.Path, req.URL.Path)
    rt := t.Transport
    if rt == nil {
        rt = http.DefaultTransport
    }
    return rt.RoundTrip(req)
}

type HandlerTransport struct{ h http.Handler }

func (t HandlerTransport) RoundTrip(req *http.Request) (*http.Response, error) {
    r, w := io.Pipe()
    resp := &http.Response{
        Proto:      "HTTP/1.1",
        ProtoMajor: 1,
        ProtoMinor: 1,
        Header:     make(http.Header),
        Body:       r,
        Request:    req,
    }
    ready := make(chan struct{})
    prw := &pipeResponseWriter{r, w, resp, ready}
    go func() {
        defer w.Close()
        t.h.ServeHTTP(prw, req)
    }()
    <-ready
    return resp, nil
}

type pipeResponseWriter struct {
    r     *io.PipeReader
    w     *io.PipeWriter
    resp  *http.Response
    ready chan<- struct{}
}

func (w *pipeResponseWriter) Header() http.Header {
    return w.resp.Header
}

func (w *pipeResponseWriter) Write(p []byte) (int, error) {
    if w.ready != nil {
        w.WriteHeader(http.StatusOK)
    }
    return w.w.Write(p)
}

func (w *pipeResponseWriter) WriteHeader(status int) {
    if w.ready == nil {
        // already called
        return
    }
    w.resp.StatusCode = status
    w.resp.Status = fmt.Sprintf("%d %s", status, http.StatusText(status))
    close(w.ready)
    w.ready = nil
}