How to test http calls in Go using httptest

I have the following code:

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"time"
)

type twitterResult struct {
	Results []struct {
		Text     string `json:"text"`
		Ids      string `json:"id_str"`
		Name     string `json:"from_user_name"`
		Username string `json:"from_user"`
		UserId   string `json:"from_user_id_str"`
	}
}

var (
  twitterUrl = "http://search.twitter.com/search.json?q=%23UCL"
  pauseDuration = 5 * time.Second
)

func retrieveTweets(c chan<- *twitterResult) {
	for {
		resp, err := http.Get(twitterUrl)
		if err != nil {
			log.Fatal(err)
		}

		defer resp.Body.Close()
		body, err := ioutil.ReadAll(resp.Body)
		r := new(twitterResult) //or &twitterResult{} which returns *twitterResult
		err = json.Unmarshal(body, &r)
		if err != nil {
			log.Fatal(err)
		}
		c <- r
		time.Sleep(pauseDuration)
	}

}

func displayTweets(c chan *twitterResult) {
	tweets := <-c
	for _, v := range tweets.Results {
		fmt.Printf("%v:%v\n", v.Username, v.Text)
	}

}

func main() {
	c := make(chan *twitterResult)
	go retrieveTweets(c)
	for {
		displayTweets(c)
	}

}

I'd like to write some tests for it, but I'm not sure how to use the httptest package http://golang.org/pkg/net/http/httptest/ would appreciate some pointers

I came up with this (shamelessly copied from the tests for go OAuth https://code.google.com/p/goauth2/source/browse/oauth/oauth_test.go):

var request = struct {
	path, query       string // request
	contenttype, body string // response
}{
	path:        "/search.json?",
	query:       "q=%23Kenya",
	contenttype: "application/json",
	body:        twitterResponse,
}

var (
	twitterResponse = `{ 'results': [{'text':'hello','id_str':'34455w4','from_user_name':'bob','from_user_id_str':'345424'}]}`
)

func TestRetrieveTweets(t *testing.T) {
	handler := func(w http.ResponseWriter, r *http.Request) {

		w.Header().Set("Content-Type", request.contenttype)
		io.WriteString(w, request.body)
	}

	server := httptest.NewServer(http.HandlerFunc(handler))
	defer server.Close()

	resp, err := http.Get(server.URL)
	if err != nil {
		t.Fatalf("Get: %v", err)
	}
	checkBody(t, resp, twitterResponse)
}

func checkBody(t *testing.T, r *http.Response, body string) {
	b, err := ioutil.ReadAll(r.Body)
	if err != nil {
		t.Error("reading reponse body: %v, want %q", err, body)
	}
	if g, w := string(b), body; g != w {
		t.Errorf("request body mismatch: got %q, want %q", g, w)
	}
}

httptest does two types of tests: response and server

Response test:

func TestHeader3D(t *testing.T) {
    resp := httptest.NewRecorder()

    uri := "/3D/header/?"
    path := "/home/test"
    unlno := "997225821"

    param := make(url.Values)
    param["param1"] = []string{path}
    param["param2"] = []string{unlno}

    req, err := http.NewRequest("GET", uri+param.Encode(), nil)
    if err != nil {
            t.Fatal(err)
    }

    http.DefaultServeMux.ServeHTTP(resp, req)
    if p, err := ioutil.ReadAll(resp.Body); err != nil {
            t.Fail()
    } else {
            if strings.Contains(string(p), "Error") {
                    t.Errorf("header response shouldn't return error: %s", p)
            } else if !strings.Contains(string(p), `expected result`) {
                    t.Errorf("header response doen't match:\n%s", p)
            }
    }
}

Server test (which is what you need to use):

func TestIt(t *testing.T){
    ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Content-Type", "application/json")
        fmt.Fprintln(w, `{"fake twitter json string"}`)
    }))
    defer ts.Close()

    twitterUrl = ts.URL
    c := make(chan *twitterResult)
    go retrieveTweets(c)
    
    tweet := <-c
    if tweet != expected1 {
        t.Fail()
    }
    tweet = <-c
    if tweet != expected2 {
        t.Fail()
    }
}

BTW, you don't need to pass in the pointer of r, because it's already a pointer.

err = json.Unmarshal(body, r)

EDIT: for my recorder test, I could use my http handler like this:

handler(resp, req)

But my original code is not using the default mux (but from Gorilla/mux), and I have some wrapping around the mux, e.g. insert server logging, and adding request context (Gorilla/context), so I had to start from mux and call ServeHTTP

Originally this code snippet was found on GitHub Gist, but while trying to apply the concept to one of my projects I realized that I had to modify the main code significantly that I decided to test these calls using an integration test by using docker and curl.

If you want to test your program, it is often best to write it with testing in mind. For instance, if you extracted the inner loop of your retrieveTweets function into something like this:

func downloadTweets(tweetsUrl string) (*twitterResult, error)

You could invoke it with the URL of a test server you've set up using the httptest package without having to worry about the sleeps or repeated requests.