为什么http.Get导致两个请求而不是一个请求？

Question

Create a test file ./bower_components/index.html and run go test in ./.

Why does the following print two lines instead of just the first one?

./bower_components/index.html                                                  
./bower_components/

Output:

=== RUN   TestRootHandler                                                      
./bower_components/index.html                                                  
./bower_components/             ???                                            
--- PASS: TestRootHandler (0.00s)                                              
        main_test.go:32: 200 - ./bower_components/Hello World.html             
PASS                                                                           
ok

Code:

// RootHandler for HTTP
func RootHandler(root string, h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        _, err := os.Open(root + r.URL.Path)
        if err != nil {
            fmt.Println(err.Error())
            h.ServeHTTP(w, r)
            return
        }
        fmt.Println(root + r.URL.Path)
        r.URL.Path = root + r.URL.Path
        h.ServeHTTP(w, r)
    })
}

// TestRootHandler
func TestRootHandler(t *testing.T) {
    ts := httptest.NewServer(RootHandler("./bower_components", http.FileServer(http.Dir("./"))))
    defer ts.Close()
    res, err := http.Get(ts.URL + "/index.html")
    if err != nil {
        t.Fatal(err)
    }
    body, err := ioutil.ReadAll(res.Body)
    res.Body.Close()
    if err != nil {
        t.Fatal(err)
    }
    t.Logf("%d - %s", res.StatusCode, body)
}

Let me know if you don't understand the question then I will setup a github repository so you can just run the go test command to see what I mean.

Answer 1

That's just the way how http.FileServer() is written. Quoting from its doc:

As a special case, the returned file server redirects any request ending in "/index.html" to the same path, without the final "index.html".

This is what you experience: you were requesting /bower_components/index.html, so the handler returned by http.FileServer() sends a redirect:

HTTP/1.1 301 Moved Permanently
Location: ./

And http.Get() being well-behaved, follows this redirect, and performs another HTTP GET, now without the index.html, and the http.FileServer() handler will try and serve the index.html in such case.