How to force error on reading response body - unit-testing

Question

I've written http client wrapper in go and I need to test it thoroughly.
I'm reading the response body with ioutil.ReadAll within the wrapper. I'm having a bit of trouble figuring out how I can force a read from the response body to fail with the help of httptest.
package req
func GetContent(url string) ([]byte, error) {
response, err := httpClient.Get(url)
// some header validation goes here
body, err := ioutil.ReadAll(response.Body)
defer response.Body.Close()
if err != nil {
errStr := fmt.Sprintf("Unable to read from body %s", err)
return nil, errors.New(errStr)
}
return body, nil
}
I'm assuming I can set up a fake server as such:
package req_test
func Test_GetContent_RequestBodyReadError(t *testing.T) {
handler := func(w http.ResponseWriter, r *http.Request) {
w.WriteHeader(http.StatusOK)
}
ts := httptest.NewServer(http.HandlerFunc(handler))
defer ts.Close()
_, err := GetContent(ts.URL)
if err != nil {
t.Log("Body read failed as expected.")
} else {
t.Fatalf("Method did not fail as expected")
}
}
I'm assuming I need to modify the ResposeWriter. Now, is there any way for me to modify the responseWriter and thereby force the ioutil.ReadAll in the wrapper to fail?
I realize that you seem to think it's a duplicate of this post and while you may believe so or it might be, just marking it as a duplicate doesn't really help me. The code provided in the answers in the "duplicate" post makes very little sense to me in this context.

Answer 1

Check the documentation of Response.Body to see when reading from it might return an error:
// Body represents the response body.
//
// The response body is streamed on demand as the Body field
// is read. If the network connection fails or the server
// terminates the response, Body.Read calls return an error.
//
// The http Client and Transport guarantee that Body is always
// non-nil, even on responses without a body or responses with
// a zero-length body. It is the caller's responsibility to
// close Body. The default HTTP client's Transport may not
// reuse HTTP/1.x "keep-alive" TCP connections if the Body is
// not read to completion and closed.
//
// The Body is automatically dechunked if the server replied
// with a "chunked" Transfer-Encoding.
Body io.ReadCloser
The easiest way is to generate an invalid HTTP response from the test handler.
How to do that? There are many ways, a simple one is to "lie" about the content length:
handler := func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Length", "1")
}
This handler tells it has 1 byte body, but actually it sends none. So at the other end (the client) when attempting to read 1 byte from it, obviously that won't succeed, and will result in the following error:
Unable to read from body unexpected EOF
See related question if you would need to simulate error reading from a request body (not from a response body): How do I test an error on reading from a request body?

Answer 2

To expand upon icza's awesome answer, you can also do this with an httptest.Server object:
bodyErrorServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Length", "1")
}))
defer bodyErrorServer.Close()
You can then pass bodyErrorServer.URL in your tests like normal, and you'll always get an EOF error:
package main
import (
"bytes"
"fmt"
"io/ioutil"
"net/http"
"net/http/httptest"
"testing"
"time"
)
func getBodyFromURL(service string, clientTimeout int) (string, error) {
var netClient = &http.Client{
Timeout: time.Duration(clientTimeout) * time.Millisecond,
}
rsp, err := netClient.Get(service)
if err != nil {
return "", err
}
defer rsp.Body.Close()
if rsp.StatusCode != 200 {
return "", fmt.Errorf("HTTP request error. Response code: %d", rsp.StatusCode)
}
buf, err := ioutil.ReadAll(rsp.Body)
if err != nil {
return "", err
}
return string(bytes.TrimSpace(buf)), nil
}
func TestBodyError(t *testing.T) {
bodyErrorServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Length", "1")
}))
_, err := getBodyFromURL(bodyErrorServer.URL, 1000)
if err.Error() != "unexpected EOF" {
t.Error("GOT AN ERROR")
} else if err == nil {
t.Error("GOT NO ERROR, THATS WRONG!")
} else {
t.Log("Got an unexpected EOF as expected, horray!")
}
}
Playground example here: https://play.golang.org/p/JzPmatibgZn