How to force error on reading response body

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