net/http/httptest: guarantee ResponseRecorder.Result returns a non-nil body

The doc for ResponseRecorder.Result guarantees that the body of the returned
http.Response will be non-nil, but this only holds true if the caller's body is
non-nil. With this change, if the caller's body is nil then the returned
response's body will be an empty io.ReadCloser.

Fixes #26442

Change-Id: I3b2fe4a2541caf9997dbb8978bbaf1f58cd1f471
GitHub-Last-Rev: d802967d89
GitHub-Pull-Request: golang/go#26453
Reviewed-on: https://go-review.googlesource.com/124875
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httptest/recorder.go

@@ -184,6 +184,8 @@ func (rw *ResponseRecorder) Result() *http.Response {
 	res.Status = fmt.Sprintf("%03d %s", res.StatusCode, http.StatusText(res.StatusCode))
 	if rw.Body != nil {
 		res.Body = ioutil.NopCloser(bytes.NewReader(rw.Body.Bytes()))
+	} else {
+		res.Body = http.NoBody
 	}
 	res.ContentLength = parseContentLength(res.Header.Get("Content-Length"))
 

src/net/http/httptest/recorder_test.go

@@ -7,6 +7,7 @@ package httptest
 import (
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net/http"
 	"testing"
 )
@@ -39,6 +40,19 @@ func TestRecorder(t *testing.T) {
 			return nil
 		}
 	}
+	hasResultContents := func(want string) checkFunc {
+		return func(rec *ResponseRecorder) error {
+			contentBytes, err := ioutil.ReadAll(rec.Result().Body)
+			if err != nil {
+				return err
+			}
+			contents := string(contentBytes)
+			if contents != want {
+				return fmt.Errorf("Result().Body = %s; want %s", contents, want)
+			}
+			return nil
+		}
+	}
 	hasContents := func(want string) checkFunc {
 		return func(rec *ResponseRecorder) error {
 			if rec.Body.String() != want {
@@ -273,6 +287,15 @@ func TestRecorder(t *testing.T) {
 			},
 			check(hasStatus(200), hasContents("Some body"), hasContentLength(9)),
 		},
+		{
+			"nil ResponseRecorder.Body", // Issue 26642
+			func(w http.ResponseWriter, r *http.Request) {
+				w.(*ResponseRecorder).Body = nil
+				io.WriteString(w, "hi")
+			},
+			check(hasResultContents("")), // check we don't crash reading the body
+
+		},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
 			r, _ := http.NewRequest("GET", "http://foo.com/", nil)