net/http/httputil: permit nil request body in ReverseProxy

Accepting a request with a nil body was never explicitly supported but
happened to work in the past.

This doesn't happen in most cases because usually people pass
a Server's incoming Request to the ReverseProxy's ServeHTTP method,
and incoming server requests are guaranteed to have non-nil bodies.

Still, it's a regression, so fix.

Fixes #12344

Change-Id: Id9a5a47aea3f2875d195b66c9a5f8581c4ca2aed
Reviewed-on: https://go-review.googlesource.com/13935
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -105,7 +105,7 @@ type requestCanceler interface {
 }
 
 type runOnFirstRead struct {
-	io.Reader
+	io.Reader // optional; nil means empty body
 
 	fn func() // Run before first Read, then set to nil
 }
@@ -115,6 +115,9 @@ func (c *runOnFirstRead) Read(bs []byte) (int, error) {
 		c.fn()
 		c.fn = nil
 	}
+	if c.Reader == nil {
+		return 0, io.EOF
+	}
 	return c.Reader.Read(bs)
 }
 

src/net/http/httputil/reverseproxy_test.go

@@ -7,6 +7,7 @@
 package httputil
 
 import (
+	"bufio"
 	"io/ioutil"
 	"log"
 	"net/http"
@@ -281,3 +282,41 @@ func TestReverseProxyCancellation(t *testing.T) {
 		t.Fatal("DefaultClient.Do() returned nil error")
 	}
 }
+
+func req(t *testing.T, v string) *http.Request {
+	req, err := http.ReadRequest(bufio.NewReader(strings.NewReader(v)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return req
+}
+
+// Issue 12344
+func TestNilBody(t *testing.T) {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("hi"))
+	}))
+	defer backend.Close()
+
+	frontend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		backURL, _ := url.Parse(backend.URL)
+		rp := NewSingleHostReverseProxy(backURL)
+		r := req(t, "GET / HTTP/1.0\r\n\r\n")
+		r.Body = nil // this accidentally worked in Go 1.4 and below, so keep it working
+		rp.ServeHTTP(w, r)
+	}))
+	defer frontend.Close()
+
+	res, err := http.Get(frontend.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	slurp, err := ioutil.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(slurp) != "hi" {
+		t.Errorf("Got %q; want %q", slurp, "hi")
+	}
+}