net/http: fix EOF handling on response body

http.Response is currently returning 0, nil on EOF.

R=golang-dev, bradfitz, bradfitz
CC=golang-dev
https://golang.org/cl/5394047

src/pkg/net/http/client_test.go

@@ -26,6 +26,31 @@ var robotsTxtHandler = HandlerFunc(func(w ResponseWriter, r *Request) {
 	fmt.Fprintf(w, "User-agent: go\nDisallow: /something/")
 })
 
+// pedanticReadAll works like ioutil.ReadAll but additionally
+// verifies that r obeys the documented io.Reader contract.
+func pedanticReadAll(r io.Reader) (b []byte, err error) {
+	var bufa [64]byte
+	buf := bufa[:]
+	for {
+		n, err := r.Read(buf)
+		if n == 0 && err == nil {
+			return nil, fmt.Errorf("Read: n=0 with err=nil")
+		}
+		b = append(b, buf[:n]...)
+		if err == io.EOF {
+			n, err := r.Read(buf)
+			if n != 0 || err != io.EOF {
+				return nil, fmt.Errorf("Read: n=%d err=%#v after EOF", n, err)
+			}
+			return b, nil
+		}
+		if err != nil {
+			return b, err
+		}
+	}
+	panic("unreachable")
+}
+
 func TestClient(t *testing.T) {
 	ts := httptest.NewServer(robotsTxtHandler)
 	defer ts.Close()
@@ -33,7 +58,7 @@ func TestClient(t *testing.T) {
 	r, err := Get(ts.URL)
 	var b []byte
 	if err == nil {
-		b, err = ioutil.ReadAll(r.Body)
+		b, err = pedanticReadAll(r.Body)
 		r.Body.Close()
 	}
 	if err != nil {

src/pkg/net/http/transfer.go

@@ -537,7 +537,9 @@ func (b *body) Read(p []byte) (n int, err error) {
 
 	// Read the final trailer once we hit EOF.
 	if err == io.EOF && b.hdr != nil {
-		err = b.readTrailer()
+		if e := b.readTrailer(); e != nil {
+			err = e
+		}
 		b.hdr = nil
 	}
 	return n, err