http: fix regression permitting io.Copy on HEAD response

With the ReadFrom change in the sendfile CL, it became
possible to illegally send a response to a HEAD request if you
did it via io.Copy.

Fixes #1939

R=rsc
CC=golang-dev
https://golang.org/cl/4584049

src/pkg/http/serve_test.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	. "http"
 	"http/httptest"
+	"io"
 	"io/ioutil"
 	"log"
 	"os"
@@ -495,6 +496,12 @@ func TestHeadResponses(t *testing.T) {
 		if err != ErrBodyNotAllowed {
 			t.Errorf("on Write, expected ErrBodyNotAllowed, got %v", err)
 		}
+
+		// Also exercise the ReaderFrom path
+		_, err = io.Copy(w, strings.NewReader("Ignored body"))
+		if err != ErrBodyNotAllowed {
+			t.Errorf("on Copy, expected ErrBodyNotAllowed, got %v", err)
+		}
 	}))
 	defer ts.Close()
 	res, err := Head(ts.URL)

src/pkg/http/server.go

@@ -129,7 +129,7 @@ func (r *response) ReadFrom(src io.Reader) (n int64, err os.Error) {
 	// WriteHeader if it hasn't been called yet, and WriteHeader
 	// is what sets r.chunking.
 	r.Flush()
-	if !r.chunking {
+	if !r.chunking && r.bodyAllowed() {
 		if rf, ok := r.conn.rwc.(io.ReaderFrom); ok {
 			n, err = rf.ReadFrom(src)
 			r.written += n
@@ -335,6 +335,15 @@ func (w *response) WriteHeader(code int) {
 	io.WriteString(w.conn.buf, "\r\n")
 }
 
+// bodyAllowed returns true if a Write is allowed for this response type.
+// It's illegal to call this before the header has been flushed.
+func (w *response) bodyAllowed() bool {
+	if !w.wroteHeader {
+		panic("")
+	}
+	return w.status != StatusNotModified && w.req.Method != "HEAD"
+}
+
 func (w *response) Write(data []byte) (n int, err os.Error) {
 	if w.conn.hijacked {
 		log.Print("http: response.Write on hijacked connection")
@@ -346,9 +355,7 @@ func (w *response) Write(data []byte) (n int, err os.Error) {
 	if len(data) == 0 {
 		return 0, nil
 	}
-
+	if !w.bodyAllowed() {
-	if w.status == StatusNotModified || w.req.Method == "HEAD" {
-		// Must not have body.
 		return 0, ErrBodyNotAllowed
 	}