net/http: fix double Content-Length in response

Fixes #8180

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/105040043

src/pkg/net/http/response.go

@@ -266,7 +266,10 @@ func (r *Response) Write(w io.Writer) error {
 		return err
 	}
 
-	if r1.ContentLength == 0 && !chunked(r1.TransferEncoding) {
+	// contentLengthAlreadySent may have been already sent for
+	// POST/PUT requests, even if zero length. See Issue 8180.
+	contentLengthAlreadySent := tw.shouldSendContentLength()
+	if r1.ContentLength == 0 && !chunked(r1.TransferEncoding) && !contentLengthAlreadySent {
 		if _, err := io.WriteString(w, "Content-Length: 0\r\n"); err != nil {
 			return err
 		}

src/pkg/net/http/responsewrite_test.go

@@ -191,6 +191,22 @@ func TestResponseWrite(t *testing.T) {
 				"Foo: Bar Baz\r\n" +
 				"\r\n",
 		},
+
+		// Want a single Content-Length header. Fixing issue 8180 where
+		// there were two.
+		{
+			Response{
+				StatusCode:       StatusOK,
+				ProtoMajor:       1,
+				ProtoMinor:       1,
+				Request:          &Request{Method: "POST"},
+				Header:           Header{},
+				ContentLength:    0,
+				TransferEncoding: nil,
+				Body:             nil,
+			},
+			"HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n",
+		},
 	}
 
 	for i := range respWriteTests {

src/pkg/net/http/transfer.go

@@ -155,7 +155,9 @@ func (t *transferWriter) WriteHeader(w io.Writer) error {
 	// function of the sanitized field triple (Body, ContentLength,
 	// TransferEncoding)
 	if t.shouldSendContentLength() {
-		io.WriteString(w, "Content-Length: ")
+		if _, err := io.WriteString(w, "Content-Length: "); err != nil {
+			return err
+		}
 		if _, err := io.WriteString(w, strconv.FormatInt(t.ContentLength, 10)+"\r\n"); err != nil {
 			return err
 		}