net/http: enable Transfer-Encoding: identity without Content-Length for HTTP 1.1.

Use case is SSE recommended configuration: http://www.w3.org/TR/eventsource/#notes
Removes a TODO.

LGTM=bradfitz
R=golang-codereviews, bradfitz, tommi.virtanen
CC=golang-codereviews
https://golang.org/cl/100000044

src/net/http/serve_test.go

@@ -778,6 +778,35 @@ func TestChunkedResponseHeaders(t *testing.T) {
 	}
 }
 
+func TestIdentityResponseHeaders(t *testing.T) {
+	defer afterTest(t)
+	log.SetOutput(ioutil.Discard) // is noisy otherwise
+	defer log.SetOutput(os.Stderr)
+
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Transfer-Encoding", "identity")
+		w.(Flusher).Flush()
+		fmt.Fprintf(w, "I am an identity response.")
+	}))
+	defer ts.Close()
+
+	res, err := Get(ts.URL)
+	if err != nil {
+		t.Fatalf("Get error: %v", err)
+	}
+	defer res.Body.Close()
+
+	if g, e := res.TransferEncoding, []string(nil); !reflect.DeepEqual(g, e) {
+		t.Errorf("expected TransferEncoding of %v; got %v", e, g)
+	}
+	if _, haveCL := res.Header["Content-Length"]; haveCL {
+		t.Errorf("Unexpected Content-Length")
+	}
+	if !res.Close {
+		t.Errorf("expected Connection: close; got %v", res.Close)
+	}
+}
+
 // Test304Responses verifies that 304s don't declare that they're
 // chunking in their response headers and aren't allowed to produce
 // output.

src/net/http/server.go

@@ -839,13 +839,20 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 	} else if hasCL {
 		delHeader("Transfer-Encoding")
 	} else if w.req.ProtoAtLeast(1, 1) {
-		// HTTP/1.1 or greater: use chunked transfer encoding
-		// to avoid closing the connection at EOF.
-		// TODO: this blows away any custom or stacked Transfer-Encoding they
-		// might have set.  Deal with that as need arises once we have a valid
-		// use case.
-		cw.chunking = true
-		setHeader.transferEncoding = "chunked"
+		// HTTP/1.1 or greater: Transfer-Encoding has been set to identity,  and no
+		// content-length has been provided. The connection must be closed after the
+		// reply is written, and no chunking is to be done. This is the setup
+		// recommended in the Server-Sent Events candidate recommendation 11,
+		// section 8.
+		if hasTE && te == "identity" {
+			cw.chunking = false
+			w.closeAfterReply = true
+		} else {
+			// HTTP/1.1 or greater: use chunked transfer encoding
+			// to avoid closing the connection at EOF.
+			cw.chunking = true
+			setHeader.transferEncoding = "chunked"
+		}
 	} else {
 		// HTTP version < 1.1: cannot do chunked transfer
 		// encoding and we don't know the Content-Length so