[release-branch.go1.16] net/http: do not send Transfer-Encoding: identity in responses

Server handlers may set a "Transfer-Encoding: identity" header on
responses to disable chunking, but this header should not be sent
on the wire.

For #49194.
Fixes #49567.

Change-Id: I46a9e3b8ff9d93edd7d1c34d264fc309fa322ad5
Reviewed-on: https://go-review.googlesource.com/c/go/+/359176
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit b69b2f63d6)
Reviewed-on: https://go-review.googlesource.com/c/go/+/368086
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/net/http/clientserver_test.go

@@ -1582,3 +1582,37 @@ func TestH12_WebSocketUpgrade(t *testing.T) {
 		},
 	}.run(t)
 }
+
+func TestIdentityTransferEncoding_h1(t *testing.T) { testIdentityTransferEncoding(t, h1Mode) }
+func TestIdentityTransferEncoding_h2(t *testing.T) { testIdentityTransferEncoding(t, h2Mode) }
+
+func testIdentityTransferEncoding(t *testing.T, h2 bool) {
+	setParallel(t)
+	defer afterTest(t)
+
+	const body = "body"
+	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
+		gotBody, _ := io.ReadAll(r.Body)
+		if got, want := string(gotBody), body; got != want {
+			t.Errorf("got request body = %q; want %q", got, want)
+		}
+		w.Header().Set("Transfer-Encoding", "identity")
+		w.WriteHeader(StatusOK)
+		w.(Flusher).Flush()
+		io.WriteString(w, body)
+	}))
+	defer cst.close()
+	req, _ := NewRequest("GET", cst.ts.URL, strings.NewReader(body))
+	res, err := cst.c.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	gotBody, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := string(gotBody), body; got != want {
+		t.Errorf("got response body = %q; want %q", got, want)
+	}
+}

src/net/http/server.go

@@ -1407,11 +1407,11 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 		hasCL = false
 	}
 
-	if w.req.Method == "HEAD" || !bodyAllowedForStatus(code) {
-		// do nothing
-	} else if code == StatusNoContent {
+	if w.req.Method == "HEAD" || !bodyAllowedForStatus(code) || code == StatusNoContent {
+		// Response has no body.
 		delHeader("Transfer-Encoding")
 	} else if hasCL {
+		// Content-Length has been provided, so no chunking is to be done.
 		delHeader("Transfer-Encoding")
 	} else if w.req.ProtoAtLeast(1, 1) {
 		// HTTP/1.1 or greater: Transfer-Encoding has been set to identity, and no
@@ -1422,6 +1422,7 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 		if hasTE && te == "identity" {
 			cw.chunking = false
 			w.closeAfterReply = true
+			delHeader("Transfer-Encoding")
 		} else {
 			// HTTP/1.1 or greater: use chunked transfer encoding
 			// to avoid closing the connection at EOF.