net/http: pass through server side Transfer-Encoding headers

Fixes #16063

Change-Id: I2e8695beb657b0aef067e83f086828d8857787ed
Reviewed-on: https://go-review.googlesource.com/24130
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -9,6 +9,7 @@ package http_test
 import (
 	"bufio"
 	"bytes"
+	"compress/gzip"
 	"context"
 	"crypto/tls"
 	"encoding/json"
@@ -4200,6 +4201,24 @@ func TestHandlerSetTransferEncodingChunked(t *testing.T) {
 	}
 }
 
+// https://golang.org/issue/16063
+func TestHandlerSetTransferEncodingGzip(t *testing.T) {
+	defer afterTest(t)
+	ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Transfer-Encoding", "gzip")
+		gz := gzip.NewWriter(w)
+		gz.Write([]byte("hello"))
+		gz.Close()
+	}))
+	resp := ht.rawResponse("GET / HTTP/1.1\nHost: foo")
+	for _, v := range []string{"gzip", "chunked"} {
+		hdr := "Transfer-Encoding: " + v
+		if n := strings.Count(resp, hdr); n != 1 {
+			t.Errorf("want 1 occurrence of %q in response, got %v\nresponse: %v", hdr, n, resp)
+		}
+	}
+}
+
 func BenchmarkClientServer(b *testing.B) {
 	b.ReportAllocs()
 	b.StopTimer()

src/net/http/server.go

@@ -1147,7 +1147,10 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 			// to avoid closing the connection at EOF.
 			cw.chunking = true
 			setHeader.transferEncoding = "chunked"
-			delHeader("Transfer-Encoding")
+			if hasTE && te == "chunked" {
+				// We will send the chunked Transfer-Encoding header later.
+				delHeader("Transfer-Encoding")
+			}
 		}
 	} else {
 		// HTTP version < 1.1: cannot do chunked transfer