net/http: check for CloseWrite interface, not TCPConn implementation

Fixes #8724

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/148040043

src/net/http/export_test.go

@@ -77,3 +77,7 @@ var DefaultUserAgent = defaultUserAgent
 func SetPendingDialHooks(before, after func()) {
 	prePendingDial, postPendingDial = before, after
 }
+
+var ExportServerNewConn = (*Server).newConn
+
+var ExportCloseWriteAndWait = (*conn).closeWriteAndWait

src/net/http/serve_test.go

@@ -2607,6 +2607,29 @@ func TestServerConnStateNew(t *testing.T) {
 	}
 }
 
+type closeWriteTestConn struct {
+	rwTestConn
+	didCloseWrite bool
+}
+
+func (c *closeWriteTestConn) CloseWrite() error {
+	c.didCloseWrite = true
+	return nil
+}
+
+func TestCloseWrite(t *testing.T) {
+	var srv Server
+	var testConn closeWriteTestConn
+	c, err := ExportServerNewConn(&srv, &testConn)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ExportCloseWriteAndWait(c)
+	if !testConn.didCloseWrite {
+		t.Error("didn't see CloseWrite call")
+	}
+}
+
 func BenchmarkClientServer(b *testing.B) {
 	b.ReportAllocs()
 	b.StopTimer()

src/net/http/server.go

@@ -1064,15 +1064,21 @@ func (c *conn) close() {
 // This timeout is somewhat arbitrary (~latency around the planet).
 const rstAvoidanceDelay = 500 * time.Millisecond
 
+type closeWriter interface {
+	CloseWrite() error
+}
+
+var _ closeWriter = (*net.TCPConn)(nil)
+
 // closeWrite flushes any outstanding data and sends a FIN packet (if
 // client is connected via TCP), signalling that we're done.  We then
-// pause for a bit, hoping the client processes it before `any
+// pause for a bit, hoping the client processes it before any
 // subsequent RST.
 //
 // See http://golang.org/issue/3595
 func (c *conn) closeWriteAndWait() {
 	c.finalFlush()
-	if tcp, ok := c.rwc.(*net.TCPConn); ok {
+	if tcp, ok := c.rwc.(closeWriter); ok {
 		tcp.CloseWrite()
 	}
 	time.Sleep(rstAvoidanceDelay)