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

Fixes #8724

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

1 files changed, 8 insertions, 2 deletions

diff --git a/src/net/http/server.go b/src/net/http/server.go
index 8f2b777b29..7ad0bcbc20 100644
--- a/src/net/http/server.go
+++ b/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)