Drain inbound traffic before closing to avoid TCP RST data loss

Half-close with CloseWrite() and drain with io.Copy before conn.Close()
so late peer packets don't trigger RST. Also fix a pre-existing data
race in Ping() where s.pingId was read outside pingLock.

Signed-off-by: Andreas Erz <andreas.erz@sap.com>

Author: anluerz

connection.go

@@ -280,8 +280,8 @@ func NewConnectionWithOptions(conn net.Conn, server bool, opts ...spdy.FramerOpt
 // Ping sends a ping frame across the connection and
 // returns the response time
 func (s *Connection) Ping() (time.Duration, error) {
-	pid := s.pingId
 	s.pingLock.Lock()
+	pid := s.pingId
 	if s.pingId > 0x7ffffffe {
 		s.pingId = s.pingId - 0x7ffffffe
 	} else {
@@ -740,6 +740,19 @@ func (s *Connection) shutdown(closeTimeout time.Duration) {
 	var err error
 	select {
 	case <-streamsClosed:
+		// Drain inbound traffic before closing so the kernel sees FIN, not RST.
+		// If a peer packet (e.g. a SPDY PING) arrives at our socket after Close(),
+		// the kernel responds with RST and discards anything still queued in OUR
+		// send buffer. i.e. bytes the application already wrote and counted as sent
+		// never reach the wire. Half-closing first lets the peer drain and stop
+		// sending; io.Copy consumes anything already in flight.
+		if cw, ok := s.conn.(interface{ CloseWrite() error }); ok {
+			if err := cw.CloseWrite(); err == nil {
+				_ = s.conn.SetReadDeadline(time.Now().Add(10 * time.Second))
+				n, _ := io.Copy(io.Discard, s.conn)
+				debugMessage("(%p) drained %d bytes during shutdown", s, n)
+			}
+		}
 		// No active streams, close should be safe
 		err = s.conn.Close()
 	case <-timeout: