xmpp: proxy ConnectionState through wrapped conns

When we wrap a connection during stream negotiation (eg. to add
compression, or some other transformation), proxy through the
ConnectionState method. This way the TLS connection state isn't hidden
from the session which will need it to pass to SASL mechanisms that use
it.

See #45

Signed-off-by: Sam Whited <sam@samwhited.com>

conn.go

@@ -5,15 +5,23 @@
 package xmpp
 
 import (
+	"crypto/tls"
 	"io"
 	"net"
 	"time"
 )
 
+type tlsConn interface {
+	ConnectionState() tls.ConnectionState
+}
+
+var _ tlsConn = (*conn)(nil)
+
 // conn is a net.Conn created for the purpose of establishing an XMPP session.
 type conn struct {
-	c  net.Conn
-	rw io.ReadWriter
+	c         net.Conn
+	rw        io.ReadWriter
+	connState func() tls.ConnectionState
 }
 
 // newConn wraps an io.ReadWriter in a Conn.
@@ -25,10 +33,31 @@ func newConn(rw io.ReadWriter, prev net.Conn) net.Conn {
 		return c
 	}
 
-	nc := &conn{rw: rw, c: prev}
+	// Pull out a connection state function if possible.
+	tc, ok := rw.(tlsConn)
+	if !ok {
+		tc, _ = prev.(tlsConn)
+	}
+	var cs func() tls.ConnectionState
+	if tc != nil {
+		cs = tc.ConnectionState
+	}
+
+	nc := &conn{
+		rw:        rw,
+		c:         prev,
+		connState: cs,
+	}
 	return nc
 }
 
+func (c *conn) ConnectionState() tls.ConnectionState {
+	if c.connState == nil {
+		return tls.ConnectionState{}
+	}
+	return c.connState()
+}
+
 // Close closes the connection.
 func (c *conn) Close() error {
 	if c.c != nil {