fix race in getOrHandshake (#400)

We missed this race with #396 (and I think this is also the crash in
issue #226). We need to lock a little higher in the getOrHandshake
method, before we reset hostinfo.ConnectionInfo. Previously, two
routines could enter this section and confuse the handshake process.

This could result in the other side sending a recv_error that also has
a race with setting hostinfo.ConnectionInfo back to nil. So we make sure
to grab the lock in handleRecvError as well.

Neither of these code paths are in the hot path (handling packets
between two hosts over an active tunnel) so there should be no
performance concerns.

inside.go

@@ -91,6 +91,17 @@ func (f *Interface) getOrHandshake(vpnIp uint32) *HostInfo {
 		return hostinfo
 	}
 
+	// Handshake is not ready, we need to grab the lock now before we start
+	// the handshake process
+	hostinfo.Lock()
+	defer hostinfo.Unlock()
+
+	// Double check, now that we have the lock
+	ci = hostinfo.ConnectionState
+	if ci != nil && ci.eKey != nil && ci.ready {
+		return hostinfo
+	}
+
 	if ci == nil {
 		// if we don't have a connection state, then send a handshake initiation
 		ci = f.newConnectionState(true, noise.HandshakeIX, []byte{}, 0)
@@ -102,10 +113,6 @@ func (f *Interface) getOrHandshake(vpnIp uint32) *HostInfo {
 	}
 
 	// If we have already created the handshake packet, we don't want to call the function at all.
-	if !hostinfo.HandshakeReady {
-		hostinfo.Lock()
-		defer hostinfo.Unlock()
-
 	if !hostinfo.HandshakeReady {
 		ixHandshakeStage0(f, vpnIp, hostinfo)
 		// FIXME: Maybe make XX selectable, but probably not since psk makes it nearly pointless for us.
@@ -120,7 +127,6 @@ func (f *Interface) getOrHandshake(vpnIp uint32) *HostInfo {
 			}
 		}
 	}
-	}
 
 	return hostinfo
 }

outside.go

@@ -329,6 +329,9 @@ func (f *Interface) handleRecvError(addr *udpAddr, h *Header) {
 		return
 	}
 
+	hostinfo.Lock()
+	defer hostinfo.Unlock()
+
 	if !hostinfo.RecvErrorExceeded() {
 		return
 	}