all: Support multiple device connections (fixes #141) (#8918)

This adds the ability to have multiple concurrent connections to a single device. This is primarily useful when the network has multiple physical links for aggregated bandwidth. A single connection will never see a higher rate than a single link can give, but multiple connections are load-balanced over multiple links.

It is also incidentally useful for older multi-core CPUs, where bandwidth could be limited by the TLS performance of a single CPU core -- using multiple connections achieves concurrency in the required crypto calculations...

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: tomasz1986 <twilczynski@naver.com>
Co-authored-by: bt90 <btom1990@googlemail.com>

lib/connections/structs.go

@@ -42,6 +42,7 @@ type internalConn struct {
 	isLocal       bool
 	priority      int
 	establishedAt time.Time
+	connectionID  string // set after Hello exchange
 }
 
 type connType int
@@ -88,12 +89,13 @@ func (t connType) Transport() string {
 }
 
 func newInternalConn(tc tlsConn, connType connType, isLocal bool, priority int) internalConn {
+	now := time.Now()
 	return internalConn{
 		tlsConn:       tc,
 		connType:      connType,
 		isLocal:       isLocal,
 		priority:      priority,
-		establishedAt: time.Now().Truncate(time.Second),
+		establishedAt: now.Truncate(time.Second),
 	}
 }
 
@@ -138,12 +140,16 @@ func (c internalConn) EstablishedAt() time.Time {
 	return c.establishedAt
 }
 
+func (c internalConn) ConnectionID() string {
+	return c.connectionID
+}
+
 func (c internalConn) String() string {
 	t := "WAN"
 	if c.isLocal {
 		t = "LAN"
 	}
-	return fmt.Sprintf("%s-%s/%s/%s/%s-P%d", c.LocalAddr(), c.RemoteAddr(), c.Type(), c.Crypto(), t, c.Priority())
+	return fmt.Sprintf("%s-%s/%s/%s/%s-P%d-%s", c.LocalAddr(), c.RemoteAddr(), c.Type(), c.Crypto(), t, c.Priority(), c.connectionID)
 }
 
 type dialerFactory interface {
@@ -160,6 +166,7 @@ type commonDialer struct {
 	lanChecker        *lanChecker
 	lanPriority       int
 	wanPriority       int
+	allowsMultiConns  bool
 }
 
 func (d *commonDialer) RedialFrequency() time.Duration {
@@ -173,10 +180,15 @@ func (d *commonDialer) Priority(host string) int {
 	return d.wanPriority
 }
 
+func (d *commonDialer) AllowsMultiConns() bool {
+	return d.allowsMultiConns
+}
+
 type genericDialer interface {
 	Dial(context.Context, protocol.DeviceID, *url.URL) (internalConn, error)
 	RedialFrequency() time.Duration
 	Priority(host string) int
+	AllowsMultiConns() bool
 }
 
 type listenerFactory interface {
@@ -212,10 +224,7 @@ type genericListener interface {
 type Model interface {
 	protocol.Model
 	AddConnection(conn protocol.Connection, hello protocol.Hello)
-	NumConnections() int
-	Connection(remoteID protocol.DeviceID) (protocol.Connection, bool)
 	OnHello(protocol.DeviceID, net.Addr, protocol.Hello) error
-	GetHello(protocol.DeviceID) protocol.HelloIntf
 	DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics, error)
 }