lib: Factor out getting IP address from net.Addr (#8538)

... and add fast paths for common cases.

lib/connections/quic_misc.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"github.com/lucas-clemente/quic-go"
+	"github.com/syncthing/syncthing/lib/osutil"
 )
 
 var (
@@ -65,9 +66,7 @@ func (q *quicTlsConn) ConnectionState() tls.ConnectionState {
 }
 
 func packetConnUnspecified(conn interface{}) bool {
-	// Since QUIC connections are wrapped, we can't do a simple typecheck
-	// on *net.UDPAddr here.
 	addr := conn.(net.PacketConn).LocalAddr()
-	host, _, err := net.SplitHostPort(addr.String())
-	return err == nil && net.ParseIP(host).IsUnspecified()
+	ip, err := osutil.IPFromAddr(addr)
+	return err == nil && ip.IsUnspecified()
 }

lib/connections/structs.go

@@ -18,6 +18,7 @@ import (
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/connections/registry"
 	"github.com/syncthing/syncthing/lib/nat"
+	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/stats"
 
@@ -117,14 +118,10 @@ func (c internalConn) Crypto() string {
 
 func (c internalConn) Transport() string {
 	transport := c.connType.Transport()
-	host, _, err := net.SplitHostPort(c.LocalAddr().String())
+	ip, err := osutil.IPFromAddr(c.LocalAddr())
 	if err != nil {
 		return transport
 	}
-	ip := net.ParseIP(host)
-	if ip == nil {
-		return transport
-	}
 	if ip.To4() != nil {
 		return transport + "4"
 	}