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

... and add fast paths for common cases.

lib/osutil/net.go

@@ -0,0 +1,51 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package osutil
+
+import (
+	"net"
+)
+
+func GetLans() ([]*net.IPNet, error) {
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+	addrs := []net.Addr{}
+	for _, currentIf := range ifs {
+		if currentIf.Flags&net.FlagUp != net.FlagUp {
+			continue
+		}
+		currentAddrs, err := currentIf.Addrs()
+		if err != nil {
+			return nil, err
+		}
+		addrs = append(addrs, currentAddrs...)
+	}
+
+	nets := make([]*net.IPNet, 0, len(addrs))
+
+	for _, addr := range addrs {
+		net, ok := addr.(*net.IPNet)
+		if ok {
+			nets = append(nets, net)
+		}
+	}
+	return nets, nil
+}
+
+func IPFromAddr(addr net.Addr) (net.IP, error) {
+	switch a := addr.(type) {
+	case *net.TCPAddr:
+		return a.IP, nil
+	case *net.UDPAddr:
+		return a.IP, nil
+	default:
+		host, _, err := net.SplitHostPort(addr.String())
+		return net.ParseIP(host), err
+	}
+}