refactor(beacon, osutil, upnp, netutil): only use anet on Android (#10211)

Add a wrapper that uses anet on Android, but net on other platforms.

### Purpose

Fixes
https://forum.syncthing.net/t/workaround-for-android-local-discovery/20403/12

### Testing

Run two Syncthing instances with Global Discovery disabled. Pair them
with each other, don't hardcode their addresses, and verify they
connect.

lib/beacon/broadcast.go

@@ -11,7 +11,7 @@ import (
 	"net"
 	"time"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 )
 
 func NewBroadcast(port int) Interface {
@@ -46,7 +46,7 @@ func writeBroadcasts(ctx context.Context, inbox <-chan []byte, port int) error {
 			return doneCtx.Err()
 		}
 
-		intfs, err := anet.Interfaces()
+		intfs, err := netutil.Interfaces()
 		if err != nil {
 			l.Debugln("Failed to list interfaces:", err)
 			// net.Interfaces() is broken on Android. see https://github.com/golang/go/issues/40569
@@ -61,7 +61,7 @@ func writeBroadcasts(ctx context.Context, inbox <-chan []byte, port int) error {
 				continue
 			}
 
-			addrs, err := anet.InterfaceAddrsByInterface(&intf)
+			addrs, err := netutil.InterfaceAddrsByInterface(&intf)
 			if err != nil {
 				l.Debugln("Failed to list interface addresses:", err)
 				// Interface discovery might work while retrieving the addresses doesn't. So log the error and carry on.

lib/beacon/multicast.go

@@ -12,7 +12,7 @@ import (
 	"net"
 	"time"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 
 	"golang.org/x/net/ipv6"
 )
@@ -61,7 +61,7 @@ func writeMulticasts(ctx context.Context, inbox <-chan []byte, addr string) erro
 			return doneCtx.Err()
 		}
 
-		intfs, err := anet.Interfaces()
+		intfs, err := netutil.Interfaces()
 		if err != nil {
 			l.Debugln(err)
 			return err
@@ -119,7 +119,7 @@ func readMulticasts(ctx context.Context, outbox chan<- recv, addr string) error
 		conn.Close()
 	}()
 
-	intfs, err := anet.Interfaces()
+	intfs, err := netutil.Interfaces()
 	if err != nil {
 		l.Debugln(err)
 		return err

lib/netutil/interfaces_android.go

@@ -0,0 +1,21 @@
+// Copyright (C) 2025 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 netutil
+
+import (
+	"net"
+
+	"github.com/wlynxg/anet"
+)
+
+func Interfaces() ([]net.Interface, error) {
+	return anet.Interfaces()
+}
+
+func InterfaceAddrsByInterface(intf *net.Interface) ([]net.Addr, error) {
+	return anet.InterfaceAddrsByInterface(intf)
+}

lib/netutil/interfaces_other.go

@@ -0,0 +1,19 @@
+// Copyright (C) 2025 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/.
+
+//go:build !android
+
+package netutil
+
+import "net"
+
+func Interfaces() ([]net.Interface, error) {
+	return net.Interfaces()
+}
+
+func InterfaceAddrsByInterface(intf *net.Interface) ([]net.Addr, error) {
+	return intf.Addrs()
+}

lib/osutil/net.go

@@ -10,13 +10,13 @@ import (
 	"net"
 	"strings"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 )
 
 // GetInterfaceAddrs returns the IP networks of all interfaces that are up.
 // Point-to-point interfaces are exluded unless includePtP is true.
 func GetInterfaceAddrs(includePtP bool) ([]*net.IPNet, error) {
-	intfs, err := anet.Interfaces()
+	intfs, err := netutil.Interfaces()
 	if err != nil {
 		return nil, err
 	}
@@ -33,7 +33,7 @@ func GetInterfaceAddrs(includePtP bool) ([]*net.IPNet, error) {
 			// which, for our purposes, do not qualify as LANs.
 			continue
 		}
-		intfAddrs, err := anet.InterfaceAddrsByInterface(&intf)
+		intfAddrs, err := netutil.InterfaceAddrsByInterface(&intf)
 		if err != nil {
 			return nil, err
 		}

lib/upnp/igd_service.go

@@ -40,7 +40,7 @@ import (
 	"net"
 	"time"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 
 	"github.com/syncthing/syncthing/lib/nat"
 )
@@ -67,7 +67,7 @@ func (s *IGDService) AddPinhole(ctx context.Context, protocol nat.Protocol, intA
 		return nil, errors.New("no interface")
 	}
 
-	addrs, err := anet.InterfaceAddrsByInterface(s.Interface)
+	addrs, err := netutil.InterfaceAddrsByInterface(s.Interface)
 	if err != nil {
 		return nil, err
 	}

lib/upnp/upnp.go

@@ -48,7 +48,7 @@ import (
 	"sync"
 	"time"
 
-	"github.com/wlynxg/anet"
+	"github.com/syncthing/syncthing/lib/netutil"
 
 	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/dialer"
@@ -106,7 +106,7 @@ const (
 func Discover(ctx context.Context, _, timeout time.Duration) []nat.Device {
 	var results []nat.Device
 
-	interfaces, err := anet.Interfaces()
+	interfaces, err := netutil.Interfaces()
 	if err != nil {
 		l.Infoln("Listing network interfaces:", err)
 		return results
@@ -388,7 +388,7 @@ func parseResponse(ctx context.Context, deviceType string, addr *net.UDPAddr, re
 }
 
 func localIPv4(netInterface *net.Interface) (net.IP, error) {
-	addrs, err := anet.InterfaceAddrsByInterface(netInterface)
+	addrs, err := netutil.InterfaceAddrsByInterface(netInterface)
 	if err != nil {
 		return nil, err
 	}
@@ -631,7 +631,7 @@ func soapRequestWithIP(ctx context.Context, url, service, function, message stri
 }
 
 func interfaceHasGUAIPv6(intf net.Interface) (bool, error) {
-	addrs, err := anet.InterfaceAddrsByInterface(&intf)
+	addrs, err := netutil.InterfaceAddrsByInterface(&intf)
 	if err != nil {
 		return false, err
 	}