[client] Wait for the overlay address before binding the file drop receiver

iOS applies the tunnel address out of band, after handing the engine the
tun fd, so the receiver bound to that address a moment too early and failed
with EADDRNOTAVAIL. Nothing retried it, leaving the peer able to send but
never to receive for the life of the connection.

Wait for the address to appear and bind then, and let a rebind start a
receiver that is down rather than skipping it. Only the subsystems that
asked to be bound are bound, so the SSH sessions and DNS queries that other
listeners are carrying are left alone.

The wait itself is iOS-only: every other platform assigns the address in the
call chain that creates the interface, or is handed one that already carries
it, so there it compiles down to nothing.

client/internal/engine_filedrop.go

@@ -15,6 +15,8 @@ import (
 	"github.com/netbirdio/netbird/client/internal/peer"
 )
 
+const fileDropWatchName = "the file drop receiver"
+
 type filedropResolver struct {
 	status *peer.Status
 }
@@ -45,6 +47,12 @@ func (e *Engine) startFileDrop() {
 	}
 
 	wgAddr := e.wgInterface.Address()
+	if !e.overlayAddrReady(wgAddr.IP) {
+		log.Infof("file drop receiver waits for the overlay address %s", wgAddr.IP)
+		e.armOverlayWatch(fileDropWatchName, e.restartFileDrop)
+		return
+	}
+
 	addr := netip.AddrPortFrom(wgAddr.IP, fileDropListenPort())
 	resolver := filedropResolver{status: e.statusRecorder}
 
@@ -63,6 +71,7 @@ func (e *Engine) startFileDrop() {
 	if v6 := wgAddr.IPv6; v6.IsValid() {
 		if err := e.fileDrop.AddReceiverListener(e.ctx, netip.AddrPortFrom(v6, bound)); err != nil {
 			log.Warnf("failed to add IPv6 file drop listener: %v", err)
+			e.armOverlayWatch(fileDropWatchName, e.restartFileDrop)
 		}
 	}
 
@@ -142,13 +151,19 @@ func (e *Engine) setFileDropTunnel() {
 	e.fileDrop.SetTunnel(dial, e.statusRecorder.GetLocalPeerState().FQDN)
 }
 
-// restartFileDrop rebinds the receiver after the platform replaced the tunnel
-// device. The listeners are bound to the overlay address of the interface being
-// swapped out and do not survive it: Android renews the tun on every route
-// change, which leaves the IPv4 listener dead with accept4: invalid argument.
-// No-op when it is not running. See Engine.rebindOverlayListeners.
+// restartFileDrop gives the receiver listeners on the interface as it is now.
+// The listeners are bound to the overlay address of the interface being swapped
+// out and do not survive it: Android renews the tun on every route change, which
+// leaves the IPv4 listener dead with accept4: invalid argument. A receiver that
+// is not running is started rather than skipped, since the reason it is down may
+// be the very bind this rebind can now make. See Engine.rebindOverlayListeners.
 func (e *Engine) restartFileDrop() error {
-	if e.fileDrop == nil || !e.fileDropRunning || e.wgInterface == nil {
+	if e.fileDrop == nil || e.wgInterface == nil {
+		return nil
+	}
+
+	if !e.fileDropRunning {
+		e.startFileDrop()
 		return nil
 	}