Support monitoring tethered interface

This would be useful to be used in together with Instant Tethering + Turn off hotspot automatically.

Refine #26, #53.

mobile/src/main/java/be/mygod/vpnhotspot/net/Routing.kt

@@ -41,6 +41,11 @@ class Routing(private val caller: Any, private val downstream: String) : IpNeigh
          */
         val IPTABLES = if (Build.VERSION.SDK_INT >= 26) "iptables -w 1" else "iptables -w"
 
+        /**
+         * For debugging: check that we do not start a Routing for the same interface twice.
+         */
+        private var downstreams = mutableSetOf<String>()
+
         fun clean() {
             TrafficRecorder.clean()
             RootSession.use {
@@ -54,6 +59,7 @@ class Routing(private val caller: Any, private val downstream: String) : IpNeigh
                 it.execQuiet("while ip rule del priority $RULE_PRIORITY_UPSTREAM; do done")
                 it.execQuiet("while ip rule del priority $RULE_PRIORITY_UPSTREAM_FALLBACK; do done")
             }
+            downstreams.clear()
         }
 
         private fun RootSession.Transaction.iptables(command: String, revert: String) {
@@ -82,6 +88,10 @@ class Routing(private val caller: Any, private val downstream: String) : IpNeigh
         override val message: String get() = app.getString(R.string.exception_interface_not_found)
     }
 
+    init {
+        check(downstreams.add(downstream)) { "Double routing detected" }
+    }
+
     private val hostAddress = try {
         NetworkInterface.getByName(downstream)!!.interfaceAddresses!!.asSequence().single { it.address is Inet4Address }
     } catch (e: Exception) {
@@ -302,5 +312,6 @@ class Routing(private val caller: Any, private val downstream: String) : IpNeigh
         fallbackUpstream.subrouting?.transaction?.revert()
         upstream.subrouting?.transaction?.revert()
         transaction.revert()
+        check(downstreams.remove(downstream)) { "Double reverting detected" }
     }
 }