android: whitelist - working recheck backoff, fewer probe rounds

- scheduled rechecks are forced: the 15/30 s steps were below FRESH_MS,
  so the unforced recheck returned early and the backoff chain died
- signal-strength-only capability changes are no longer network events
  (each one reset the backoff and launched a check)
- tunnel state transitions force a new round at most every 15 s

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/frontends/android/app/src/main/java/org/strongswan/android/logic/connectivity/WhiteListManager.kt

@@ -44,6 +44,7 @@ import org.json.JSONArray
 import org.json.JSONException
 import org.json.JSONObject
 import org.strongswan.android.logic.VpnStateService
+import java.util.concurrent.ConcurrentHashMap
 
 /** Whether the ISP currently blocks direct access to foreign hosts. */
 enum class WhiteListStatus
@@ -177,6 +178,9 @@ class WhiteListManager private constructor(private val context: Context) : VpnSt
 	private val _state = MutableStateFlow(WhiteListState())
 	val state: StateFlow<WhiteListState> = _state
 
+	/* last classification-relevant capabilities per network (see capabilityKey) */
+	private val lastCapabilities = ConcurrentHashMap<Network, String>()
+
 	/* event trigger: the underlying network changed, the ISP may classify differently now */
 	private val networkCallback = object : ConnectivityManager.NetworkCallback()
 	{
@@ -186,12 +190,19 @@ class WhiteListManager private constructor(private val context: Context) : VpnSt
 		}
 
 		override fun onCapabilitiesChanged(network: Network, capabilities: NetworkCapabilities)
-		{
+		{	/* fired on every signal-strength change too: only a change that
-			onNetworkEvent()
+			 * can alter the ISP's classification counts as an event (it also
+			 * resets the recheck backoff) */
+			val key = capabilityKey(capabilities)
+			if (lastCapabilities.put(network, key) != key)
+			{
+				onNetworkEvent()
+			}
 		}
 
 		override fun onLost(network: Network)
 		{
+			lastCapabilities.remove(network)
 			onNetworkEvent()
 		}
 	}
@@ -238,6 +249,7 @@ class WhiteListManager private constructor(private val context: Context) : VpnSt
 		recheckJob?.cancel()
 		recheckJob = null
 		unregisterNetworkCallback()
+		lastCapabilities.clear()
 		service?.unregisterListener(this)
 		try
 		{
@@ -364,7 +376,10 @@ class WhiteListManager private constructor(private val context: Context) : VpnSt
 			return
 		}
 		backoffAttempt = 0
-		scope.launch { runCheck(force = true) }
+		/* a flapping tunnel (CONNECTING -> DISCONNECTING -> DISABLED -> ...)
+		 * must not trigger a fresh direct probe round on every transition */
+		val force = SystemClock.elapsedRealtime() - lastCheckElapsed >= TRANSITION_MIN_GAP_MS
+		scope.launch { runCheck(force = force) }
 	}
 
 	private fun onNetworkEvent()
@@ -453,7 +468,9 @@ class WhiteListManager private constructor(private val context: Context) : VpnSt
 		recheckJob?.cancel()
 		recheckJob = scope.launch {
 			delay(delayMs)
-			runCheck(force = false)
+			/* forced: the first backoff steps are shorter than FRESH_MS, an
+			 * unforced recheck returned early and ended the chain */
+			runCheck(force = true)
 		}
 	}
 
@@ -598,6 +615,22 @@ class WhiteListManager private constructor(private val context: Context) : VpnSt
 		/** Cached results younger than this are not re-probed. */
 		private const val FRESH_MS = 60_000L
 
+		/** State transitions force a new round only if the last one is at least this old. */
+		private const val TRANSITION_MIN_GAP_MS = 15_000L
+
+		/** The parts of the capabilities that matter for the classification. */
+		@JvmStatic
+		internal fun capabilityKey(caps: NetworkCapabilities): String
+		{
+			val transports = intArrayOf(NetworkCapabilities.TRANSPORT_WIFI, NetworkCapabilities.TRANSPORT_CELLULAR,
+				NetworkCapabilities.TRANSPORT_ETHERNET, NetworkCapabilities.TRANSPORT_VPN)
+				.filter { caps.hasTransport(it) }
+			val capabilities = intArrayOf(NetworkCapabilities.NET_CAPABILITY_INTERNET,
+				NetworkCapabilities.NET_CAPABILITY_VALIDATED, NetworkCapabilities.NET_CAPABILITY_NOT_METERED)
+				.filter { caps.hasCapability(it) }
+			return "$transports/$capabilities"
+		}
+
 		/**
 		 * A cached ACTIVE status stops suppressing reconnects once no recheck
 		 * has confirmed it for this long (fail open). Kept above the backoff