Clear some memory when the system is running low

Closes #209

app/src/main/java/com/frostnerd/smokescreen/service/DnsVpnService.kt

@@ -3,10 +3,7 @@ package com.frostnerd.smokescreen.service
 import android.app.NotificationManager
 import android.app.PendingIntent
 import android.app.Service
-import android.content.BroadcastReceiver
-import android.content.Context
-import android.content.Intent
-import android.content.SharedPreferences
+import android.content.*
 import android.content.pm.PackageManager
 import android.net.*
 import android.os.*
@@ -98,6 +95,8 @@ class DnsVpnService : VpnService(), Runnable {
     private var lastVPNStopTime:Long? = null
     private val coroutineScope:CoroutineContext = SupervisorJob()
     private var queryCount = 0
+    private var dnsCache:SimpleDnsCache? = null
+    private var localResolver:LocalResolver? = null
     private val addressResolveScope:CoroutineScope by lazy {
         CoroutineScope(newSingleThreadContext("service-resolve-retry"))
     }
@@ -525,6 +524,15 @@ class DnsVpnService : VpnService(), Runnable {
         return if (destroyed) Service.START_NOT_STICKY else Service.START_STICKY
     }
 
+    override fun onTrimMemory(level: Int) {
+        if(level >= ComponentCallbacks2.TRIM_MEMORY_BACKGROUND ||
+                level == ComponentCallbacks2.TRIM_MEMORY_RUNNING_CRITICAL ||
+                level == ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW) {
+            dnsCache?.clear()
+            localResolver?.cleanup()
+        }
+    }
+
     private fun setServerConfiguration(intent: Intent?) {
         log("Updating server configuration..")
         userServerConfig = BackgroundVpnConfigureActivity.readServerInfoFromIntent(intent)
@@ -1020,12 +1028,15 @@ class DnsVpnService : VpnService(), Runnable {
         }
         log("Creating DNS proxy with ${1 + handles.size} handles")
 
+        dnsCache = createDnsCache()
+        localResolver = createLocalResolver()
+
         dnsProxy = SmokeProxy(
             defaultHandle!!,
             handles + createProxyBypassHandlers(),
-            createDnsCache(),
+            dnsCache,
             createQueryLogger(),
-            createLocalResolver()
+            localResolver
         )
         log("DnsProxy created, creating VPN proxy")
         vpnProxy = RetryingVPNTunnelProxy(dnsProxy!!, vpnService = this, coroutineScope = CoroutineScope(

app/src/main/java/com/frostnerd/smokescreen/util/proxy/DnsRuleResolver.kt

@@ -236,7 +236,13 @@ class DnsRuleResolver(context: Context) : LocalResolver(false) {
         } ?: throw IllegalStateException()
     }
 
-    override fun cleanup() {}
+    override fun cleanup() {
+        cachedWildcardWhitelisted.clear()
+        cachedNonWildcardWhitelisted.clear()
+        cachedResolved.clear()
+        cachedWildcardResolved.clear()
+        cachedNonIncluded.clear()
+    }
 
     // Handle CNAME Cloaking
     // Does not need to handle whitelist as the query has already been forwarded