Prioritize Android WebSocket downlink packets

clients/android/app/build.gradle

@@ -30,8 +30,8 @@ android {
         applicationId "su.cin.rapvpn"
         minSdk 26
         targetSdk 35
-        versionCode 201
-        versionName "0.2.201"
+        versionCode 202
+        versionName "0.2.202"
         buildConfigField "String", "DEFAULT_BACKEND_URL", "\"${normalizeGradleString(defaultBackendUrl)}\""
         buildConfigField "String", "DEFAULT_CLUSTER_ID", "\"${normalizeGradleString(defaultClusterId)}\""
         buildConfigField "String", "DEFAULT_ORGANIZATION_ID", "\"${normalizeGradleString(defaultOrganizationId)}\""

clients/android/app/src/main/java/su/cin/rapvpn/VpnPacketWebSocketRelay.java

@@ -31,6 +31,7 @@ final class VpnPacketWebSocketRelay {
     private final VpnService vpnService;
     private final OkHttpClient httpClient;
     private final FabricServiceChannel fabricServiceChannel;
+    private final BlockingQueue<List<byte[]>> priorityIncoming = new ArrayBlockingQueue<>(512);
     private final BlockingQueue<List<byte[]>> incoming = new ArrayBlockingQueue<>(2048);
     private final Object lock = new Object();
 
@@ -153,7 +154,19 @@ final class VpnPacketWebSocketRelay {
         connect(clusterId, vpnConnectionId);
         awaitOpen(Math.min(OPEN_WAIT_MS, Math.max(1, timeoutMs)));
         int waitMs = Math.max(1, timeoutMs);
-        List<byte[]> packets = incoming.poll(waitMs, TimeUnit.MILLISECONDS);
+        List<byte[]> packets = priorityIncoming.poll();
+        if (packets != null) {
+            return packets;
+        }
+        packets = incoming.poll(Math.min(2, waitMs), TimeUnit.MILLISECONDS);
+        if (packets != null) {
+            return packets;
+        }
+        packets = priorityIncoming.poll();
+        if (packets != null) {
+            return packets;
+        }
+        packets = incoming.poll(Math.max(1, waitMs - 2), TimeUnit.MILLISECONDS);
         return packets == null ? new ArrayList<>() : packets;
     }
 
@@ -167,6 +180,7 @@ final class VpnPacketWebSocketRelay {
         open = false;
         connecting = false;
         connectingSinceMs = 0;
+        priorityIncoming.clear();
         incoming.clear();
         if (webSocket != null) {
             try {
@@ -238,10 +252,7 @@ final class VpnPacketWebSocketRelay {
             if (packets.isEmpty()) {
                 return;
             }
-            if (!incoming.offer(packets)) {
-                incoming.poll();
-                incoming.offer(packets);
-            }
+            offerIncomingPacketBatch(packets);
         }
 
         @Override
@@ -333,6 +344,38 @@ final class VpnPacketWebSocketRelay {
         return packets;
     }
 
+    private void offerIncomingPacketBatch(List<byte[]> packets) {
+        BlockingQueue<List<byte[]>> target = containsTCPControlPacket(packets) ? priorityIncoming : incoming;
+        if (!target.offer(packets)) {
+            target.poll();
+            target.offer(packets);
+        }
+    }
+
+    private static boolean containsTCPControlPacket(List<byte[]> packets) {
+        if (packets == null) {
+            return false;
+        }
+        for (byte[] packet : packets) {
+            if (isTCPControlPacket(packet)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private static boolean isTCPControlPacket(byte[] packet) {
+        if (packet == null || packet.length < 20 || (packet[0] >> 4) != 4) {
+            return false;
+        }
+        int ihl = (packet[0] & 0x0f) * 4;
+        if (ihl < 20 || packet.length < ihl + 20 || packet[9] != 6) {
+            return false;
+        }
+        int flags = packet[ihl + 13] & 0xff;
+        return (flags & 0x17) != 0;
+    }
+
     private static String trimRight(String value) {
         if (value == null) {
             return "";