Fix VPN memory pressure monitor (#3535)

Task/Issue URL: https://app.asana.com/0/414235014887631/1208695924903429/f
Tech Design URL:
CC:

Description:

This PR fixes the memory pressure monitor. Previously, the memory source and queue were not being retained in any way, so they would deallocate instantly. Now, the memory source and queue are kept as properties on the packet tunnel provider.

Author: samsymons

PacketTunnelProvider/NetworkProtection/NetworkProtectionPacketTunnelProvider.swift

@@ -479,28 +479,33 @@ final class NetworkProtectionPacketTunnelProvider: PacketTunnelProvider {
         APIRequest.Headers.setUserAgent(DefaultUserAgentManager.duckDuckGoUserAgent)
     }
 
+    deinit {
+        memoryPressureSource?.cancel()
+        memoryPressureSource = nil
+    }
+
+    private var memoryPressureSource: DispatchSourceMemoryPressure?
+    private let memoryPressureQueue = DispatchQueue(label: "com.duckduckgo.mobile.ios.NetworkExtension.memoryPressure")
+
     private func startMonitoringMemoryPressureEvents() {
-        let source = DispatchSource.makeMemoryPressureSource(eventMask: .all, queue: nil)
-
-        let queue = DispatchQueue.init(label: "com.duckduckgo.mobile.ios.alpha.NetworkExtension.memoryPressure")
-        queue.async {
-            source.setEventHandler {
-                let event: DispatchSource.MemoryPressureEvent  = source.mask
-                print(event)
-                switch event {
-                case DispatchSource.MemoryPressureEvent.normal:
-                    break
-                case DispatchSource.MemoryPressureEvent.warning:
-                    DailyPixel.fire(pixel: .networkProtectionMemoryWarning)
-                case DispatchSource.MemoryPressureEvent.critical:
-                    DailyPixel.fire(pixel: .networkProtectionMemoryCritical)
-                default:
-                    break
-                }
+        let source = DispatchSource.makeMemoryPressureSource(eventMask: .all, queue: memoryPressureQueue)
 
+        source.setEventHandler { [weak source] in
+            guard let source else { return }
+
+            let event = source.data
+
+            if event.contains(.warning) {
+                Logger.networkProtectionMemory.warning("Received memory pressure warning")
+                DailyPixel.fire(pixel: .networkProtectionMemoryWarning)
+            } else if event.contains(.critical) {
+                Logger.networkProtectionMemory.warning("Received memory pressure critical warning")
+                DailyPixel.fire(pixel: .networkProtectionMemoryCritical)
             }
-            source.resume()
         }
+
+        self.memoryPressureSource = source
+        source.activate()
     }
 
     private func observeServerChanges() {