Fix re-entrancy issue with MQTTListeners.notify (#131)

* Copy listener array when inside lock

process copied listener array outside lock

* Remove commented out code

Sources/MQTTNIO/MQTTClient.swift

@@ -411,9 +411,9 @@ public final class MQTTClient {
     }
 
     var connectionParameters = ConnectionParameters()
-    var publishListeners = MQTTListeners<MQTTPublishInfo>()
-    var closeListeners = MQTTListeners<Void>()
-    var shutdownListeners = MQTTListeners<Void>()
+    let publishListeners = MQTTListeners<MQTTPublishInfo>()
+    let closeListeners = MQTTListeners<Void>()
+    let shutdownListeners = MQTTListeners<Void>()
     private var _connection: MQTTConnection?
     private var lock = NIOLock()
 }

Sources/MQTTNIO/MQTTListeners.swift

@@ -14,14 +14,15 @@
 import NIO
 import NIOConcurrencyHelpers
 
-class MQTTListeners<ReturnType> {
+final class MQTTListeners<ReturnType> {
     typealias Listener = (Result<ReturnType, Error>) -> Void
 
     func notify(_ result: Result<ReturnType, Error>) {
-        self.lock.withLock {
-            listeners.values.forEach { listener in
-                listener(result)
-            }
+        let listeners = self.lock.withLock {
+            return self.listeners
+        }
+        listeners.values.forEach { listener in
+            listener(result)
         }
     }
 
@@ -38,7 +39,9 @@ class MQTTListeners<ReturnType> {
     }
 
     func removeAll() {
-        self.listeners = [:]
+        self.lock.withLock {
+            self.listeners = [:]
+        }
     }
 
     private let lock = NIOLock()