MQTT3 clean shutdown (#535)

* Add MqttConnectionCore: an auxiliary class handling lifetimes of the MqttConnection object and the underlying C connection.

* Extract MqttConnection class from MqttClient.cpp into separate source files.

* Add MqttConnectionOptions class to pass all possible options on MqttConnection object creation uniformly.

* Add MqttTypes.h with MQTT3-related types used in multiple places.

include/aws/crt/http/HttpRequestResponse.h

@@ -18,7 +18,8 @@ namespace Aws
         namespace Mqtt
         {
             class MqttConnection;
-        }
+            class MqttConnectionCore;
+        } // namespace Mqtt
         namespace Mqtt5
         {
             class Mqtt5ClientCore;
@@ -107,7 +108,7 @@ namespace Aws
              */
             class AWS_CRT_CPP_API HttpRequest : public HttpMessage
             {
-                friend class Mqtt::MqttConnection;
+                friend class Mqtt::MqttConnectionCore;
                 friend class Mqtt5::Mqtt5ClientCore;
 
               public:

include/aws/crt/mqtt/MqttTypes.h

@@ -0,0 +1,130 @@
+#pragma once
+/**
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0.
+ */
+
+#include <aws/crt/Types.h>
+#include <aws/crt/io/SocketOptions.h>
+#include <aws/crt/io/TlsOptions.h>
+
+#include <aws/mqtt/client.h>
+#include <aws/mqtt/v5/mqtt5_client.h>
+
+#include <functional>
+
+namespace Aws
+{
+    namespace Crt
+    {
+        namespace Mqtt
+        {
+            class MqttConnection;
+
+            /**
+             * Options required to create an MqttConnection.
+             */
+            struct MqttConnectionOptions
+            {
+                const char *hostName = nullptr;
+                uint16_t port = 0;
+                Io::SocketOptions socketOptions;
+                Crt::Io::TlsContext tlsContext;
+                Crt::Io::TlsConnectionOptions tlsConnectionOptions;
+                bool useWebsocket = false;
+                bool useTls = false;
+                Allocator *allocator = nullptr;
+            };
+
+            /**
+             * Invoked upon receipt of a Publish message on a subscribed topic.
+             *
+             * @param connection The connection object.
+             * @param topic The information channel to which the payload data was published.
+             * @param payload The payload data.
+             * @param dup DUP flag. If true, this might be re-delivery of an earlier attempt to send the message.
+             * @param qos Quality of Service used to deliver the message.
+             * @param retain Retain flag. If true, the message was sent as a result of a new subscription being made by
+             * the client.
+             */
+            using OnMessageReceivedHandler = std::function<void(
+                MqttConnection &connection,
+                const String &topic,
+                const ByteBuf &payload,
+                bool dup,
+                QOS qos,
+                bool retain)>;
+
+            /**
+             * Invoked when a suback message is received.
+             *
+             * @param connection The connection object.
+             * @param packetId Packet ID of the corresponding subscribe request.
+             * @param topic The information channel to which the payload data was published.
+             * @param qos Quality of Service used to deliver the message.
+             * @param errorCode Indicating if an error occurred.
+             */
+            using OnSubAckHandler = std::function<
+                void(MqttConnection &connection, uint16_t packetId, const String &topic, QOS qos, int errorCode)>;
+
+            /**
+             * Invoked when a suback message for multiple topics is received.
+             *
+             * @param connection The connection object.
+             * @param packetId Packet ID of the corresponding subscribe request.
+             * @param topics The information channels to which the payload data was published.
+             * @param qos Quality of Service used to deliver the message.
+             * @param errorCode Indicating if an error occurred.
+             */
+            using OnMultiSubAckHandler = std::function<void(
+                MqttConnection &connection,
+                uint16_t packetId,
+                const Vector<String> &topics,
+                QOS qos,
+                int errorCode)>;
+
+            /**
+             * Invoked when an operation completes.
+             *
+             * For QoS 0, this is when the packet is passed to the tls layer. For QoS 1 (and 2, in theory) this is when
+             * the final ACK packet is received from the server.
+             *
+             * @param connection The connection object.
+             * @param packetId Packet ID of the corresponding subscribe request.
+             * @param errorCode Indicating if an error occurred.
+             */
+            using OnOperationCompleteHandler =
+                std::function<void(MqttConnection &connection, uint16_t packetId, int errorCode)>;
+
+            /**
+             * Simple statistics about the current state of the client's queue of operations.
+             */
+            struct AWS_CRT_CPP_API MqttConnectionOperationStatistics
+            {
+                /*
+                 * Total number of operations submitted to the connection that have not yet been completed. Unacked
+                 * operations are a subset of this.
+                 */
+                uint64_t incompleteOperationCount;
+
+                /*
+                 * Total packet size of operations submitted to the connection that have not yet been completed. Unacked
+                 * operations are a subset of this.
+                 */
+                uint64_t incompleteOperationSize;
+
+                /*
+                 * Total number of operations that have been sent to the server and are waiting for a corresponding ACK
+                 * before they can be completed.
+                 */
+                uint64_t unackedOperationCount;
+
+                /*
+                 * Total packet size of operations that have been sent to the server and are waiting for a corresponding
+                 * ACK before they can be completed.
+                 */
+                uint64_t unackedOperationSize;
+            };
+        } // namespace Mqtt
+    }     // namespace Crt
+} // namespace Aws