Fixed a new double-free error with user-defined persistence.

src/iclient_persistence.cpp

@@ -81,8 +81,10 @@ int iclient_persistence::persistence_get(void* handle, char* key,
 	try {
 		if (handle && key && buffer && buflen) {
 			auto sv = static_cast<iclient_persistence*>(handle)->get(key);
-			*buffer = const_cast<char*>(sv.data());
-			*buflen = (int) sv.length();
+			size_t n = sv.length();
+			*buffer = static_cast<char*>(MQTTAsync_malloc(n));
+			memcpy(*buffer, sv.data(), n);
+			*buflen = int(n);
 			return MQTTASYNC_SUCCESS;
 		}
 	}

src/samples/async_publish.cpp

@@ -8,6 +8,7 @@
 // The sample demonstrates:
 //  - Connecting to an MQTT server/broker
 //  - Publishing messages
+//  - Default file persistence
 //  - Last will and testament
 //  - Using asynchronous tokens
 //  - Implementing callbacks and action listeners
@@ -42,6 +43,7 @@ using namespace std;
 
 const std::string DFLT_SERVER_ADDRESS	{ "tcp://localhost:1883" };
 const std::string DFLT_CLIENT_ID		{ "async_publish" };
+const std::string PERSIST_DIR			{ "./persist" };
 
 const string TOPIC { "hello" };
 
@@ -127,7 +129,7 @@ int main(int argc, char* argv[])
 			clientID = (argc > 2) ? string(argv[2]) : DFLT_CLIENT_ID;
 
 	cout << "Initializing for server '" << address << "'..." << endl;
-	mqtt::async_client client(address, clientID);
+	mqtt::async_client client(address, clientID, PERSIST_DIR);
 
 	callback cb;
 	client.set_callback(cb);

src/samples/sync_publish.cpp

@@ -100,7 +100,7 @@ public:
 			<< key << "']" << std::endl;
 		std::string str;
 		for (const auto& b : bufs)
-			str += b.str();
+			str.append(b.data(), b.size());	// += b.str();
 		store_[key] = std::move(str);
 	}