QOS 2 can deliver a message twice

[daisukes]

Hello,

I have experienced an issue where my app delivers a message twice when the app has a disconnection/reconnection, even if I set the message QOS to 2.
I want to send a message every second and deliver all the messages even if the phone is disconnected for a while.

If I set cleanSession to True, it looks like the app cleans the queue and loses some messages.
If I set cleanSession to False, I got the issue.

I found that recoverSessionBy enqueues stored messages and can duplicate messages.
I think it should check if the mqueue already holds the same message when a stored message is appended.

CocoaMQTT/Source/CocoaMQTTDeliver.swift

Lines 78 to 94 in ff7c07b

	func recoverSessionBy(_ storage: CocoaMQTTStorage) {
	let frames = storage.takeAll()
	if(frames != nil){
	guard frames!.count >= 0 else {
	return
	}
	// Sync to push the frame to mqueue for avoiding overcommit
	deliverQueue.sync {
	for f in frames! {
	mqueue.append(f)
	}
	self.storage = storage
	printInfo("Deliver recover \(frames!.count) msgs")
	printDebug("Recover message \(frames!)")
	}

The way to reproduce the issue

• make an instance with cleanSession = false

  self.mqtt = CocoaMQTT(clientID: clientID, host: host, port: port)
  mqtt.cleanSession = false
  

• connect to the broker via Wi-Fi
• record delivered messages by subscribing to the message (I used Mosquitto as the broker and subscribed by a client with paho.mqtt.cpp)
• publish a message every second (keeps publishing while it is disconnected)
• disconnect intentionally by disabling Wi-Fi, wait 10 seconds, and enable Wi-Fi
• wait until the connection is recovered and it sends the all messages
• stop

Behavior

• My client received all the messages + duplicated messages published while the app was disconnected.

Expected

• My client received all the messages without any duplication

Thank you!