Calls to stop before connected cause lockups.

[bleckers]

Describe the bug
Calling sslClient.stop() when !sslClient.connected() causes buffer overflow, even after previously successful connection (and disconnect).

To Reproduce
EthernetClient mqttEthClient;
Ethernet.init(14);
SSLClient sslClient(mqttEthClient, TAs_HTTPS, (size_t)TAs_NUM_HTTPS, RANDOM_DATA_PIN);
sslClient.stop();

Expected behavior
Calls to stop should exit gracefully or ignore the request if already exited.

Context (please complete the following information):

• Device Type - ESP32/Teensy4
• Arduino Core Version - Teensyduino 1.52
• Relevant Library Versions - PubSubClient
• SSLClient Version - v1.6.6

Additional context
Discovered while trying to make the library robust to internet connectivity issues. Removing jump_handshake in br_ssl_engine_close prevents this (not a fix).

[prototypicalpro]

Is the behavior a crash or a freeze? In my experience BearSSL attempts to close the connection by sending a close_notify (which is usually ignored) and then times out waiting for a response.

[prototypicalpro]

I think 4707ea4 should fix the close_notify issue, would you give master latest a shot?

[bleckers]

The behaviour is a crash rather than a freeze. The new commit fixes this with a fresh stop before connect, after a client has connected and disconnected AND if the client is connected.

[bleckers]

Oh just to add, if you have network connectivity, a connected socket, but temporarily no route to the server and are using the Arduino Ethernet 2.0 library, you will need the following change in the EthernetClient.cpp for this commit to fix these issues (as calls to get_arduino_client().flush() in this configuration will hang indefinitely when there's no route). This is also true for calls to flush in the m_update_engine of SSLClient, and considering the lack of development effort in Ethernet, it might have to be a recommendation in SSLClient (particularly if using PubSubClient because of the regular pings which are susceptible to minor lapses in network connectivity).

void EthernetClient::flush()
{
	unsigned long start = millis();
	while (sockindex < MAX_SOCK_NUM) {
		if (millis() - start > _timeout) 
		{
			stop();
			return;
		}
		uint8_t stat = Ethernet.socketStatus(sockindex);
		if (stat != SnSR::ESTABLISHED && stat != SnSR::CLOSE_WAIT) return;
		if (Ethernet.socketSendAvailable(sockindex) >= W5100.SSIZE) return;
	}
}

This has been PRed into Paul's "fork" of the Arduino repo - PaulStoffregen/Ethernet@master...bleckers:patch-1

[prototypicalpro]

Hmmm yeah, I don't think I have a good way of enforcing a timeout on Client::flush without modifying the client implementation. I will add a note in the README about this, and patch EthernetLarge as soon as your pull request is merged.