Poco c++ Websocket server connection reset by peer - websocket

Question

I am writing a kind of chat server app where a message received from one websocket client is sent out to all other websocket clients. To do this, I keep the connected clients in a list. When a client disconnects, I need to remove it from the list (so that future "sends" do not fail).
However, sometimes when a client disconnects, the server just gets an exception "connection reset by peer", and the code does not get chance to remove from the client list. Is there a way to guarantee a "nice" notification that the connection has been reset?
My code is:
void WsRequestHandler::handleRequest(HTTPServerRequest &req, HTTPServerResponse &resp)
{
int n;
Poco::Timespan timeOut(5,0);
try
{
req.set("Connection","Upgrade"); // knock out any extra tokens firefox may send such as "keep-alive"
ws = new WebSocket(req, resp);
ws->setKeepAlive(false);
connectedSockets->push_back(this);
do
{
flags = 0;
if (!ws->poll(timeOut,Poco::Net::Socket::SELECT_READ || Poco::Net::Socket::SELECT_ERROR))
{
// cout << ".";
}
else
{
n = ws->receiveFrame(buffer, sizeof(buffer), flags);
if (n > 0)
{
if ((flags & WebSocket::FRAME_OP_BITMASK) == WebSocket::FRAME_OP_BINARY)
{
// process and send out to all other clients
DoReceived(ws, buffer, n);
}
}
}
}
while ((flags & WebSocket::FRAME_OP_BITMASK) != WebSocket::FRAME_OP_CLOSE);
// client has closed, so remove from list
for (vector<WsRequestHandler *>::iterator it = connectedSockets->begin() ; it != connectedSockets->end(); ++it)
{
if (*it == this)
{
connectedSockets->erase(it);
logger->information("Connection closed %s", ws->peerAddress().toString());
break;
}
}
delete(ws);
ws = NULL;
}
catch (WebSocketException& exc)
{
//never gets called
}
}

Answer 1

See receiveFrame() documentation:
Returns the number of bytes received. A return value of 0 means that the peer has shut down or closed the connection.
So if receiveFrame() call returns zero, you can act acordingly.

Answer 2

I do not know if this is an answer to the question, but the implementation you have done does not deal with PING frames. This is currently (as of my POCO version: 1.7.5) not done automatically by the POCO framework. I put up a question about that recently. According to the RFC (6465), the ping and pong frames are used (among others) as a keep-alive function. This may therefore be critical to get right in order to get your connection stable over time. Much of this is guess-work from my side as I am experimenting with this now myself.
#Alex, you are a main developer of POCO I believe, a comment on my answer would be much appreciated.

Answer 3

I extended the catch, to do some exception handling for "Connection reset by peer".
catch (Poco::Net::WebSocketException& exc)
{
// Do something
}
catch (Poco::Exception& e)
{
// This is where the "Connection reset by peer" lands
}

Answer 4

A bit late to the party here... but I am using Poco and Websockets as well - and properly handling disconnects was tricky.
I ended up implementing a simple ping functionality myself where the client side sends an ACK message for every WS Frame it receives. A separate thread on the server side tries to read the ACK messages - and it will now detect when the client has disconnected by looking at flags | WebSocket::FRAME_OP_CLOSE.
//Serverside - POCO. Start thread for receiving ACK packages. Needed in order to detect when websocket is closed!
thread t0([&]()->void{
while((!KillFlag && ws!= nullptr && flags & WebSocket::FRAME_OP_BITMASK) != WebSocket::FRAME_OP_CLOSE && machineConnection != nullptr){
try{
if(ws == nullptr){
return;
}
if(ws->available() > 0){
int len = ws->receiveFrame(buffer, sizeof(buffer), flags);
}
else{
Util::Sleep(10);
}
}
catch(Poco::Exception &pex){
flags = flags | WebSocket::FRAME_OP_CLOSE;
return;
}
catch(...){
//log::info(string("Unknown exception in ACK Thread drained"));
return;
}
}
log::debug("OperatorWebHandler::HttpRequestHandler() Websocket Acking thread DONE");
});
on the client side I just send a dummy "ACK" message back to the server (JS) every time I receive a WS frame from the server (POCO).
websocket.onmessage = (evt) => {
_this.receivedData = JSON.parse(evt.data);
websocket.send("ACK");
};

Answer 5

It is not about disconnect handling, rather about the stability of the connection.
Had some issues with POCO Websocket server in StreamSocket mode and C# client. Sometimes the client sends Pong messages with zero length payload and disconnect occurs so I added Ping and Pong handling code.
int WebSocketImpl::receiveBytes(void* buffer, int length, int)
{
char mask[4];
bool useMask;
_frameFlags = 0;
for (;;) {
int payloadLength = receiveHeader(mask, useMask);
int frameOp = _frameFlags & WebSocket::FRAME_OP_BITMASK;
if (frameOp == WebSocket::FRAME_OP_PONG || frameOp ==
WebSocket::FRAME_OP_PING) {
std::vector<char> tmp(payloadLength);
if (payloadLength != 0) {
receivePayload(tmp.data(), payloadLength, mask, useMask);
}
if (frameOp == WebSocket::FRAME_OP_PING) {
sendBytes(tmp.data(), payloadLength, WebSocket::FRAME_OP_PONG);
}
continue;
}
if (payloadLength <= 0)
return payloadLength;
if (payloadLength > length)
throw WebSocketException(Poco::format("Insufficient buffer for
payload size %d", payloadLength),
WebSocket::WS_ERR_PAYLOAD_TOO_BIG);
return receivePayload(reinterpret_cast<char*>(buffer), payloadLength,
mask, useMask);
}
}