how do i correctly fork() and exit a child process when i'm using ZeroMQ - fork

Question

I have a simple application that listens on a ZeroMQ socket. When the client connects and requests a worker node, I fork() my process, the forked child process creates a new context and a new ZeroMQ socket. The client and the worker node perform a REQ-REP formal behaviour on that socket.
My problem is how do I can gracefully handle a shutdown of my worker node.
The client sends an EXIT message to my worker node, who needs to close its socket and its context (?)
From what I can see, the child process exits however, new clients cannot now talk to my original parent process.
Psuedo Code
while (looping) {
zmq::message_t request;
try {
socket.recv(&request); // Wait
string reqStr = string(static_cast<char *>(request.data()), request.size());
if (reqStr.compare("exit") == 0) {
LOG(INFO) << "exiting.." << endl;
looping = false;
}
LOG(INFO) << "******************************************************************" << endl;
LOG(INFO) << "Received request for an endpoint " << reqStr << endl;
int port = doFork(reqStr);
if (port > 0) {
LOG(INFO) << "Returning endPoint: " << reqStr << " on port: " << port << endl;
string result = NumberToString(port);
zmq::message_t reply(result.length());
memcpy((void *) reply.data(), result.c_str(), result.length());
socket.send(reply);
}
else {
// Child Process exiting OR error in Fork"
looping = false;
child = true;
}
}
catch (zmq::error_t &e) {
LOG(INFO) << "W: Caught Exception OR Interrupt: " << e.what() << " : and pid is " << getpid() << endl;
}
}
if (!child) {
socket.close();
context.close();
LOG(INFO) << "Closed socket and context for pid " << getpid() << endl;
}}
int Forker::doFork(string reqStr) {
pid_t pid;
int port = ++startingPort;
switch (pid = fork()) {
case -1:
LOG(INFO) << "Error in fork";
return -1;
case 0:
LOG(INFO) << "Created child process with pid: " << getpid() << endl;
{
ServicePtr servicePtr(new Service(NumberToString(port)));
LOG(INFO) << "Spawning on port: " << port << endl;
servicePtr->spawn();
}
LOG(INFO) << "Child Process exiting on port: " << port << endl;
return 0;
default:
LOG(INFO) << "Parent process. My process id is " << getpid() << endl;
}
return port;
}

Answer 1

Your psuedocode isn't enough to get much of anywhere - notably you don't define your sockets there, so I don't even know which side is the REQ and which side is the REP (or, indeed, that you're actually using those socket types), or which side binds and which side connects.
But, my first guess is that you've got an uneven send/receive pairing, and something like the following is happening:
Client binds on REQ socket, worker connects on REP socket
Client sends a request (client expects recv next)
Worker sends response (client expects send next)
Client sends a follow up message (client expects recv next)
Worker shuts down
New worker spins up
Client sends a request (ERROR - REQ sockets are strictly send/recv ordering)
IF that's what's causing your issue, you can either make sure to respond back to reset the client socket, or you can use a DEALER socket instead of REQ.