client hung when the open files reach limit

[HarrisChu]

# in graph server
ulimit -n 400
scripts/nebula.service start graphd

# in client
# create a pool and then initial session in loop, e.g.

func main() {
	hostAddress := nebula.HostAddress{Host: address, Port: port}
	hostList := []nebula.HostAddress{hostAddress}
	testPoolConfig := nebula.GetDefaultConf()
	testPoolConfig.MaxConnPoolSize = 100

	pool, err := nebula.NewConnectionPool(hostList, testPoolConfig, log)
	if err != nil {
		log.Fatal(fmt.Sprintf("Fail to initialize the connection pool, host: %s, port: %d, %s", address, port, err.Error()))
	}
	defer pool.Close()
	for i := 0; i < testPoolConfig.MaxConnPoolSize; i++ {
		log.Info(fmt.Sprintf("begin get session %d", i))
		session, err := pool.GetSession(username, password)
		if err != nil {
			log.Fatal(fmt.Sprintf("Fail to create a new session from connection pool, username: %s, password: %s, %s",
				username, password, err.Error()))

		}
		session.Execute("USE SF1")
	}
}

client would hung up forever, and the sever log as below:

E0713 22:18:30.224404 38653 AsyncServerSocket.cpp:961] accept failed: out of file descriptors; entering accept back-off state
E0713 22:18:30.224467 38521 Acceptor.cpp:396] error accepting on acceptor socket: accept() failed
E0713 22:18:31.225469 38653 AsyncServerSocket.cpp:961] accept failed: out of file descriptors; entering accept back-off state
E0713 22:18:31.225526 38522 Acceptor.cpp:396] error accepting on acceptor socket: accept() failed

I think we should return error if cannot make connections.

[HarrisChu]

just as a known issue, client set timeout configuration would raise error.