GenMyPtyHandlerId generates a random session ID string.

WaitForTerminal is called from apihandler.handleAttach as a goroutine Waits for the SockJS connection to be opened by the client the session to be bound in handleMyPtyHandler.

SessionMap stores a map of all MyPtyHandler objects and a sessLock to avoid concurrent conflict.

PtyHandler is what remotecommand expects from a pty.