scratchClient singleton reworked

scratchClient needs to run alone, as a ‘singleton’, on one computer. The reason is that concurrent access to GPIO, SPI or other resources could result in unpredictable results.

First approach was to use a pid-file with some logic. At startup, scratchClient writes its own process identification ‘pid’ into a file When another scratchClient is started, it looks for existing pid-file, doublechecks if a process with the recorded pid is running. In this situation, the new started process stopped.

In practice, this situation arised quite often when kids started scratchClient with wrong config file and simply closed the terminal. On a fresh start in a new terminal, the ‘old’ pid file was still existing or even the previous process was still running somewhere. These situations asked too much linux knowledge from kids and teachers and often resulted in unnecessary reboots of the computers.

A better solution is that a new start of scratchClient causes previously started processes to stop. This should work inside one computer and work independent from user permissions on the process: a new scratchClient started by ‘pi’ should be able to stop a previously started process by ‘root’.

The new singleton mechanism uses a socket to achieve this behavior. On startup of scratchClient, socket 42003 is opened as a TCPServer and listens to commands.
A new started process will get errors on opening same socket as a listener, switches to client mode and issues a shutdown command to the server. Next step is then to open TCPServer on this socket.

The video shows this effect with two terminal sessions.

In upper window, scratchClient is started. Another start in lower window causes the first process to stop.

This behavior now is default for scratchClient.

The command line allows to select the singleton mechanics:

-singletonPID        when multiple instances are running, report other instance and 
-singletonIPC        when multiple instances are running, terminate other instance
                     used port 42003 (default from 2017-02-14)
-singletonNONE       no singleton policy applied. For debug only

Summary: new singleton mechanism introduced in scratchClient since Feb 2017.