SvcD class and "svcd" command to run persistent service programmes.

Latest release 20210316: Serialise the Popen calls to avoid entirely hypothetical subprocess.Popen MT bug that may be making portfwd coredump.

This provides the features one wants from a daemon for arbitrary commands providing a service:

• process id (pid) files for both svcd and the service command
• filesystem visible status (command running, service enabled) via cs.app.flag <https://pypi.org/project/cs.app.flag/>_
• command restart if the command exits
• command control (stop, restart, disable) via cs.app.flag <https://pypi.org/project/cs.app.flag/>_
• test function to monitor for service viability; if the test function fails, do not run the service. This typically monitors something like network routing (suspend service while laptop offline) or a ping (suspend ssh tunnel while target does not answer pings).
• signature function to monitor for service restart; if the signature changes, restart the service. This typically monitors something like file contents (restart service on configuration change) or network routing (restart ssh tunnel on network change)
• callbacks for service command start and end, for example to display desktop notifications

I use this to run persistent ssh port forwards and a small collection of other personal services. I have convenient shell commands to look up service status and to start/stop/restart services.

See cs.app.portfwd <https://pypi.org/project/cs.app.portfwd/>_ which I use to manage my ssh tunnels; it is a single Python programme running multiple ssh commands, each via its own SvcD instance.

Function callproc(*a, **kw)

Workalike for subprocess.call, using LockedPopen.

Function LockedPopen(*a, **kw)

Serialise the Popen calls.

My long term multithreaded SvcD programmes sometimes coredump. My working theory is that Popen, maybe only on MacOS, is slightly not multithead safe. This function exists to test that theory.

Function main(argv=None)

Command line main programme.

Class SvcD(cs.app.flag.FlaggedMixin)

A process based service.

Method SvcD.__init__(self, argv, name=None, environ=None, flags=None, group_name=None, pidfile=None, sig_func=None, test_flags=None, test_func=None, test_rate=None, restart_delay=None, once=False, quiet=False, trace=False, on_spawn=None, on_reap=None)

Initialise the SvcD.

Parameters:

• argv: command to run as a subprocess.
• flags: a cs.app.flag.Flags -like object, default None; if None the default flags will be used.
• group_name: alert group name, default "SVCD " + name.
• pidfile: path to pid file, default $VARRUN/{name}.pid.
• sig_func: signature function to compute a string which causes a restart if it changes
• test_flags: map of {flagname: truthiness} which should be monitored at test time; truthy flags must be true and untruthy flags must be false
• test_func: test function with must return true if the comannd can run
• test_rate: frequency of tests, default TEST_RATE
• restart_delay: delay before start of an exiting command, default RESTART_DELAY
• once: if true, run the command only once
• quiet: if true, do not issue alerts
• trace: trace actions, default False
• on_spawn: to be called after a new subprocess is spawned
• on_reap: to be called after a subprocess is reaped

Method SvcD.alert(self, msg, *a)

Issue an alert message via the "alert" command.

Method SvcD.dbg(self, msg, *a)

Log a debug message if not tracing.

Method SvcD.disable(self)

Turn on the disable flag.

Method SvcD.enable(self)

Turn of the disable flag.

Method SvcD.probe(self)

Probe the subprocess: true if running.

Method SvcD.reap(self)

Collect the subprocess status after termination.

Calls the on_reap function if any.

Method SvcD.restart(self)

Set the restart flag, will be cleared by the restart.

Method SvcD.spawn(self)

Spawn the subprocess.

Calls the on_spwan function if any.

Method SvcD.start(self)

Start the subprocess and its monitor.

Method SvcD.stop(self)

Set the stop flag.

Method SvcD.test(self)

Test whther the service should run.

In order:

• True if the override flag is true.
• False if the disable flag is true.
• False if any of the specified test flags are false.
• False if the test function fails.
• Otherwise True.

Method SvcD.wait(self)

Wait for the subprocess by waiting for the monitor.

Release Log

Release 20210316: Serialise the Popen calls to avoid entirely hypothetical subprocess.Popen MT bug that may be making portfwd coredump.

Release 20190729: Get DEVNULL via cs.py3 instead of directly from subprocess.

Release 20190602.2: Another doc tweak.

Release 20190602.1: Improve module documentation formatting.

Release 20190602:

• Support alert groups.
• Catch and report exceptions from the monitor signature function.
• Python 2 port fix for DEVNULL.

Release 20171118: Bugfix for su invocation in setuid mode. Improved signature command tracing with -x option.

Release 20171026: Improved logic around signature changes.

Release 20171025: New "-F flag,..." option for svcd. Improve stop logic. Other small fixes.

Release 20170906: Initial PyPI release.