Add a non-interactive function to use softioc in deamon mode

[XavSPM]

I propose this modification, following the initial request #155

Reminder of the initial proposal:

I propose adding the non_interactive_ioc function to allow the use of softioc in daemon mode. This mode can, for example, be used with systemd. It is compatible with asyncio and cothread.

To ensure that information is correctly logged in the journal, it is recommended to execute the Python script with the -u option or to set the environment variable PYTHONUNBUFFERED=TRUE.

Here is an example of a service:

[Unit]
Description=IOC
After=network.target

[Service]
User=root
Group=root
WorkingDirectory=/opt
ExecStart=/opt/softioc/bin/python -u /opt/example.py
Environment="PATH=/opt/softioc/bin"
Restart=always

[Install]
WantedBy=multi-user.target

And an example of a script:

# Import the basic framework components.
from softioc import softioc, builder
import cothread

# Set the record prefix
builder.SetDeviceName("MY-DEVICE-PREFIX")

# Create some records
ai = builder.aIn('AI', initial_value=5)
ao = builder.aOut('AO', initial_value=12.45, on_update=lambda v: ai.set(v))

# Boilerplate to get the IOC started
builder.LoadDatabase()
softioc.iocInit()

# Start processes required to be run after iocInit
def update():
    while True:
        ai.set(ai.get() + 1)
        print(ai.get())
        cothread.Sleep(1)

cothread.Spawn(update)

if __name__ == "__main__":
    softioc.non_interactive_ioc()

[Araneidae]

The wait_for_quit stuff looks straightforward enough (I've suggested a couple of minor code cleanups) ... but I don't see any mechanism for actually quitting (other than a signal).

[AlexanderWells-diamond]

Thank you for the suggested code. It's a useful feature that we've previously done without due to never actually running as a daemon process in any of our deployments. It will be useful to have this option going forward.

I've made some code change suggestions. There's also a couple of odds and ends that need cleaning up:

• Please add an entry to the "Unreleased" section of the CHANGELOG.rst file, following the pattern (and rather picky syntax) of previous entries.
• Please add to the documentation. This should probably go directly below the section for interactive_ioc, i.e. define it at about line 123 of the file docs/reference/api.rst. Docs can be built using pipenv run docs, and viewed at build/html/index.html
• I would normally ask for a test or two to be written, but I think testing this will be tricky. I may add some tests at a later time.

[AlexanderWells-diamond]

Apologies for the delay in reviewing this. It's looking good, just a couple of minor cleanup requests.

When we merge this we will also want to clean up the commit history - you are welcome to do this as part of these last cleanups (i.e squash into a single commit) or we'll do it as part of the merge.

[XavSPM]

Hello, @AlexanderWells-diamond

I've made the requested changes and also reduced the number of commits.

Is there anything else I can do ?

[Araneidae]

I have squashed this work into a single commit, fixed a couple of trivial whitespace issues, and pushed to master. This is now accepted (the "conflicts" are just my whitespace fixes), so will close this PR as completed.

Thank you for this.