Playing with the RPi WDT

watchtest.py

#!/usr/bin/env python3
"""
Why bother with a service that hits the watchdog? The watchdog package seems
kinda daft if you're worried about your service locking up and/or not doing
something it should be doing.

Maybe could use something like this in conjuction with the watchdog daemon,
but don't want both to reset the WDT or that would be near pointless. Ideally
on my service start I'd like to permanently put the WDT into a state where
it's active, which seems to be done by opening the `/dev` file. Python
"unfortunately" generally wants to clean up dangling file handles (doubly so
when using the context manager like below), so maybe there's a way to
intentionally leak it on startup? Another process?

## Demo

```
nick@lappy386:~$ ssh pi@192.168.1.127
pi@192.168.1.127's password:
Linux raspberrypi 4.14.79-v7+ #1159 SMP Sun Nov 4 17:50:20 GMT 2018 armv7l
...
pi@raspberrypi:~ $ sudo python3 watchtest.py
OK.
0...OK.
0...1...OK.
0...1...2...OK.
0...1...2...3...OK.
0...1...2...3...4...OK.
0...1...2...3...4...5...OK.
0...1...2...3...4...5...6...OK.
0...1...2...3...4...5...6...7...OK.
0...1...2...3...4...5...6...7...8...OK.
0...1...2...3...4...5...6...7...8...9...OK.
0...1...2...3...4...5...6...7...8...9...10...OK.
0...1...2...3...4...5...6...7...8...9...10...11...OK.
0...1...2...3...4...5...6...7...8...9...10...11...12...OK.
0...1...2...3...4...5...6...7...8...9...10...11...12...13...OK.
0...1...2...3...4...5...6...7...8...9...10...11...12...13...14...OK.
0...1...2...3...4...5...6...7...8...9...10...11...12...13...14...15...Write failed: Broken pipe
nick@lappy386:~$ # aaaand it's dead.
```
"""

import sys
import time

for x in range(30):
    with open("/dev/watchdog", "w") as f:
        f.write('x')
        for n in range(x):
            print('{}...'.format(n), end='')
            sys.stdout.flush()
            time.sleep(1)
        print('OK.')
        f.write('x')