test mode (satpy_launcher -m) fails instantly with ValueError when a plugin with a timeout is used.

[gerritholl]

Describe the bug

Running satpy_launcher -m ... fails instantly with ValueError

To Reproduce

satpy_launcher.py -m /tmp/test-message /opt/pytroll/pytroll_inst/config/trollflow2-viirs-direkt.yaml

The configuration files has timeouts configured. It seems attempting to set the timeout is triggering the failure.

Expected behavior

I expect trollflow2 to process the message in the test file as it would if this message were received with posttroll in normal operating mode.

Actual results

[WARNING: 2022-03-21 16:29:35 : satpy_launcher] Launching Satpy-based runner.
[INFO: 2022-03-21 16:29:35 : trollflow2.launcher] Launching trollflow2 with threads
[DEBUG: 2022-03-21 16:29:35 : trollflow2.launcher] Distributed processing not configured, using default scheduler
[ERROR: 2022-03-21 16:29:35 : trollflow2.launcher] Process crashed
Traceback (most recent call last):
  File "/opt/pytroll/pytroll_inst/mambaforge/envs/py39/lib/python3.9/site-packages/trollflow2/launcher.py", line 370, in process
    signal.signal(signal.SIGALRM, _timeout_handler)
  File "/opt/pytroll/pytroll_inst/mambaforge/envs/py39/lib/python3.9/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread of the main interpreter
[DEBUG: 2022-03-21 16:29:35 : trollflow2.launcher] Cleaning up
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/opt/pytroll/pytroll_inst/mambaforge/envs/py39/lib/python3.9/threading.py", line 973, in _bootstrap_inner
    self.run()
  File "/opt/pytroll/pytroll_inst/mambaforge/envs/py39/lib/python3.9/threading.py", line 910, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/pytroll/pytroll_inst/mambaforge/envs/py39/lib/python3.9/site-packages/trollflow2/launcher.py", line 370, in process
    signal.signal(signal.SIGALRM, _timeout_handler)
  File "/opt/pytroll/pytroll_inst/mambaforge/envs/py39/lib/python3.9/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread of the main interpreter
[INFO: 2022-03-21 16:29:35 : trollflow2.launcher] All 0 files produced nominally in 0:00:00.145675

Environment Info:

• Trollflow2 Version: 0.13.5.dev8+gde82238
• Satpy Version: 0.35.1.dev14+gc5ea104c