Improved unit test coverage of mig lib daemon (#367)

* Use a shared `before_each` init to isolate test configuration etc.
Add coverage of several previously missing branches and corner cases.
- Direct event helper testing
- Signal handler verification
- Concurrent event handling
- Nap timing accuracy checks
- Multiple reset cycles
- Error cases like invalid arguments
- Various sleep interruption scenarios

* Implement a new helper to unregister all registered signals and use it in unit
tests to increase isolation and be able to test handling when no handlers where
registered.

* Added unit test coverage:
 - Immediate sleep exit with max_secs < nap_secs
 - Positional argument handling
 - Negative max_secs handling
 - Zero max_secs handling
 - Unregistered signal safety
 - Consecutive signal handling
 - Verify explicit unregistration of signal handlers
 - Test sleep terminating after a condition becomes true
 - Edge case test for equal max/sleep durations
 - Test error handling when break conditions raise exceptions
 - Direct tests of state reset and setting helpers
 - Integration test with real signal delivery

* Switch dummy conf and logger to the default ones from `_provide_configuration`.
Move used_signal accounting to class-level. Perhaps we want to maintain it on
actual use and add helpers to track active ones eventually.

* Address the review comment on PR367 about the edge cases not really being
verified by inserting missing `assert`s.

* Adjust conflicting handler test to behave as expected with only the last handler
assigned being active.

* Increase test coverage to 100% with more corner case tests of sleep intervals,
unregister, etc.

* Remove useless and unused result from `interruptible_sleep`, also called out by
pylint.

Author: jonasbardino

mig/lib/daemon.py

@@ -68,9 +68,23 @@ def register_run_handler(configuration, run_signal=signal.SIGCONT):
     signal.signal(run_signal, run_handler)
 
 
+def unregister_signal_handlers(configuration, target_signals=None):
+    """Remove any handlers registered to react on provided target_signals list.
+    Default to SIGCONT, SIGUSR1 and SIGUSR2 if target_signals is left to None.
+    """
+    if target_signals is None:
+        target_signals = [signal.SIGCONT, signal.SIGUSR1, signal.SIGUSR2]
+    for target in target_signals:
+        original_handler = signal.getsignal(target)
+        if original_handler:
+            signal.signal(target, signal.SIG_IGN)
+
+
 def interruptible_sleep(configuration, max_secs, break_checks, nap_secs=0.1):
     """Idle loop for max_secs or until one or more break_checks succeed"""
-    assert max_secs >= nap_secs, "Invalid max_secs value smaller than nap_secs"
+    assert nap_secs > 0.0, "Invalid non-positive nap_secs value"
+    if max_secs < nap_secs:
+        return
     for _ in range(int(max_secs / nap_secs)):
         for check in break_checks:
             if check():