Fix for #217

The cron scheduler has been made more robust against changes in the
system clock (caused by e.g. DST changes, NTP syncs or manual setting).

A more granular management for cronjob events has been introduced, now
supporting a `TIME_SYNC` event besides the usual `STOP`. When the cron
scheduler detects a system clock drift (i.e. the timestamp offset before
and after a blocking wait is >1 sec) then all the cronjobs are notified
and forced to refresh their state.

tests/etc/scripts/test_cron.py

@@ -2,25 +2,34 @@ import datetime
 
 from platypush.cron import cron
 
-from tests.test_cron import tmp_files, tmp_files_ready, \
-    test_timeout, expected_cron_file_content
+from tests.test_cron import test_timeout, cron_queue
+
+
+def make_cron_expr(cron_time: datetime.datetime):
+    return '{min} {hour} {day} {month} * {sec}'.format(
+        min=cron_time.minute,
+        hour=cron_time.hour,
+        day=cron_time.day,
+        month=cron_time.month,
+        sec=cron_time.second,
+    )
+
 
 # Prepare a cronjob that should start test_timeout/2 seconds from the application start
-cron_time = datetime.datetime.now() + datetime.timedelta(seconds=test_timeout/2)
-cron_expr = '{min} {hour} {day} {month} * {sec}'.format(
-    min=cron_time.minute, hour=cron_time.hour, day=cron_time.day,
-    month=cron_time.month, sec=cron_time.second)
+cron_time = datetime.datetime.now() + datetime.timedelta(seconds=test_timeout / 2)
 
 
-@cron(cron_expr)
+@cron(make_cron_expr(cron_time))
 def cron_test(**_):
-    """
-    Simple cronjob that awaits for ``../test_cron.py`` to be ready and writes the expected
-    content to the monitored temporary file.
-    """
-    files_ready = tmp_files_ready.wait(timeout=test_timeout)
-    assert files_ready, \
-        'The test did not prepare the temporary files within {} seconds'.format(test_timeout)
+    cron_queue.put('cron_test')
 
-    with open(tmp_files[0], 'w') as f:
-        f.write(expected_cron_file_content)
+
+# Prepare another cronjob that should start 1hr + test_timeout/2 seconds from the application start
+cron_time = datetime.datetime.now() + datetime.timedelta(
+    hours=1, seconds=test_timeout / 2
+)
+
+
+@cron(make_cron_expr(cron_time))
+def cron_1hr_test(**_):
+    cron_queue.put('cron_1hr_test')