SyncProducer: Prevent from starting another periodic task (fixes canopen-python#582) (canopen-python#600)

Raise a RuntimeError when a SYNC transmission task has previously been
started.  Make sure to clear the internal _task attribute when
stopping, to make the check work.

Author: acolomb

canopen/sync.py

@@ -15,32 +15,42 @@ class SyncProducer:
     def __init__(self, network: canopen.network.Network):
         self.network = network
         self.period: Optional[float] = None
-        self._task = None
+        self._task: Optional[canopen.network.PeriodicMessageTask] = None
 
     def transmit(self, count: Optional[int] = None):
         """Send out a SYNC message once.
 
         :param count:
             Counter to add in message.
+        :raises ValueError:
+            If the counter value does not fit in one byte.
         """
-        data = [count] if count is not None else []
+        data = bytes([count]) if count is not None else b""
         self.network.send_message(self.cob_id, data)
 
     def start(self, period: Optional[float] = None):
         """Start periodic transmission of SYNC message in a background thread.
 
         :param period:
             Period of SYNC message in seconds.
+        :raises RuntimeError:
+            If a periodic transmission is already started.
+        :raises ValueError:
+            If no period is set via argument nor the instance attribute.
         """
+        if self._task is not None:
+            raise RuntimeError("Periodic SYNC transmission task already running")
+
         if period is not None:
             self.period = period
 
         if not self.period:
             raise ValueError("A valid transmission period has not been given")
 
-        self._task = self.network.send_periodic(self.cob_id, [], self.period)
+        self._task = self.network.send_periodic(self.cob_id, b"", self.period)
 
     def stop(self):
         """Stop periodic transmission of SYNC message."""
         if self._task is not None:
             self._task.stop()
+        self._task = None

test/test_sync.py

@@ -74,6 +74,16 @@ def periodicity():
         if msg is not None:
             self.assertIsNone(self.net.bus.recv(TIMEOUT))
 
+    def test_sync_producer_restart(self):
+        self.sync.start(PERIOD)
+        self.addCleanup(self.sync.stop)
+        # Cannot start again while running
+        with self.assertRaises(RuntimeError):
+            self.sync.start(PERIOD)
+        # Can restart after stopping
+        self.sync.stop()
+        self.sync.start(PERIOD)
+
 
 if __name__ == "__main__":
     unittest.main()