NmtMaster: Delay heartbeat callbacks after state update (canopen-python#601)

Reduce the amount of code within the critical section holding the
lock.  Unpack and log first.  Update timestamp and state while locked
and notify condition waiters.  Invoke callbacks after releasing the
lock.

Author: acolomb

canopen/nmt.py

@@ -120,14 +120,13 @@ def __init__(self, node_id: int):
         self._callbacks: list[Callable[[int], None]] = []
 
     def on_heartbeat(self, can_id, data, timestamp):
+        new_state, = struct.unpack_from("B", data)
+        # Mask out toggle bit
+        new_state &= 0x7F
+        logger.debug("Received heartbeat can-id %d, state is %d", can_id, new_state)
+
         with self.state_update:
             self.timestamp = timestamp
-            new_state, = struct.unpack_from("B", data)
-            # Mask out toggle bit
-            new_state &= 0x7F
-            logger.debug("Received heartbeat can-id %d, state is %d", can_id, new_state)
-            for callback in self._callbacks:
-                callback(new_state)
             if new_state == 0:
                 # Boot-up, will go to PRE-OPERATIONAL automatically
                 self._state = 127
@@ -136,6 +135,9 @@ def on_heartbeat(self, can_id, data, timestamp):
             self._state_received = new_state
             self.state_update.notify_all()
 
+        for callback in self._callbacks:
+            callback(new_state)
+
     def send_command(self, code: int):
         """Send an NMT command code to the node.