fix race condition handling (NVIDIA#2728)

nvflare/fuel/utils/pipe/pipe_handler.py

@@ -296,11 +296,19 @@ def _read(self):
     def _try_read(self):
         self._last_heartbeat_received_time = time.time()
         while not self.asked_to_stop:
+            time.sleep(self.read_interval)
             if self._pause:
-                time.sleep(self.read_interval)
                 continue
 
-            msg = self.pipe.receive()
+            # we assign self.pipe to p and access pipe methods through p
+            # this is because self.pipe could be set to None at any moment (e.g. the abort process could
+            # stop the pipe handler at any time).
+            p = self.pipe
+            if not p:
+                # the pipe handler is most likely stopped, but we leave it for the while loop to decide
+                continue
+
+            msg = p.receive()
             now = time.time()
 
             if msg:
@@ -315,7 +323,7 @@ def _try_read(self):
             else:
                 # is peer gone?
                 # ask the pipe for the last known active time of the peer
-                last_peer_active_time = self.pipe.get_last_peer_active_time()
+                last_peer_active_time = p.get_last_peer_active_time()
                 if last_peer_active_time > self._last_heartbeat_received_time:
                     self._last_heartbeat_received_time = last_peer_active_time
 
@@ -331,7 +339,6 @@ def _try_read(self):
                     )
                     break
 
-            time.sleep(self.read_interval)
         self.reader = None
 
     def _heartbeat(self):