fix (ProcessPool): add more safeguard when processing result queue

src/DIRAC/Core/Utilities/ProcessPool.py

@@ -93,6 +93,7 @@
 executing same type of callables in subprocesses and  hence you are expecting the same type of results
 everywhere.
 """
+
 import errno
 import inspect
 import multiprocessing
@@ -911,8 +912,14 @@ def processResults(self):
                 if processed == 0:
                     log.debug("Process results, but queue is empty...")
                 break
-            # get task
-            task = self.__resultsQueue.get()
+            # In principle, there should be a task right away. However,
+            # queue.empty can't be trusted (https://docs.python.org/3/library/queue.html#queue.Queue.empty)
+            try:
+                task = self.__resultsQueue.get(block=False)
+            except queue.Empty:
+                log.warn("Queue.empty lied to us again...")
+                return 0
+
             log.debug("__resultsQueue.get", f"t={time.time() - start:.2f}")
             # execute callbacks
             try: