Make the FileSummaryRecord.periodic_flush test more reliable

Kernel/FileSummaryRecord/tests/python/FSRTests/periodic_flush.py

 ###############################################################################
-# (c) Copyright 2022 CERN for the benefit of the LHCb Collaboration           #
+# (c) Copyright 2022-2023 CERN for the benefit of the LHCb Collaboration      #
 #                                                                             #
 # This software is distributed under the terms of the GNU General Public      #
 # Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   #
@@ -28,37 +28,37 @@ def checkDiff(a, b):
 
 
 def keep_intermediate_files(filename):
-    from time import sleep
-    wait_time = 0.1
-    max_time = 90
-    max_iter = int(max_time / wait_time)
-    counter = 0
-    while max_iter and counter < 6:
+    print("===> starting thread to copy FSR dumps", flush=True)
+    from time import sleep, time
+    wait_time = 0.01
+    max_time = time() + 300
+    counter = -1
+    # when to stop looking for new FSR dumps after event 5
+    # or if we have been running for too long
+    while counter < 5 and time() < max_time:
         try:
             data = open(filename).read()
             parsed_data = json.loads(data)  # check that the file can be parser
-            # we make a copy of the FSR file on the first appearance and whenever
-            # we see a change, but only after the guid of the output file is included
-            # (if the test job is very slow we may end up with an FSR file with counter
-            # of 1 and no guid followed by a file with counter of 1 and a guid, but the
-            # validation fails if we have a file with counter of 1 and no guid)
-            if counter == 0 or (
-                    data != open(f"{filename}.~{counter-1}~").read()
-                    and "guid" in parsed_data):
+            # we make a copy of the FSR file for each event, only if the output file guid is filled
+            if not parsed_data.get("guid"):
+                continue
+            counter = parsed_data.get("EvtCounter.count", {}).get(
+                "nEntries", -1)
+            if counter >= 0 and not os.path.exists(f"{filename}.~{counter}~"):
                 with open(f"{filename}.~{counter}~", "w") as f:
                     f.write(data)
                     print("=== FSR begin ===")
                     print(data.rstrip())
                     print("=== FSR  end  ===")
                     print(f"---> wrote {filename}.~{counter}~", flush=True)
-                counter += 1
         except FileNotFoundError:
             pass
         except json.JSONDecodeError:
             continue  # the file is corrupted (it might be being written)
 
         sleep(wait_time)
-        max_iter -= 1
+
+    print("===> stopping thread to copy FSR dumps", flush=True)
 
 
 def config():