Merge "Serialize update of changes in timer driver"

zuul/driver/timer/__init__.py

@@ -17,6 +17,7 @@
 import logging
 import threading
 import time
+from collections import defaultdict
 from uuid import uuid4
 
 from apscheduler.schedulers.background import BackgroundScheduler
@@ -42,6 +43,10 @@ class TimerDriver(Driver, TriggerInterface):
         self.election = None
         self.election_thread = None
         self.election_won = False
+        # Mapping of locks: canonical project name -> lock
+        # The lock are used to avoid concurrent update errors when a
+        # lot of periodic pipelines are triggering simultanously.
+        self.project_update_locks = defaultdict(threading.Lock)
         self.stop_event = threading.Event()
         self.stopped = False
 
@@ -175,7 +180,8 @@ class TimerDriver(Driver, TriggerInterface):
                 event.timestamp = time.time()
                 # Refresh the branch in order to update the item in the
                 # change cache.
-                project.source.getChange(event, refresh=True)
+                with self.project_update_locks[project.canonical_name]:
+                    project.source.getChange(event, refresh=True)
                 log = get_annotated_logger(self.log, event)
                 log.debug("Adding event")
                 self.sched.addTriggerEvent(self.name, event)