[tor-commits] [ooni-probe/master] Fix #612 deadlock confirmed by TestScheduler.test_thundering_herd

[art at torproject.org]

commit 15bfb0312030a79c1985564dcbf7ee18101388b4
Author: Leonid Evdokimov <leon at darkk.net.ru>
Date:   Mon Oct 10 23:06:50 2016 +0300

    Fix #612 deadlock confirmed by TestScheduler.test_thundering_herd
    
    The trivial fix makes sense as current SchedulerService ticks every 30
    seconds and imprecise scheduling is acceptable.
---
 ooni/agent/scheduler.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/ooni/agent/scheduler.py b/ooni/agent/scheduler.py
index f6b52b7..969602a 100644
--- a/ooni/agent/scheduler.py
+++ b/ooni/agent/scheduler.py
@@ -61,6 +61,11 @@ class DidNotRun(Exception):
 
 
 class ScheduledTask(object):
+    """
+    Two ScheduledTask instances with same identifier are not permited to run
+    concurrently.  There should be no ScheduledTask queue waiting for the lock
+    as SchedulerService ticks quite often.
+    """
     _time_format = "%Y-%m-%dT%H:%M:%SZ"
     schedule = None
     identifier = None
@@ -117,6 +122,9 @@ class ScheduledTask(object):
 
     @defer.inlineCallbacks
     def run(self):
+        if self._last_run_lock.locked:
+            # do not allow the queue to grow forever
+            raise DidNotRun
         yield self._last_run_lock.acquire()
         if not self.should_run:
             self._last_run_lock.release()