Ensured that jitter is always positive (#416)

Fixes #291.

3 files changed, 6 insertions, 35 deletions

diff --git a/apscheduler/triggers/base.py b/apscheduler/triggers/base.py
index ce2526a..55d010d 100644
--- a/apscheduler/triggers/base.py
+++ b/apscheduler/triggers/base.py
@@ -22,27 +22,16 @@ class BaseTrigger(six.with_metaclass(ABCMeta)):
 
     def _apply_jitter(self, next_fire_time, jitter, now):
         """
-        Randomize ``next_fire_time`` by adding or subtracting a random value (the jitter). If the
-        resulting datetime is in the past, returns the initial ``next_fire_time`` without jitter.
-
-        ``next_fire_time - jitter <= result <= next_fire_time + jitter``
+        Randomize ``next_fire_time`` by adding a random value (the jitter).
 
         :param datetime.datetime|None next_fire_time: next fire time without jitter applied. If
             ``None``, returns ``None``.
-        :param int|None jitter: maximum number of seconds to add or subtract to
-            ``next_fire_time``. If ``None`` or ``0``, returns ``next_fire_time``
+        :param int|None jitter: maximum number of seconds to add to ``next_fire_time``
+            (if ``None`` or ``0``, returns ``next_fire_time``)
         :param datetime.datetime now: current datetime
         :return datetime.datetime|None: next fire time with a jitter.
         """
         if next_fire_time is None or not jitter:
             return next_fire_time
 
-        next_fire_time_with_jitter = next_fire_time + timedelta(
-                seconds=random.uniform(-jitter, jitter))
-
-        if next_fire_time_with_jitter < now:
-            # Next fire time with jitter is in the past.
-            # Ignore jitter to avoid false misfire.
-            return next_fire_time
-
-        return next_fire_time_with_jitter
+        return next_fire_time + timedelta(seconds=random.uniform(0, jitter))
diff --git a/docs/versionhistory.rst b/docs/versionhistory.rst
index 623b3e2..7df2888 100644
--- a/docs/versionhistory.rst
+++ b/docs/versionhistory.rst
@@ -12,6 +12,8 @@ APScheduler, see the :doc:`migration section <migration>`.
 * Pinned ``tzlocal`` to a version compatible with pytz
 * Fixed deprecation warnings on the MongoDB job store and increased the minimum PyMongo
   version to 3.0
+* Ensured that jitter is always non-negative to prevent triggers from firing more often than
+  intended
 
 
 3.6.3
diff --git a/tests/test_triggers.py b/tests/test_triggers.py
index a637485..ea0d1c1 100644
--- a/tests/test_triggers.py
+++ b/tests/test_triggers.py
@@ -50,16 +50,6 @@ class TestJitter(object):
         trigger = _DummyTriggerWithJitter(dt, 60)
         assert trigger.get_next_fire_time(None, now) == expected_dt
 
-    def test_jitter_in_past_but_initial_date_in_future(self, monkeypatch):
-        monkeypatch.setattr(random, 'uniform', lambda a, b: -30.)
-
-        now = datetime(2017, 5, 25, 13, 40, 44)
-        dt = datetime(2017, 5, 25, 13, 40, 47)
-        expected_dt = dt
-
-        trigger = _DummyTriggerWithJitter(dt, 60)
-        assert trigger.get_next_fire_time(None, now) == expected_dt
-
     def test_jitter_in_future_but_initial_date_in_past(self, monkeypatch):
         monkeypatch.setattr(random, 'uniform', lambda a, b: 30.)
 
@@ -70,16 +60,6 @@ class TestJitter(object):
         trigger = _DummyTriggerWithJitter(dt, 60)
         assert trigger.get_next_fire_time(None, now) == expected_dt
 
-    def test_jitter_misfire(self, monkeypatch):
-        monkeypatch.setattr(random, 'uniform', lambda a, b: -30.)
-
-        now = datetime(2017, 5, 25, 13, 40, 44)
-        dt = datetime(2017, 5, 25, 13, 40, 40)
-        expected_dt = dt
-
-        trigger = _DummyTriggerWithJitter(dt, 60)
-        assert trigger.get_next_fire_time(None, now) == expected_dt
-
     def test_jitter_is_now(self, monkeypatch):
         monkeypatch.setattr(random, 'uniform', lambda a, b: 4.)