Avoid croniter infinite loop (#5957)

* Avoid croniter loop

* Update test_clocks.py

* Create PR5957.yaml

* mypy

* Update clocks.py

changes/PR5957.yaml

@@ -0,0 +1,21 @@
+# An example changelog entry
+#
+# 1. Choose one (or more if a PR encompasses multiple changes) of the following headers:
+#   - feature
+#   - enhancement
+#   - task
+#   - fix
+#   - deprecation
+#   - breaking (for breaking changes)
+#
+# 2. Fill in one (or more) bullet points under the heading, describing the change.
+#    Markdown syntax may be used.
+#
+# 3. If you would like to be credited as helping with this release, add a
+#    contributor section with your name and github username.
+#
+# Here's an example of a PR that adds an enhancement
+
+fix:
+  - "Fix bug with infinite loop when parsing DST cron schedules - [#5957](https://github.com/PrefectHQ/prefect/pull/5957)"
+

src/prefect/schedules/clocks.py

@@ -317,13 +317,31 @@ def events(self, after: datetime = None) -> Iterable[ClockEvent]:
         cron = croniter(self.cron, after_localized, day_or=self.day_or)  # type: ignore
         dates = set()  # type: Set[datetime]
 
+        iterations = 0
         while True:
+            iterations += 1
+            # escape hatch
+            if iterations > 1000:
+                break
             next_date = pendulum.instance(cron.get_next(datetime))
             # because of croniter's rounding behavior, we want to avoid
             # issuing the after date; we also want to avoid duplicates caused by
             # DST boundary issues
-            if next_date.in_tz("UTC") == after.in_tz("UTC") or next_date in dates:
+            stuck_count = 0
+            while next_date.in_tz("UTC") == after.in_tz("UTC") or next_date in dates:
                 next_date = pendulum.instance(cron.get_next(datetime))
+                stuck_count += 1
+
+                # if we are stuck in a DST loop, create a new cron instance
+                # that starts after this date. This brute force avoids
+                # croniter bugs like the Santiago DST boundary
+                if stuck_count == 3:
+                    after = next_date
+                    cron = croniter(self.cron, after, day_or=self.day_or)  # type: ignore
+                    next_date = pendulum.instance(cron.get_next(datetime))
+                    # if we're still seeing the issue, escape!
+                    if next_date in dates:
+                        iterations = 1000
 
             if self.end_date and next_date > self.end_date:
                 break

tests/schedules/test_clocks.py

@@ -640,6 +640,26 @@ def test_cron_clock_daily_start_daylight_savings_time_backward(self, serialize):
         ]
         assert [t.start_time.in_tz("UTC").hour for t in next_4] == [13, 13, 14, 14]
 
+    @pytest.mark.parametrize("serialize", [True, False])
+    def test_cron_clock_santiago(self, serialize):
+        """
+        On 9/4/2022, at 12am, America/Santiago switches clocks back an hour.
+
+        Croniter gets stuck in an infinite loop at midnight
+        """
+        dt = pendulum.datetime(2022, 8, 21, 10, tz="America/Santiago")
+        c = clocks.CronClock("0 0 * * 0", dt)
+        if serialize:
+            c = ClockSchema().load(ClockSchema().dump(c))
+        next_4 = islice(c.events(after=dt), 4)
+        # no infinite loop on the 4th
+        assert [t.start_time.in_tz("America/Santiago").day for t in next_4] == [
+            28,
+            4,
+            11,
+            18,
+        ]
+
 
 class TestDatesClock:
     def test_create_dates_clock_one_date(self):