bpo-29097: Forego fold detection on windows for low timestamp values (GH-2385)

On Windows, passing a negative value to local results in an OSError because localtime_s on Windows does not support negative timestamps. Unfortunately this means that fold detection for timestamps between 0 and max_fold_seconds will result in this OSError since we subtract max_fold_seconds from the timestamp to detect a fold. However, since we know there haven't been any folds in the interval [0, max_fold_seconds) in any timezone, we can hackily just forego fold detection for this time range on Windows.

1 files changed, 6 insertions, 1 deletions

diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 7d4cdac9f4..f647a232f4 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -70,7 +70,7 @@ class TestModule(unittest.TestCase):
                     if not name.startswith('__') and not name.endswith('__'))
         allowed = set(['MAXYEAR', 'MINYEAR', 'date', 'datetime',
                        'datetime_CAPI', 'time', 'timedelta', 'timezone',
-                       'tzinfo'])
+                       'tzinfo', 'sys'])
         self.assertEqual(names - allowed, set([]))
 
     def test_divide_and_round(self):
@@ -4955,6 +4955,11 @@ class TestLocalTimeDisambiguation(unittest.TestCase):
         self.assertEqual(t0.fold, 0)
         self.assertEqual(t1.fold, 1)
 
+    def test_fromtimestamp_low_fold_detection(self):
+        # Ensure that fold detection doesn't cause an
+        # OSError for really low values, see bpo-29097
+        self.assertEqual(datetime.fromtimestamp(0).fold, 0)
+
     @support.run_with_tz('EST+05EDT,M3.2.0,M11.1.0')
     def test_timestamp(self):
         dt0 = datetime(2014, 11, 2, 1, 30)