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.
diff --git a/Lib/datetime.py b/Lib/datetime.py
index 5e922c8..cff9203 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -6,6 +6,7 @@
import time as _time
import math as _math
+import sys
def _cmp(x, y):
return 0 if x == y else 1 if x > y else -1
@@ -1572,6 +1573,14 @@
# 23 hours at 1969-09-30 13:00:00 in Kwajalein.
# Let's probe 24 hours in the past to detect a transition:
max_fold_seconds = 24 * 3600
+
+ # On Windows localtime_s throws an OSError for negative values,
+ # thus we can't perform fold detection for values of time less
+ # than the max time fold. See comments in _datetimemodule's
+ # version of this method for more details.
+ if t < max_fold_seconds and sys.platform.startswith("win"):
+ return result
+
y, m, d, hh, mm, ss = converter(t - max_fold_seconds)[:6]
probe1 = cls(y, m, d, hh, mm, ss, us, tz)
trans = result - probe1 - timedelta(0, max_fold_seconds)