gh-145703: Fix asyncio.BaseEventLoop low clock resolution (#145706)

Author: CaptainFlint

Lib/asyncio/base_events.py

@@ -19,14 +19,15 @@
 import errno
 import heapq
 import itertools
+import math
 import os
 import socket
 import stat
 import subprocess
+import sys
 import threading
 import time
 import traceback
-import sys
 import warnings
 import weakref
 
@@ -2022,7 +2023,10 @@ def _run_once(self):
         event_list = None
 
         # Handle 'later' callbacks that are ready.
-        end_time = self.time() + self._clock_resolution
+        now = self.time()
+        # Ensure that `end_time` is strictly increasing
+        # when the clock resolution is too small.
+        end_time = now + max(self._clock_resolution, math.ulp(now))
         while self._scheduled:
             handle = self._scheduled[0]
             if handle._when >= end_time:

Misc/NEWS.d/next/Library/2026-03-09-19-59-05.gh-issue-145703.4EEP7J.rst

@@ -0,0 +1,3 @@
+:mod:`asyncio`: Make sure that :meth:`loop.call_at <asyncio.loop.call_at>` and
+:meth:`loop.call_later <asyncio.loop.call_later>` trigger scheduled events on
+time when the clock resolution becomes too small.