Fix timeout resets to use the call-time clock

linode_api4/polling.py

@@ -35,14 +35,17 @@ def __init__(self, timeout_seconds=120):
         self._start_time = datetime.datetime.now()
         self._timeout_seconds = timeout_seconds
 
-    def start(self, start_time=datetime.datetime.now()):
+    def start(self, start_time=None):
         """
         Sets the timeout start time to the current time.
 
-        :param start_time: The moment when the context started.
+        :param start_time: The moment when the context started. If omitted,
+                           use the current time when this method is called.
         :type start_time: datetime
         """
-        self._start_time = start_time
+        self._start_time = (
+            datetime.datetime.now() if start_time is None else start_time
+        )
 
     def extend(self, seconds: int):
         """

test/unit/polling_test.py

@@ -0,0 +1,37 @@
+import datetime
+from unittest import TestCase
+from unittest.mock import patch
+
+from linode_api4.polling import TimeoutContext
+
+
+class TimeoutContextTest(TestCase):
+    def test_start_restarts_an_expired_timeout(self):
+        started_at = datetime.datetime(2026, 1, 1)
+        with patch(
+            "linode_api4.polling.datetime.datetime", wraps=datetime.datetime
+        ) as clock:
+            clock.now.return_value = started_at
+            context = TimeoutContext(timeout_seconds=10)
+            context.start(started_at)
+
+            for elapsed in (30, 60):
+                clock.now.return_value = started_at + datetime.timedelta(
+                    seconds=elapsed
+                )
+                self.assertTrue(context.expired)
+                context.start()
+                self.assertEqual(context.seconds_remaining, 10)
+                self.assertTrue(context.valid)
+
+    def test_start_preserves_explicit_start_time(self):
+        started_at = datetime.datetime(2026, 1, 1)
+        now = started_at + datetime.timedelta(seconds=7)
+        with patch(
+            "linode_api4.polling.datetime.datetime", wraps=datetime.datetime
+        ) as clock:
+            clock.now.return_value = now
+            context = TimeoutContext(timeout_seconds=10)
+            context.start(started_at)
+            self.assertEqual(context.seconds_remaining, 3)
+            self.assertTrue(context.valid)