Prevent MockClock from mutating the current runner

newsfragments/3205.bugfix.rst

@@ -0,0 +1 @@
+Don't mutate the global runner when MockClock is created.

src/trio/_core/_mock_clock.py

@@ -70,7 +70,7 @@ def __init__(self, rate: float = 0.0, autojump_threshold: float = inf) -> None:
         self._real_base = 0.0
         self._virtual_base = 0.0
         self._rate = 0.0
-        self._autojump_threshold = 0.0
+
         # kept as an attribute so that our tests can monkeypatch it
         self._real_clock = time.perf_counter
 
@@ -119,7 +119,8 @@ def _try_resync_autojump_threshold(self) -> None:
         except AttributeError:
             pass
         else:
-            runner.clock_autojump_threshold = self._autojump_threshold
+            if runner.clock is self:
+                runner.clock_autojump_threshold = self._autojump_threshold
 
     # Invoked by the run loop when runner.clock_autojump_threshold is
     # exceeded.

src/trio/_core/_tests/test_mock_clock.py

@@ -8,6 +8,7 @@
 from ... import _core
 from .. import wait_all_tasks_blocked
 from .._mock_clock import MockClock
+from .._run import GLOBAL_RUN_CONTEXT
 from .tutil import slow
 
 
@@ -175,3 +176,18 @@ async def waiter() -> None:
         nursery.start_soon(waiter)
 
     assert record == ["waiter done", "yawn"]
+
+
+async def test_initialization_doesnt_mutate_runner() -> None:
+    before = (
+        GLOBAL_RUN_CONTEXT.runner.clock,
+        GLOBAL_RUN_CONTEXT.runner.clock_autojump_threshold,
+    )
+
+    MockClock(autojump_threshold=2, rate=3)
+
+    after = (
+        GLOBAL_RUN_CONTEXT.runner.clock,
+        GLOBAL_RUN_CONTEXT.runner.clock_autojump_threshold,
+    )
+    assert before == after