test: synchronize watchdog log rotation assertions

tests/unit/test_event_loop_diagnostics.py

@@ -1,6 +1,5 @@
 import asyncio
 import threading
-import time
 
 import pytest
 
@@ -52,11 +51,18 @@ async def test_event_loop_watchdog_stops_worker_thread():
 
 
 @pytest.mark.asyncio
-async def test_event_loop_watchdog_writes_rotating_log(tmp_path):
+async def test_event_loop_watchdog_writes_rotating_log(tmp_path, monkeypatch):
     """The watchdog should write to and rotate its log file."""
     log_path = tmp_path / "logs" / "event_loop_watchdog.log"
     log_path.parent.mkdir()
     log_path.write_text("x" * 8, encoding="utf-8")
+    stack_written = threading.Event()
+    print_stack = diagnostics.traceback.print_stack
+    monkeypatch.setattr(
+        diagnostics.traceback,
+        "print_stack",
+        lambda *args, **kwargs: (print_stack(*args, **kwargs), stack_written.set()),
+    )
 
     task = asyncio.create_task(
         diagnostics.event_loop_watchdog(
@@ -66,11 +72,13 @@ async def test_event_loop_watchdog_writes_rotating_log(tmp_path):
             max_bytes=4,
         )
     )
-    await asyncio.sleep(0)
-    time.sleep(0.05)  # noqa: ASYNC251 - Intentionally block the event loop.
-    await asyncio.sleep(0.02)
-    task.cancel()
-    await asyncio.gather(task, return_exceptions=True)
+    try:
+        await asyncio.sleep(0)
+        # Keep the test frame on the stack until the watchdog writes it.
+        assert stack_written.wait(timeout=2)
+    finally:
+        task.cancel()
+        await asyncio.gather(task, return_exceptions=True)
 
     log_content = log_path.read_text(encoding="utf-8")
     assert "Event loop stalled for" in log_content