Fix run_engine fixture file handler leak and add test to check for it (#1681)

* Added test to check for memory leak of run_engine

* Adjusted variable names

* Fixed test

* Added small threshold to trigger test fail, increased number of iterations

* First at attempt at fixing memory leak. Single instance of run_engine but is cleared between each use.

* fixed global

* Correct spelling

* Remove pytest_sessionfinish

* Correct _ensure_running_bluesky_event_loop to use same RunEngine

* Rename test to test_run_engine_fixture_has_no_file_handler_leak

* Fixed test, corrected error message to file handler

Author: oliwenmandiamond

src/dodal/testing/fixtures/run_engine.py

@@ -10,22 +10,25 @@
 from bluesky.run_engine import RunEngine
 from bluesky.simulators import RunEngineSimulator
 
+_run_engine = RunEngine()
+
 
 @pytest.fixture(scope="session", autouse=True)
 async def _ensure_running_bluesky_event_loop():
-    run_engine = RunEngine()
     # make sure the event loop is thoroughly up and running before we try to create
     # any ophyd_async devices which might need it
     timeout = time.monotonic() + 1
-    while not run_engine.loop.is_running():
+    while not _run_engine.loop.is_running():
         await asyncio.sleep(0)
         if time.monotonic() > timeout:
             raise TimeoutError("This really shouldn't happen but just in case...")
 
 
 @pytest.fixture()
-async def run_engine():
-    yield RunEngine()
+def run_engine():
+    global _run_engine
+    _run_engine.reset()
+    return _run_engine
 
 
 @pytest.fixture

tests/testing/__init__.py

@@ -0,0 +1,33 @@
+import os
+
+import pytest
+from bluesky import RunEngine
+
+_baseline_n_open_files = None
+threshold = 5
+
+
+@pytest.mark.parametrize("iteration", range(11))
+def test_run_engine_fixture_has_no_file_handler_leak(
+    run_engine: RunEngine, iteration: int
+) -> None:
+    global _baseline_n_open_files
+    pid = os.getpid()
+    n_open_files = len(os.listdir(f"/proc/{pid}/fd"))
+    if iteration == 0:
+        _baseline_n_open_files = n_open_files
+    else:
+        if _baseline_n_open_files is None:
+            raise Exception(
+                "Unable to determine number of file handlers as it is None."
+            )
+        try:
+            delta = n_open_files - _baseline_n_open_files
+            # Allow a small threshold from other processes e.g from pytest itself.
+            assert delta < threshold
+        except AssertionError as exc:
+            raise AssertionError(
+                "Detected file handle leak, the number of open files has increased from "
+                f"{_baseline_n_open_files} to {n_open_files} when calling the "
+                "run_engine fixture",
+            ) from exc