test(fastapi): add GC-based app collection test; refactor tracking to WeakSet and safe discard

Author: artemziborev

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -359,8 +359,8 @@ def uninstrument_app(app: fastapi.FastAPI):
         app._is_instrumented_by_opentelemetry = False
 
         # Remove the app from the set of instrumented apps to prevent memory leaks
-        if app in _InstrumentedFastAPI._instrumented_fastapi_apps:
-            _InstrumentedFastAPI._instrumented_fastapi_apps.remove(app)
+        # Use discard to avoid KeyError if already GC'ed
+        _InstrumentedFastAPI._instrumented_fastapi_apps.discard(app)
 
     def instrumentation_dependencies(self) -> Collection[str]:
         return _instruments
@@ -414,7 +414,10 @@ class _InstrumentedFastAPI(fastapi.FastAPI):
     _http_capture_headers_sanitize_fields: list[str] | None = None
     _exclude_spans: list[Literal["receive", "send"]] | None = None
 
-    _instrumented_fastapi_apps = set()
+    # Track instrumented app instances using weak references to avoid GC leaks
+    from weakref import WeakSet as _WeakSet  # type: ignore
+
+    _instrumented_fastapi_apps: "_InstrumentedFastAPI._WeakSet" = _WeakSet()
     _sem_conv_opt_in_mode = _StabilityMode.DEFAULT
 
     def __init__(self, *args, **kwargs):
@@ -435,8 +438,8 @@ def __init__(self, *args, **kwargs):
         _InstrumentedFastAPI._instrumented_fastapi_apps.add(self)
 
     def __del__(self):
-        if self in _InstrumentedFastAPI._instrumented_fastapi_apps:
-            _InstrumentedFastAPI._instrumented_fastapi_apps.remove(self)
+        # Best-effort cleanup; WeakSet clears references on GC automatically
+        _InstrumentedFastAPI._instrumented_fastapi_apps.discard(self)
 
 
 def _get_route_details(scope):

instrumentation/opentelemetry-instrumentation-fastapi/tests/test_fastapi_instrumentation.py

@@ -1407,6 +1407,19 @@ def test_mark_span_internal_in_presence_of_span_from_other_framework(self):
         )
 
 
+class TestFastAPIGarbageCollection(unittest.TestCase):
+    def test_fastapi_app_is_collected_after_instrument(self):
+        import gc
+        import weakref
+
+        app = fastapi.FastAPI()
+        otel_fastapi.FastAPIInstrumentor().instrument_app(app)
+        app_ref = weakref.ref(app)
+        del app
+        gc.collect()
+        self.assertIsNone(app_ref())
+
+
 @patch.dict(
     "os.environ",
     {