Use a weak reference to sqlalchemy Engine to avoid memory leak (#1771)

* Use a weak reference to sqlalchemy Engine to avoid memory leak

Closes #1761

By using a weak reference to the `Engine` object, we can avoid the memory leak as disposed `Engines` get properly deallocated. Whenever `SQLAlchemy` is uninstrumented, we only trigger a removal for those event listeners which are listening for objects that haven't been garbage-collected yet.

* Made a mistake in resolving the weak reference

* Fixed formatting issues

* Updated changelog

* Added unit test to check that engine was garbage collected

* Do not save engine in EngineTracer to avoid memory leak

* Add an empty line to satisfy black formatter

* Fix isort complaints

* Fixed the issue when pool name is not set and =None

* Fix formatting issue

* Rebased after changes in a recent commit

* Updated PR number in changelog

---------

Co-authored-by: Shalev Roda <[email protected]>

CHANGELOG.md

@@ -40,6 +40,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `opentelemetry-instrumentation-system-metrics` Add `process.` prefix to `runtime.memory`, `runtime.cpu.time`, and `runtime.gc_count`. Change `runtime.memory` from count to UpDownCounter. ([#1735](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1735))
 - Add request and response hooks for GRPC instrumentation (client only)
   ([#1706](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1706))
+- Fix memory leak in SQLAlchemy instrumentation where disposed `Engine` does not get garbage collected
+  ([#1771](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1771)
 - `opentelemetry-instrumentation-pymemcache` Update instrumentation to support pymemcache >4
   ([#1764](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1764))
 - `opentelemetry-instrumentation-confluent-kafka` Add support for higher versions of confluent_kafka

instrumentation/opentelemetry-instrumentation-sqlalchemy/src/opentelemetry/instrumentation/sqlalchemy/engine.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 import os
 import re
+import weakref
 
 from sqlalchemy.event import (  # pylint: disable=no-name-in-module
     listen,
@@ -99,11 +100,11 @@ def __init__(
         commenter_options=None,
     ):
         self.tracer = tracer
-        self.engine = engine
         self.connections_usage = connections_usage
         self.vendor = _normalize_vendor(engine.name)
         self.enable_commenter = enable_commenter
         self.commenter_options = commenter_options if commenter_options else {}
+        self._engine_attrs = _get_attributes_from_engine(engine)
         self._leading_comment_remover = re.compile(r"^/\*.*?\*/")
 
         self._register_event_listener(
@@ -118,23 +119,11 @@ def __init__(
         self._register_event_listener(engine, "checkin", self._pool_checkin)
         self._register_event_listener(engine, "checkout", self._pool_checkout)
 
-    def _get_connection_string(self):
-        drivername = self.engine.url.drivername or ""
-        host = self.engine.url.host or ""
-        port = self.engine.url.port or ""
-        database = self.engine.url.database or ""
-        return f"{drivername}://{host}:{port}/{database}"
-
-    def _get_pool_name(self):
-        if self.engine.pool.logging_name is not None:
-            return self.engine.pool.logging_name
-        return self._get_connection_string()
-
     def _add_idle_to_connection_usage(self, value):
         self.connections_usage.add(
             value,
             attributes={
-                "pool.name": self._get_pool_name(),
+                **self._engine_attrs,
                 "state": "idle",
             },
         )
@@ -143,7 +132,7 @@ def _add_used_to_connection_usage(self, value):
         self.connections_usage.add(
             value,
             attributes={
-                "pool.name": self._get_pool_name(),
+                **self._engine_attrs,
                 "state": "used",
             },
         )
@@ -169,12 +158,21 @@ def _pool_checkout(
     @classmethod
     def _register_event_listener(cls, target, identifier, func, *args, **kw):
         listen(target, identifier, func, *args, **kw)
-        cls._remove_event_listener_params.append((target, identifier, func))
+        cls._remove_event_listener_params.append(
+            (weakref.ref(target), identifier, func)
+        )
 
     @classmethod
     def remove_all_event_listeners(cls):
-        for remove_params in cls._remove_event_listener_params:
-            remove(*remove_params)
+        for (
+            weak_ref_target,
+            identifier,
+            func,
+        ) in cls._remove_event_listener_params:
+            # Remove an event listener only if saved weak reference points to an object
+            # which has not been garbage collected
+            if weak_ref_target() is not None:
+                remove(weak_ref_target(), identifier, func)
         cls._remove_event_listener_params.clear()
 
     def _operation_name(self, db_name, statement):
@@ -300,3 +298,22 @@ def _get_attributes_from_cursor(vendor, cursor, attrs):
             if info.port:
                 attrs[SpanAttributes.NET_PEER_PORT] = int(info.port)
     return attrs
+
+
+def _get_connection_string(engine):
+    drivername = engine.url.drivername or ""
+    host = engine.url.host or ""
+    port = engine.url.port or ""
+    database = engine.url.database or ""
+    return f"{drivername}://{host}:{port}/{database}"
+
+
+def _get_attributes_from_engine(engine):
+    """Set metadata attributes of the database engine"""
+    attrs = {}
+
+    attrs["pool.name"] = getattr(
+        getattr(engine, "pool", None), "logging_name", None
+    ) or _get_connection_string(engine)
+
+    return attrs

instrumentation/opentelemetry-instrumentation-sqlalchemy/tests/test_sqlalchemy.py

@@ -307,3 +307,26 @@ def test_no_op_tracer_provider(self):
         cnx.execute("SELECT 1 + 1;").fetchall()
         spans = self.memory_exporter.get_finished_spans()
         self.assertEqual(len(spans), 0)
+
+    def test_no_memory_leakage_if_engine_diposed(self):
+        SQLAlchemyInstrumentor().instrument()
+        import gc
+        import weakref
+
+        from sqlalchemy import create_engine
+
+        callback = mock.Mock()
+
+        def make_shortlived_engine():
+            engine = create_engine("sqlite:///:memory:")
+            # Callback will be called if engine is deallocated during garbage
+            # collection
+            weakref.finalize(engine, callback)
+            with engine.connect() as conn:
+                conn.execute("SELECT 1 + 1;").fetchall()
+
+        for _ in range(0, 5):
+            make_shortlived_engine()
+
+        gc.collect()
+        assert callback.call_count == 5