fix(asm): fix cpu leak

ddtrace/appsec/_asm_request_context.py

@@ -151,7 +151,7 @@ def finalise(self):
             callbacks = GLOBAL_CALLBACKS.get(_CONTEXT_CALL, []) if env.must_call_globals else []
             env.must_call_globals = False
             if env is not None and env.callbacks is not None and env.callbacks.get(_CONTEXT_CALL):
-                callbacks += env.callbacks.get(_CONTEXT_CALL)
+                callbacks = callbacks + env.callbacks.get(_CONTEXT_CALL)
             if callbacks:
                 if env is not None:
                     for function in callbacks:

releasenotes/notes/fix_incident_25768-043ae458e46c9620.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    ASM: This fix resolves an issue with Flask instrumentation causing CPU leak with ASM, 
+    API Security and Telemetry enabled.

tests/appsec/contrib_appsec/utils.py

@@ -1037,6 +1037,25 @@ def test_multiple_service_name(self, interface):
             else:
                 raise AssertionError("extra service not found")
 
+    def test_global_callback_list_length(self, interface):
+        from ddtrace.appsec import _asm_request_context
+
+        with override_global_config(
+            dict(
+                _asm_enabled=True,
+                _api_security_enabled=True,
+                _telemetry_enabled=True,
+            )
+        ):
+            self.update_tracer(interface)
+            assert ddtrace.config._remote_config_enabled
+            for _ in range(20):
+                response = interface.client.get("/new_service/awesome_test")
+            assert self.status(response) == 200
+            assert self.body(response) == "awesome_test"
+            # only two global callbacks are expected for API Security and Nested Events
+            assert len(_asm_request_context.GLOBAL_CALLBACKS.get(_asm_request_context._CONTEXT_CALL, [])) == 2
+
 
 @contextmanager
 def test_tracer():