chore(utils): stop threads in dummy writer (#15307)

## Description

Fix crash in `appsec_threats` test on macos caused by to many
NativeWriter being created. The crash is due to each native writer
instance holding onto a kqueue fd exceeding the macos ulimit (256). This
fixes the issue by shutting down the tokio runtime of the native writer
when we are done with the DummyTracer ie. when pop is called.

## Testing

Run the `appsec_threats_fastapi` test with `ulimit -n` set to 256 (by
default on mac).

---------

Co-authored-by: Christophe Papazian <[email protected]>
Co-authored-by: Christophe Papazian <[email protected]>

Author: 2 people

tests/appsec/contrib_appsec/test_fastapi.py

@@ -15,7 +15,7 @@
 
 class Test_FastAPI(utils.Contrib_TestClass_For_Threats):
     @pytest.fixture
-    def interface(self, tracer, printer):
+    def interface(self, printer):
         from fastapi.testclient import TestClient
 
         # for fastapi, test tracer needs to be set before the app is created

tests/utils.py

@@ -644,6 +644,10 @@ def pop(self):
         spans = DummyWriterMixin.pop(self)
         if self._trace_flush_enabled:
             flush_test_tracer_spans(self)
+        # Stop the writer threads in case the writer is no longer used.
+        # Otherwise we risk accumulating threads and file descriptors causing crashes
+        # In case the writer is used again it will be restarted by native side.
+        self._inner_writer.before_fork()
         return spans
 
     def recreate(self, appsec_enabled: Optional[bool] = None) -> "DummyWriter":