Fix issue with Flask instrumentation when a request spawn children th…

…reads and copie the request context

CHANGELOG.md

@@ -26,7 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#1618](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1618))
 
 ### Fixed
-
+- Fix Flask instrumentation to only close the span if it was created by the same thread. `ValueError: generator already executing` ([#1653](https://github.com/open-telemetry/opentelemetry-python-contrib/issues/1653))
 - Fix TortoiseORM instrumentation `AttributeError: type object 'Config' has no attribute 'title'`
   ([#1575](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1575))
 - Fix SQLAlchemy uninstrumentation

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

@@ -238,10 +238,10 @@ def response_hook(span: Span, status: str, response_headers: List):
 API
 ---
 """
-
 from logging import getLogger
 from time import time_ns
 from timeit import default_timer
+from threading import get_ident
 from typing import Collection
 
 import flask
@@ -397,7 +397,10 @@ def _before_request():
 
         activation = trace.use_span(span, end_on_exit=True)
         activation.__enter__()  # pylint: disable=E1101
-        flask_request_environ[_ENVIRON_ACTIVATION_KEY] = activation
+        flask_request_environ[_ENVIRON_ACTIVATION_KEY] = (
+            get_ident(),
+            activation,
+        )
         flask_request_environ[_ENVIRON_SPAN_KEY] = span
         flask_request_environ[_ENVIRON_TOKEN] = token
 
@@ -436,8 +439,10 @@ def _teardown_request(exc):
         if excluded_urls and excluded_urls.url_disabled(flask.request.url):
             return
 
-        activation = flask.request.environ.get(_ENVIRON_ACTIVATION_KEY)
-        if not activation:
+        thread_id, activation = flask.request.environ.get(
+            _ENVIRON_ACTIVATION_KEY
+        )
+        if not activation or thread_id != get_ident():
             # This request didn't start a span, maybe because it was created in
             # a way that doesn't run `before_request`, like when it is created
             # with `app.test_request_context`.