Break reference cycle created by default in Flask instances.

Flask instances with static folders were creating a reference cycle
via their "static" view function (which held a strong reference back
to the Flask instance to call its `send_static_file` method). This
prevented CPython from freeing the memory for a Flask instance
when all external references to it were released.

Now use a weakref for the back reference to avoid this.

Co-authored-by: Joshua Bronson <jab@users.noreply.github.com>

tests/test_basic.py

@@ -1,8 +1,11 @@
+import gc
 import re
 import sys
 import time
 import uuid
+import weakref
 from datetime import datetime
+from platform import python_implementation
 from threading import Thread
 
 import pytest
@@ -16,6 +19,11 @@ from werkzeug.routing import BuildError
 import flask
 
 
+require_cpython_gc = pytest.mark.skipif(
+    python_implementation() != "CPython", reason="Requires CPython GC behavior",
+)
+
+
 def test_options_work(app, client):
     @app.route("/", methods=["GET", "POST"])
     def index():
@@ -1970,3 +1978,19 @@ def test_max_cookie_size(app, client, recwarn):
 
     client.get("/")
     assert len(recwarn) == 0
+
+
+@require_cpython_gc
+def test_app_freed_on_zero_refcount():
+    # A Flask instance should not create a reference cycle that prevents CPython
+    # from freeing it when all external references to it are released (see #3761).
+    gc.disable()
+    try:
+        app = flask.Flask(__name__)
+        assert app.view_functions["static"]
+        weak = weakref.ref(app)
+        assert weak() is not None
+        del app
+        assert weak() is None
+    finally:
+        gc.enable()