Fix callback order for nested blueprints

Handlers registered via url_value_preprocessor, before_request,
context_processor, and url_defaults are called in downward order: First
on the app and last on the current blueprint.

Handlers registered via after_request and teardown_request are called
in upward order: First on the current blueprint and last on the app.

CHANGES.rst

@@ -21,6 +21,9 @@ Unreleased
     :issue:`4096`
 -   The CLI loader handles ``**kwargs`` in a ``create_app`` function.
     :issue:`4170`
+-   Fix the order of ``before_request`` and other callbacks that trigger
+    before the view returns. They are called from the app down to the
+    closest nested blueprint. :issue:`4229`
 
 
 Version 2.0.1

src/flask/app.py

@@ -745,12 +745,12 @@ class Flask(Scaffold):
         :param context: the context as a dictionary that is updated in place
                         to add extra variables.
         """
-        funcs: t.Iterable[
-            TemplateContextProcessorCallable
-        ] = self.template_context_processors[None]
+        funcs: t.Iterable[TemplateContextProcessorCallable] = []
+        if None in self.template_context_processors:
+            funcs = chain(funcs, self.template_context_processors[None])
         reqctx = _request_ctx_stack.top
         if reqctx is not None:
-            for bp in request.blueprints:
+            for bp in reversed(request.blueprints):
                 if bp in self.template_context_processors:
                     funcs = chain(funcs, self.template_context_processors[bp])
         orig_ctx = context.copy()
@@ -1806,7 +1806,9 @@ class Flask(Scaffold):
             # This is called by url_for, which can be called outside a
             # request, can't use request.blueprints.
             bps = _split_blueprint_path(endpoint.rpartition(".")[0])
-            bp_funcs = chain.from_iterable(self.url_default_functions[bp] for bp in bps)
+            bp_funcs = chain.from_iterable(
+                self.url_default_functions[bp] for bp in reversed(bps)
+            )
             funcs = chain(funcs, bp_funcs)
 
         for func in funcs:
@@ -1846,19 +1848,17 @@ class Flask(Scaffold):
         further request handling is stopped.
         """
 
-        funcs: t.Iterable[URLValuePreprocessorCallable] = self.url_value_preprocessors[
-            None
-        ]
-        for bp in request.blueprints:
-            if bp in self.url_value_preprocessors:
-                funcs = chain(funcs, self.url_value_preprocessors[bp])
+        funcs: t.Iterable[URLValuePreprocessorCallable] = []
+        for name in chain([None], reversed(request.blueprints)):
+            if name in self.url_value_preprocessors:
+                funcs = chain(funcs, self.url_value_preprocessors[name])
         for func in funcs:
             func(request.endpoint, request.view_args)
 
-        funcs: t.Iterable[BeforeRequestCallable] = self.before_request_funcs[None]
-        for bp in request.blueprints:
-            if bp in self.before_request_funcs:
-                funcs = chain(funcs, self.before_request_funcs[bp])
+        funcs: t.Iterable[BeforeRequestCallable] = []
+        for name in chain([None], reversed(request.blueprints)):
+            if name in self.before_request_funcs:
+                funcs = chain(funcs, self.before_request_funcs[name])
         for func in funcs:
             rv = self.ensure_sync(func)()
             if rv is not None:
@@ -1881,11 +1881,9 @@ class Flask(Scaffold):
         """
         ctx = _request_ctx_stack.top
         funcs: t.Iterable[AfterRequestCallable] = ctx._after_request_functions
-        for bp in request.blueprints:
-            if bp in self.after_request_funcs:
-                funcs = chain(funcs, reversed(self.after_request_funcs[bp]))
-        if None in self.after_request_funcs:
-            funcs = chain(funcs, reversed(self.after_request_funcs[None]))
+        for name in chain(request.blueprints, [None]):
+            if name in self.after_request_funcs:
+                funcs = chain(funcs, reversed(self.after_request_funcs[name]))
         for handler in funcs:
             response = self.ensure_sync(handler)(response)
         if not self.session_interface.is_null_session(ctx.session):
@@ -1917,12 +1915,10 @@ class Flask(Scaffold):
         """
         if exc is _sentinel:
             exc = sys.exc_info()[1]
-        funcs: t.Iterable[TeardownCallable] = reversed(
-            self.teardown_request_funcs[None]
-        )
-        for bp in request.blueprints:
-            if bp in self.teardown_request_funcs:
-                funcs = chain(funcs, reversed(self.teardown_request_funcs[bp]))
+        funcs: t.Iterable[TeardownCallable] = []
+        for name in chain(request.blueprints, [None]):
+            if name in self.teardown_request_funcs:
+                funcs = chain(funcs, reversed(self.teardown_request_funcs[name]))
         for func in funcs:
             self.ensure_sync(func)(exc)
         request_tearing_down.send(self, exc=exc)

tests/test_blueprints.py

@@ -837,6 +837,86 @@ def test_nested_blueprint(app, client):
     assert client.get("/parent/child/grandchild/no").data == b"Grandchild no"
 
 
+def test_nested_callback_order(app, client):
+    parent = flask.Blueprint("parent", __name__)
+    child = flask.Blueprint("child", __name__)
+
+    @app.before_request
+    def app_before1():
+        flask.g.setdefault("seen", []).append("app_1")
+
+    @app.teardown_request
+    def app_teardown1(e=None):
+        assert flask.g.seen.pop() == "app_1"
+
+    @app.before_request
+    def app_before2():
+        flask.g.setdefault("seen", []).append("app_2")
+
+    @app.teardown_request
+    def app_teardown2(e=None):
+        assert flask.g.seen.pop() == "app_2"
+
+    @app.context_processor
+    def app_ctx():
+        return dict(key="app")
+
+    @parent.before_request
+    def parent_before1():
+        flask.g.setdefault("seen", []).append("parent_1")
+
+    @parent.teardown_request
+    def parent_teardown1(e=None):
+        assert flask.g.seen.pop() == "parent_1"
+
+    @parent.before_request
+    def parent_before2():
+        flask.g.setdefault("seen", []).append("parent_2")
+
+    @parent.teardown_request
+    def parent_teardown2(e=None):
+        assert flask.g.seen.pop() == "parent_2"
+
+    @parent.context_processor
+    def parent_ctx():
+        return dict(key="parent")
+
+    @child.before_request
+    def child_before1():
+        flask.g.setdefault("seen", []).append("child_1")
+
+    @child.teardown_request
+    def child_teardown1(e=None):
+        assert flask.g.seen.pop() == "child_1"
+
+    @child.before_request
+    def child_before2():
+        flask.g.setdefault("seen", []).append("child_2")
+
+    @child.teardown_request
+    def child_teardown2(e=None):
+        assert flask.g.seen.pop() == "child_2"
+
+    @child.context_processor
+    def child_ctx():
+        return dict(key="child")
+
+    @child.route("/a")
+    def a():
+        return ", ".join(flask.g.seen)
+
+    @child.route("/b")
+    def b():
+        return flask.render_template_string("{{ key }}")
+
+    parent.register_blueprint(child)
+    app.register_blueprint(parent)
+    assert (
+        client.get("/a").data == b"app_1, app_2, parent_1, parent_2, child_1, child_2"
+    )
+    assert client.get("/b").data == b"child"
+
+
 @pytest.mark.parametrize(
     "parent_init, child_init, parent_registration, child_registration",
     [