Alter ensure_sync implementation to support extensions

This allows extensions to override the Flask.ensure_sync method and
have the change apply to blueprints as well. Without this change it is
possible for differing blueprints to have differing ensure_sync
approaches depending on the extension used - which would likely result
in event-loop blocking issues.

This also allows blueprints to have a custom ensure_sync, although
this is a by product rather than an expected use case.

tests/test_async.py

@@ -3,12 +3,20 @@ import sys
 
 import pytest
 
-from flask import abort
+from flask import Blueprint
 from flask import Flask
 from flask import request
 from flask.helpers import run_async
 
 
+class AppError(Exception):
+    pass
+
+
+class BlueprintError(Exception):
+    pass
+
+
 @pytest.fixture(name="async_app")
 def _async_app():
     app = Flask(__name__)
@@ -18,24 +26,111 @@ def _async_app():
         await asyncio.sleep(0)
         return request.method
 
+    @app.errorhandler(AppError)
+    async def handle(_):
+        return "", 412
+
     @app.route("/error")
     async def error():
-        abort(412)
+        raise AppError()
+
+    blueprint = Blueprint("bp", __name__)
+
+    @blueprint.route("/", methods=["GET", "POST"])
+    async def bp_index():
+        await asyncio.sleep(0)
+        return request.method
+
+    @blueprint.errorhandler(BlueprintError)
+    async def bp_handle(_):
+        return "", 412
+
+    @blueprint.route("/error")
+    async def bp_error():
+        raise BlueprintError()
+
+    app.register_blueprint(blueprint, url_prefix="/bp")
 
     return app
 
 
 @pytest.mark.skipif(sys.version_info < (3, 7), reason="requires Python >= 3.7")
-def test_async_request_context(async_app):
+@pytest.mark.parametrize("path", ["/", "/bp/"])
+def test_async_route(path, async_app):
     test_client = async_app.test_client()
-    response = test_client.get("/")
+    response = test_client.get(path)
     assert b"GET" in response.get_data()
-    response = test_client.post("/")
+    response = test_client.post(path)
     assert b"POST" in response.get_data()
-    response = test_client.get("/error")
+
+
+@pytest.mark.skipif(sys.version_info < (3, 7), reason="requires Python >= 3.7")
+@pytest.mark.parametrize("path", ["/error", "/bp/error"])
+def test_async_error_handler(path, async_app):
+    test_client = async_app.test_client()
+    response = test_client.get(path)
     assert response.status_code == 412
 
 
+@pytest.mark.skipif(sys.version_info < (3, 7), reason="requires Python >= 3.7")
+def test_async_before_after_request():
+    app_first_called = False
+    app_before_called = False
+    app_after_called = False
+    bp_before_called = False
+    bp_after_called = False
+
+    app = Flask(__name__)
+
+    @app.route("/")
+    def index():
+        return ""
+
+    @app.before_first_request
+    async def before_first():
+        nonlocal app_first_called
+        app_first_called = True
+
+    @app.before_request
+    async def before():
+        nonlocal app_before_called
+        app_before_called = True
+
+    @app.after_request
+    async def after(response):
+        nonlocal app_after_called
+        app_after_called = True
+        return response
+
+    blueprint = Blueprint("bp", __name__)
+
+    @blueprint.route("/")
+    def bp_index():
+        return ""
+
+    @blueprint.before_request
+    async def bp_before():
+        nonlocal bp_before_called
+        bp_before_called = True
+
+    @blueprint.after_request
+    async def bp_after(response):
+        nonlocal bp_after_called
+        bp_after_called = True
+        return response
+
+    app.register_blueprint(blueprint, url_prefix="/bp")
+
+    test_client = app.test_client()
+    test_client.get("/")
+    assert app_first_called
+    assert app_before_called
+    assert app_after_called
+    test_client.get("/bp/")
+    assert bp_before_called
+    assert bp_after_called
+
+
 @pytest.mark.skipif(sys.version_info >= (3, 7), reason="should only raise Python < 3.7")
 def test_async_runtime_error():
     with pytest.raises(RuntimeError):