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.

src/flask/app.py

@@ -2,6 +2,7 @@ import os
 import sys
 import weakref
 from datetime import timedelta
+from inspect import iscoroutinefunction
 from itertools import chain
 from threading import Lock
 
@@ -34,6 +35,7 @@ from .helpers import get_env
 from .helpers import get_flashed_messages
 from .helpers import get_load_dotenv
 from .helpers import locked_cached_property
+from .helpers import run_async
 from .helpers import url_for
 from .json import jsonify
 from .logging import create_logger
@@ -1517,6 +1519,19 @@ class Flask(Scaffold):
         """
         return False
 
+    def ensure_sync(self, func):
+        """Ensure that the returned function is sync and calls the async func.
+
+        .. versionadded:: 2.0
+
+        Override if you wish to change how asynchronous functions are
+        run.
+        """
+        if iscoroutinefunction(func):
+            return run_async(func)
+
+        return func
+
     def make_response(self, rv):
         """Convert the return value from a view function to an instance of
         :attr:`response_class`.

src/flask/blueprints.py

@@ -1,3 +1,4 @@
+from collections import defaultdict
 from functools import update_wrapper
 
 from .scaffold import _endpoint_from_view_func
@@ -235,24 +236,44 @@ class Blueprint(Scaffold):
         # Merge blueprint data into parent.
         if first_registration:
 
-            def extend(bp_dict, parent_dict):
+            def extend(bp_dict, parent_dict, ensure_sync=False):
                 for key, values in bp_dict.items():
                     key = self.name if key is None else f"{self.name}.{key}"
+
+                    if ensure_sync:
+                        values = [app.ensure_sync(func) for func in values]
+
                     parent_dict[key].extend(values)
 
-            def update(bp_dict, parent_dict):
-                for key, value in bp_dict.items():
-                    key = self.name if key is None else f"{self.name}.{key}"
-                    parent_dict[key] = value
+            for key, value in self.error_handler_spec.items():
+                key = self.name if key is None else f"{self.name}.{key}"
+                value = defaultdict(
+                    dict,
+                    {
+                        code: {
+                            exc_class: app.ensure_sync(func)
+                            for exc_class, func in code_values.items()
+                        }
+                        for code, code_values in value.items()
+                    },
+                )
+                app.error_handler_spec[key] = value
 
-            app.view_functions.update(self.view_functions)
-            extend(self.before_request_funcs, app.before_request_funcs)
-            extend(self.after_request_funcs, app.after_request_funcs)
-            extend(self.teardown_request_funcs, app.teardown_request_funcs)
+            for endpoint, func in self.view_functions.items():
+                app.view_functions[endpoint] = app.ensure_sync(func)
+
+            extend(
+                self.before_request_funcs, app.before_request_funcs, ensure_sync=True
+            )
+            extend(self.after_request_funcs, app.after_request_funcs, ensure_sync=True)
+            extend(
+                self.teardown_request_funcs,
+                app.teardown_request_funcs,
+                ensure_sync=True,
+            )
             extend(self.url_default_functions, app.url_default_functions)
             extend(self.url_value_preprocessors, app.url_value_preprocessors)
             extend(self.template_context_processors, app.template_context_processors)
-            update(self.error_handler_spec, app.error_handler_spec)
 
         for deferred in self.deferred_functions:
             deferred(state)
@@ -380,7 +401,9 @@ class Blueprint(Scaffold):
         before each request, even if outside of a blueprint.
         """
         self.record_once(
-            lambda s: s.app.before_request_funcs.setdefault(None, []).append(f)
+            lambda s: s.app.before_request_funcs.setdefault(None, []).append(
+                s.app.ensure_sync(f)
+            )
         )
         return f
 
@@ -388,7 +411,9 @@ class Blueprint(Scaffold):
         """Like :meth:`Flask.before_first_request`.  Such a function is
         executed before the first request to the application.
         """
-        self.record_once(lambda s: s.app.before_first_request_funcs.append(f))
+        self.record_once(
+            lambda s: s.app.before_first_request_funcs.append(s.app.ensure_sync(f))
+        )
         return f
 
     def after_app_request(self, f):
@@ -396,7 +421,9 @@ class Blueprint(Scaffold):
         is executed after each request, even if outside of the blueprint.
         """
         self.record_once(
-            lambda s: s.app.after_request_funcs.setdefault(None, []).append(f)
+            lambda s: s.app.after_request_funcs.setdefault(None, []).append(
+                s.app.ensure_sync(f)
+            )
         )
         return f
 
@@ -443,3 +470,14 @@ class Blueprint(Scaffold):
             lambda s: s.app.url_default_functions.setdefault(None, []).append(f)
         )
         return f
+
+    def ensure_sync(self, f):
+        """Ensure the function is synchronous.
+
+        Override if you would like custom async to sync behaviour in
+        this blueprint. Otherwise :meth:`~flask.Flask..ensure_sync` is
+        used.
+
+        .. versionadded:: 2.0
+        """
+        return f

src/flask/helpers.py

@@ -755,6 +755,7 @@ def run_async(func):
         ctx module, except it has an async inner.
         """
         ctx = None
+
         if _request_ctx_stack.top is not None:
             ctx = _request_ctx_stack.top.copy()
 

src/flask/scaffold.py

@@ -4,7 +4,6 @@ import pkgutil
 import sys
 from collections import defaultdict
 from functools import update_wrapper
-from inspect import iscoroutinefunction
 
 from jinja2 import FileSystemLoader
 from werkzeug.exceptions import default_exceptions
@@ -13,7 +12,6 @@ from werkzeug.exceptions import HTTPException
 from .cli import AppGroup
 from .globals import current_app
 from .helpers import locked_cached_property
-from .helpers import run_async
 from .helpers import send_from_directory
 from .templating import _default_template_ctx_processor
 
@@ -687,17 +685,7 @@ class Scaffold:
             return exc_class, None
 
     def ensure_sync(self, func):
-        """Ensure that the returned function is sync and calls the async func.
-
-        .. versionadded:: 2.0
-
-        Override if you wish to change how asynchronous functions are
-        run.
-        """
-        if iscoroutinefunction(func):
-            return run_async(func)
-        else:
-            return func
+        raise NotImplementedError()
 
 
 def _endpoint_from_view_func(view_func):