Split the App and Blueprint into Sansio and IO parts

This follows a similar structure in Werkzeug and allows for async
based IO projects, specifically Quart, to base themselves on
Flask.

Note that the globals, and signals are specific to Flask and hence
specific to Flask's IO. This means they cannot be moved to the sansio
part of the codebase.

src/flask/blueprints.py

@@ -0,0 +1,91 @@
+from __future__ import annotations
+
+import os
+import typing as t
+from datetime import timedelta
+
+from .globals import current_app
+from .helpers import send_from_directory
+from .sansio.blueprints import Blueprint as SansioBlueprint
+from .sansio.blueprints import BlueprintSetupState as BlueprintSetupState  # noqa
+
+if t.TYPE_CHECKING:  # pragma: no cover
+    from .wrappers import Response
+
+
+class Blueprint(SansioBlueprint):
+    def get_send_file_max_age(self, filename: str | None) -> int | None:
+        """Used by :func:`send_file` to determine the ``max_age`` cache
+        value for a given file path if it wasn't passed.
+
+        By default, this returns :data:`SEND_FILE_MAX_AGE_DEFAULT` from
+        the configuration of :data:`~flask.current_app`. This defaults
+        to ``None``, which tells the browser to use conditional requests
+        instead of a timed cache, which is usually preferable.
+
+        Note this is a duplicate of the same method in the Flask
+        class.
+
+        .. versionchanged:: 2.0
+            The default configuration is ``None`` instead of 12 hours.
+
+        .. versionadded:: 0.9
+        """
+        value = current_app.config["SEND_FILE_MAX_AGE_DEFAULT"]
+
+        if value is None:
+            return None
+
+        if isinstance(value, timedelta):
+            return int(value.total_seconds())
+
+        return value
+
+    def send_static_file(self, filename: str) -> Response:
+        """The view function used to serve files from
+        :attr:`static_folder`. A route is automatically registered for
+        this view at :attr:`static_url_path` if :attr:`static_folder` is
+        set.
+
+        Note this is a duplicate of the same method in the Flask
+        class.
+
+        .. versionadded:: 0.5
+
+        """
+        if not self.has_static_folder:
+            raise RuntimeError("'static_folder' must be set to serve static_files.")
+
+        # send_file only knows to call get_send_file_max_age on the app,
+        # call it here so it works for blueprints too.
+        max_age = self.get_send_file_max_age(filename)
+        return send_from_directory(
+            t.cast(str, self.static_folder), filename, max_age=max_age
+        )
+
+    def open_resource(self, resource: str, mode: str = "rb") -> t.IO[t.AnyStr]:
+        """Open a resource file relative to :attr:`root_path` for
+        reading.
+
+        For example, if the file ``schema.sql`` is next to the file
+        ``app.py`` where the ``Flask`` app is defined, it can be opened
+        with:
+
+        .. code-block:: python
+
+            with app.open_resource("schema.sql") as f:
+                conn.executescript(f.read())
+
+        :param resource: Path to the resource relative to
+            :attr:`root_path`.
+        :param mode: Open the file in this mode. Only reading is
+            supported, valid values are "r" (or "rt") and "rb".
+
+        Note this is a duplicate of the same method in the Flask
+        class.
+
+        """
+        if mode not in {"r", "rt", "rb"}:
+            raise ValueError("Resources can only be opened for reading.")
+
+        return open(os.path.join(self.root_path, resource), mode)