Move parts of the session code to sansio

This will allow it to be used in Quart, thereby reducing duplication,
and ensuring the APIs match.

src/flask/sansio/sessions.py

@@ -0,0 +1,215 @@
+from __future__ import annotations
+
+import collections.abc as c
+import typing as t
+from abc import ABCMeta
+from collections.abc import MutableMapping
+from datetime import datetime
+from datetime import timezone
+
+from werkzeug.datastructures import CallbackDict
+
+from .app import App
+
+if t.TYPE_CHECKING:  # pragma: no cover
+    import typing_extensions as te
+
+
+class SessionMixin(MutableMapping[str, t.Any]):
+    """Expands a basic dictionary with session attributes."""
+
+    @property
+    def permanent(self) -> bool:
+        """This reflects the ``'_permanent'`` key in the dict."""
+        return self.get("_permanent", False)
+
+    @permanent.setter
+    def permanent(self, value: bool) -> None:
+        self["_permanent"] = bool(value)
+
+    #: Some implementations can detect whether a session is newly
+    #: created, but that is not guaranteed. Use with caution. The mixin
+    # default is hard-coded ``False``.
+    new = False
+
+    #: Some implementations can detect changes to the session and set
+    #: this when that happens. The mixin default is hard coded to
+    #: ``True``.
+    modified = True
+
+    #: Some implementations can detect when session data is read or
+    #: written and set this when that happens. The mixin default is hard
+    #: coded to ``True``.
+    accessed = True
+
+
+class SecureCookieSession(CallbackDict[str, t.Any], SessionMixin):
+    """Base class for sessions based on signed cookies.
+
+    This session backend will set the :attr:`modified` and
+    :attr:`accessed` attributes. It cannot reliably track whether a
+    session is new (vs. empty), so :attr:`new` remains hard coded to
+    ``False``.
+    """
+
+    #: When data is changed, this is set to ``True``. Only the session
+    #: dictionary itself is tracked; if the session contains mutable
+    #: data (for example a nested dict) then this must be set to
+    #: ``True`` manually when modifying that data. The session cookie
+    #: will only be written to the response if this is ``True``.
+    modified = False
+
+    #: When data is read or written, this is set to ``True``. Used by
+    # :class:`.SecureCookieSessionInterface` to add a ``Vary: Cookie``
+    #: header, which allows caching proxies to cache different pages for
+    #: different users.
+    accessed = False
+
+    def __init__(
+        self,
+        initial: c.Mapping[str, t.Any] | c.Iterable[tuple[str, t.Any]] | None = None,
+    ) -> None:
+        def on_update(self: te.Self) -> None:
+            self.modified = True
+            self.accessed = True
+
+        super().__init__(initial, on_update)
+
+    def __getitem__(self, key: str) -> t.Any:
+        self.accessed = True
+        return super().__getitem__(key)
+
+    def get(self, key: str, default: t.Any = None) -> t.Any:
+        self.accessed = True
+        return super().get(key, default)
+
+    def setdefault(self, key: str, default: t.Any = None) -> t.Any:
+        self.accessed = True
+        return super().setdefault(key, default)
+
+
+class NullSession(SecureCookieSession):
+    """Class used to generate nicer error messages if sessions are not
+    available.  Will still allow read-only access to the empty session
+    but fail on setting.
+    """
+
+    def _fail(self, *args: t.Any, **kwargs: t.Any) -> t.NoReturn:
+        raise RuntimeError(
+            "The session is unavailable because no secret "
+            "key was set.  Set the secret_key on the "
+            "application to something unique and secret."
+        )
+
+    __setitem__ = __delitem__ = clear = pop = popitem = update = setdefault = _fail  # type: ignore # noqa: B950
+    del _fail
+
+
+class SessionInterface(metaclass=ABCMeta):
+    """This is a SansIO abstract base class used by Flask and Quart to
+    then define thebasic interface to implement in order to replace
+    the default session interface of the frameworks.
+
+    .. versionadded:: 3.2.0
+
+    """
+
+    #: :meth:`make_null_session` will look here for the class that should
+    #: be created when a null session is requested.  Likewise the
+    #: :meth:`is_null_session` method will perform a typecheck against
+    #: this type.
+    null_session_class = NullSession
+
+    #: A flag that indicates if the session interface is pickle based.
+    #: This can be used by Flask extensions to make a decision in regards
+    #: to how to deal with the session object.
+    #:
+    #: .. versionadded:: 0.10
+    pickle_based = False
+
+    def is_null_session(self, obj: object) -> bool:
+        """Checks if a given object is a null session.  Null sessions are
+        not asked to be saved.
+
+        This checks if the object is an instance of :attr:`null_session_class`
+        by default.
+        """
+        return isinstance(obj, self.null_session_class)
+
+    def get_cookie_name(self, app: App) -> str:
+        """The name of the session cookie. Uses``app.config["SESSION_COOKIE_NAME"]``."""
+        return app.config["SESSION_COOKIE_NAME"]  # type: ignore[no-any-return]
+
+    def get_cookie_domain(self, app: App) -> str | None:
+        """The value of the ``Domain`` parameter on the session cookie. If not set,
+        browsers will only send the cookie to the exact domain it was set from.
+        Otherwise, they will send it to any subdomain of the given value as well.
+
+        Uses the :data:`SESSION_COOKIE_DOMAIN` config.
+
+        .. versionchanged:: 2.3
+            Not set by default, does not fall back to ``SERVER_NAME``.
+        """
+        return app.config["SESSION_COOKIE_DOMAIN"]  # type: ignore[no-any-return]
+
+    def get_cookie_path(self, app: App) -> str:
+        """Returns the path for which the cookie should be valid.  The
+        default implementation uses the value from the ``SESSION_COOKIE_PATH``
+        config var if it's set, and falls back to ``APPLICATION_ROOT`` or
+        uses ``/`` if it's ``None``.
+        """
+        return app.config["SESSION_COOKIE_PATH"] or app.config["APPLICATION_ROOT"]  # type: ignore[no-any-return]
+
+    def get_cookie_httponly(self, app: App) -> bool:
+        """Returns True if the session cookie should be httponly.  This
+        currently just returns the value of the ``SESSION_COOKIE_HTTPONLY``
+        config var.
+        """
+        return app.config["SESSION_COOKIE_HTTPONLY"]  # type: ignore[no-any-return]
+
+    def get_cookie_secure(self, app: App) -> bool:
+        """Returns True if the cookie should be secure.  This currently
+        just returns the value of the ``SESSION_COOKIE_SECURE`` setting.
+        """
+        return app.config["SESSION_COOKIE_SECURE"]  # type: ignore[no-any-return]
+
+    def get_cookie_samesite(self, app: App) -> str | None:
+        """Return ``'Strict'`` or ``'Lax'`` if the cookie should use the
+        ``SameSite`` attribute. This currently just returns the value of
+        the :data:`SESSION_COOKIE_SAMESITE` setting.
+        """
+        return app.config["SESSION_COOKIE_SAMESITE"]  # type: ignore[no-any-return]
+
+    def get_cookie_partitioned(self, app: App) -> bool:
+        """Returns True if the cookie should be partitioned. By default, uses
+        the value of :data:`SESSION_COOKIE_PARTITIONED`.
+
+        .. versionadded:: 3.1
+        """
+        return app.config["SESSION_COOKIE_PARTITIONED"]  # type: ignore[no-any-return]
+
+    def get_expiration_time(self, app: App, session: SessionMixin) -> datetime | None:
+        """A helper method that returns an expiration date for the session
+        or ``None`` if the session is linked to the browser session.  The
+        default implementation returns now + the permanent session
+        lifetime configured on the application.
+        """
+        if session.permanent:
+            return datetime.now(timezone.utc) + app.permanent_session_lifetime
+        return None
+
+    def should_set_cookie(self, app: App, session: SessionMixin) -> bool:
+        """Used by session backends to determine if a ``Set-Cookie`` header
+        should be set for this session cookie for this response. If the session
+        has been modified, the cookie is set. If the session is permanent and
+        the ``SESSION_REFRESH_EACH_REQUEST`` config is true, the cookie is
+        always set.
+
+        This check is usually skipped if the session was deleted.
+
+        .. versionadded:: 0.11
+        """
+
+        return session.modified or (
+            session.permanent and app.config["SESSION_REFRESH_EACH_REQUEST"]
+        )