Limit sessions per user instead of a global LRU cap

The session store held at most 100 sessions and evicted the least
recently used one. Anyone with an account could log in 101 times and
thereby log out every other user, including the admins.

- Each user has at most 10 sessions. A new login above the limit
  removes the user's own least recently used session (by _last_seen).
- There is no global limit anymore: the store only accepts sessions
  with a user_id (raises SessionWithoutUser otherwise), so it is
  bounded by 10 * number of users. Anonymous users still create no
  store entries.
- save_session clears an invalid session before re-raising, since
  Flask saves the session again for the error response.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

max/exceptions.py

@@ -10,6 +10,10 @@ class NotFound(MaxException):
     pass
 
 
+class SessionWithoutUser(MaxException):
+    pass
+
+
 class PermissionDenied(MaxException):
     def __init__(self, message):
         super().__init__()

max/sessions.py

@@ -6,6 +6,7 @@ from itsdangerous import BadSignature, Signer, want_bytes
 from werkzeug.datastructures import CallbackDict
 
 from max.crypto import get_random_string
+from max.exceptions import SessionWithoutUser
 
 # in seconds
 SESSION_IDLE_TIMEOUT = 30 * 60
@@ -16,48 +17,42 @@ CREATED_AT_KEY = "_created_at"
 LAST_SEEN_KEY = "_last_seen"
 
 
+SESSION_MAX_PER_USER = 10
+
+
 class SessionStorage:
-    def __init__(self, maxcap=100):
-        self.maxcap = maxcap
-        self.lru_ids = []  # newest ids are on the end (so do append to add a fresh id)
+    def __init__(self):
         self.data = {}
 
     def __getitem__(self, id):
-        if id not in self.data:
-            return None
-
-        self._refresh_id(id)
-        return self.data[id]
+        return self.data.get(id)
 
     def __delitem__(self, id):
-        if id in self.data:
-            del self.data[id]
-            self._remove_id(id)
+        self.data.pop(id, None)
 
     def __setitem__(self, id, data):
-        self._refresh_id(id)
+        if data.get("user_id") is None:
+            raise SessionWithoutUser("Refusing to store a session without user_id")
+
+        if id not in self.data:
+            self._limit_user_sessions(data["user_id"])
         self.data[id] = data
 
-        while len(self.lru_ids) > self.maxcap:
-            removed_id = self.lru_ids.pop(0)  # remove first element
-            del self.data[removed_id]
-
-    def _remove_id(self, id):
-        self.lru_ids = [_id for _id in self.lru_ids if _id != id]
-
-    def _refresh_id(self, id):
-        self._remove_id(id)
-        self.lru_ids.append(id)
+    def _limit_user_sessions(self, user_id):
+        """Makes room for one new session by removing the least recently used ones"""
+        user_ids = [id for id, data in self.data.items() if data["user_id"] == user_id]
+        user_ids.sort(key=lambda id: self.data[id][LAST_SEEN_KEY])
+        for id in user_ids[: max(0, len(user_ids) - SESSION_MAX_PER_USER + 1)]:
+            del self.data[id]
 
     def remove_user_sessions(self, user_id, except_id=None):
         ids_to_remove = []
         for id, data in self.data.items():
-            if data.get("user_id", None) == user_id and id != except_id:
+            if data["user_id"] == user_id and id != except_id:
                 ids_to_remove.append(id)
 
         for id in ids_to_remove:
             del self.data[id]
-            self._remove_id(id)
 
 
 session_storage = SessionStorage()
@@ -161,11 +156,16 @@ class SessionInterface(FlaskSessionInterface):
             session.last_seen = time.monotonic()
             if session.created_at is None:
                 session.created_at = session.last_seen
+            try:
                 session_storage[session.id] = {
                     **session,
                     CREATED_AT_KEY: session.created_at,
                     LAST_SEEN_KEY: session.last_seen,
                 }
+            except SessionWithoutUser:
+                # Flask saves the session again for the error response
+                session.clear()
+                raise
             session_id = (
                 self._get_signer(app).sign(want_bytes(session.id)).decode("utf-8")
             )