Optimize permissions, use memoization

apps/permission/decorators.py

@@ -0,0 +1,56 @@
+# Copyright (C) 2018-2020 by BDE ENS Paris-Saclay
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+from functools import lru_cache
+from time import time
+
+from django.contrib.sessions.models import Session
+from note_kfet.middlewares import get_current_session
+
+
+def memoize(f):
+    """
+    Memoize results and store in sessions
+
+    This decorator is useful for permissions: they are loaded once needed, then stored for next calls.
+    The storage is contained with sessions since it depends on the selected mask.
+    """
+    sess_funs = {}
+    last_collect = time()
+
+    def collect():
+        """
+        Clear cache of results when sessions are invalid, to flush useless data.
+        This function is called every minute.
+        """
+        nonlocal sess_funs
+
+        new_sess_funs = {}
+        for sess_key in sess_funs:
+            if Session.objects.filter(session_key=sess_key).exists():
+                new_sess_funs[sess_key] = sess_funs[sess_key]
+        sess_funs = new_sess_funs
+
+    def func(*args, **kwargs):
+        nonlocal last_collect
+
+        if time() - last_collect > 60:
+            # Clear cache
+            collect()
+            last_collect = time()
+
+        # If there is no session, then we don't memoize anything.
+        sess = get_current_session()
+        if sess is None or sess.session_key is None:
+            return f(*args, **kwargs)
+
+        sess_key = sess.session_key
+        if sess_key not in sess_funs:
+            # lru_cache makes the job of memoization
+            # We store only the 512 latest data per session. It has to be enough.
+            sess_funs[sess_key] = lru_cache(512)(f)
+        return sess_funs[sess_key](*args, **kwargs)
+
+    func.func_name = f.__name__
+
+    return func