dephne compatibility of LocalCacheProxy

src/c3nav/mapdata/utils/cache/local.py

@@ -1,4 +1,5 @@
 from collections import OrderedDict
+from contextvars import ContextVar
 
 from django.core.cache import cache
 from django.conf import settings
@@ -10,42 +11,46 @@ class NoneFromCache:
 
 class LocalCacheProxy:
     # django cache, buffered using a LRU cache
-    # only usable for stuff that never changes, obviously
-    # todo: ensure thread-safety, compatible with async + daphne etc
+    # only usable for stuff that never needs to know about changes made by other cache clients, obviously
     def __init__(self, maxsize=128):
         self._maxsize = maxsize
         self._mapupdate = None
-        self._items = OrderedDict()
+        self._items: ContextVar[OrderedDict] = ContextVar("cache items")
+        # do not use a default of a dict, this can lead to same instance in different contexts
+        # we don't particularly care about this for LocalCacheProxy,
+        # but we DEFINITELY care about this for the local request cache.
+        # Most importantly, this is why the clear function always sets a new dictionary to be extra sure.
+        self.clear()
 
     def get(self, key, default=None):
         if self._mapupdate is None:
             self._check_mapupdate()
         try:
             # first check out cache
-            result = self._items[key]
+            result = self._items.get()[key]
         except KeyError:
             # not in our cache
             result = cache.get(key, default=NoneFromCache)
             if result is not NoneFromCache:
-                self._items[key] = result
+                self._items.get()[key] = result
                 self._prune()
             else:
                 result = default
         else:
-            self._items.move_to_end(key, last=True)
+            self._items.get().move_to_end(key, last=True)
         return result
 
     def _prune(self):
         # remove old items
-        while len(self._items) > self._maxsize:
-            self._items.pop(next(iter(self._items.keys())))
+        while len(self._items.get()) > self._maxsize:
+            self._items.get().pop(next(iter(self._items.get().keys())))
 
     def _check_mapupdate(self):
         # todo: thanks to enable_globally() we shouldn't need this any more
         from c3nav.mapdata.models import MapUpdate
         mapupdate = MapUpdate.current_cache_key()
         if self._mapupdate != mapupdate:
-            self._items = OrderedDict()
+            self.clear()
             self._mapupdate = mapupdate
 
     enabled = False
@@ -61,11 +66,11 @@ class LocalCacheProxy:
         self._check_mapupdate()
         cache.set(key, value, expire)
         if LocalCacheProxy.enabled:
-            self._items[key] = value
+            self._items.get()[key] = value
         self._prune()
 
     def clear(self):
-        self._items.clear()
+        self._items.set(OrderedDict())
 
 
 class RequestLocalCacheProxy(LocalCacheProxy):