Fix other re-entrancy nits for the lru_cache.

rhettinger · rhettinger · commit f2c17a9276e5 · 2013-03-04T03:34:09.000-05:00
Keep references for oldkey and oldvalue so they can't
trigger a __del__ method to reenter our thread.

Move the cache[key]=link step to the end, after the link
data is in a consistent state.

Under exotic circumstances, the cache[key]=link step could
trigger reentrancy (i.e. the key would have to have a hash
exactly equal to that for another key in the cache and the
key would need a __eq__ method that makes a reentrant call
our cached function).
diff --git a/Lib/functools.py b/Lib/functools.py
@@ -267,19 +267,23 @@ def wrapper(*args, **kwds):
                         # computed result and update the count of misses.
                         pass
                     elif full:
-                        # use root to store the new key and result
-                        root[KEY] = key
-                        root[RESULT] = result
-                        cache[key] = root
+                        # use the old root to store the new key and result
+                        oldroot = root
+                        oldroot[KEY] = key
+                        oldroot[RESULT] = result
                         # empty the oldest link and make it the new root
-                        root = root[NEXT]
-                        del cache[root[KEY]]
+                        root = oldroot[NEXT]
+                        oldkey = root[KEY]
+                        oldvalue = root[RESULT]
                         root[KEY] = root[RESULT] = None
+                        # now update the cache dictionary for the new links
+                        del cache[oldkey]
+                        cache[key] = oldroot
                     else:
                         # put result in a new link at the front of the queue
                         last = root[PREV]
                         link = [last, root, key, result]
-                        cache[key] = last[NEXT] = root[PREV] = link
+                        last[NEXT] = root[PREV] = cache[key] = link
                         currsize += 1
                         full = (currsize == maxsize)
                     misses += 1
