Change LRUCache to be tree-based so we can delete subtrees.

synapse/util/caches/treecache.py

@@ -0,0 +1,52 @@
+SENTINEL = object()
+
+
+class TreeCache(object):
+    def __init__(self):
+        self.root = {}
+
+    def __setitem__(self, key, value):
+        return self.set(key, value)
+
+    def set(self, key, value):
+        node = self.root
+        for k in key[:-1]:
+            node = node.setdefault(k, {})
+        node[key[-1]] = value
+
+    def get(self, key, default=None):
+        node = self.root
+        for k in key[:-1]:
+            node = node.get(k, None)
+            if node is None:
+                return default
+        return node.get(key[-1], default)
+
+    def clear(self):
+        self.root = {}
+
+    def pop(self, key, default=None):
+        nodes = []
+
+        node = self.root
+        for k in key[:-1]:
+            node = node.get(k, None)
+            nodes.append(node)  # don't add the root node
+            if node is None:
+                return default
+        popped = node.pop(key[-1], SENTINEL)
+        if popped is SENTINEL:
+            return default
+
+        node_and_keys = zip(nodes, key)
+        node_and_keys.reverse()
+        node_and_keys.append((self.root, None))
+
+        for i in range(len(node_and_keys) - 1):
+            n,k = node_and_keys[i]
+
+            if n:
+                break
+            node_and_keys[i+1][0].pop(k)
+
+        return popped