directories: keep track of your position as you decode netstring after netstring from an input buffer instead of copying the trailing part

This makes decoding linear in the number of netstrings instead of O(N^2).

src/allmydata/dirnode.py

@@ -204,15 +204,16 @@ class NewDirectoryNode:
         # rocap, rwcap, metadata), in which the name,rocap,metadata are in
         # cleartext. The 'name' is UTF-8 encoded. The rwcap is formatted as:
         # pack("16ss32s", iv, AES(H(writekey+iv), plaintextrwcap), mac)
-        assert isinstance(data, str)
+        assert isinstance(data, str), (repr(data), type(data))
         # an empty directory is serialized as an empty string
         if data == "":
             return {}
         writeable = not self.is_readonly()
         children = {}
-        while len(data) > 0:
-            entry, data = split_netstring(data, 1, True)
-            name, rocap, rwcapdata, metadata_s = split_netstring(entry, 4)
+        position = 0
+        while position < len(data):
+            entries, position = split_netstring(data, 1, position)
+            (name, rocap, rwcapdata, metadata_s), subpos = split_netstring(entries[0], 4)
             name = name.decode("utf-8")
             rwcap = None
             if writeable: