Refactor code to fetch missing entities once again

This is another attempt at reducing CPU usage similar to:
    1b6b4a57d9

In addition it simplifies some of the code and opens up new
ideas for the state cache as well.

telethon/entitycache.py

@@ -1,7 +1,57 @@
 import itertools
+
 from . import utils
 from .tl import types
 
+# Which updates have the following fields?
+_has_user_id = []
+_has_chat_id = []
+_has_channel_id = []
+_has_peer = []
+_has_dialog_peer = []
+_has_message = []
+
+# Note: We don't bother checking for some rare:
+# * `UpdateChatParticipantAdd.inviter_id` integer.
+# * `UpdateNotifySettings.peer` dialog peer.
+# * `UpdatePinnedDialogs.order` list of dialog peers.
+# * `UpdateReadMessagesContents.messages` list of messages.
+# * `UpdateChatParticipants.participants` list of participants.
+#
+# There are also some uninteresting `update.message` of type string.
+
+
+def _fill():
+    for name in dir(types):
+        update = getattr(types, name)
+        if getattr(update, 'SUBCLASS_OF_ID', None) == 0x9f89304e:
+            cid = update.CONSTRUCTOR_ID
+            doc = update.__init__.__doc__ or ''
+            if ':param int user_id:' in doc:
+                _has_user_id.append(cid)
+            if ':param int chat_id:' in doc:
+                _has_chat_id.append(cid)
+            if ':param int channel_id:' in doc:
+                _has_channel_id.append(cid)
+            if ':param TypePeer peer:' in doc:
+                _has_peer.append(cid)
+            if ':param TypeDialogPeer peer:' in doc:
+                _has_dialog_peer.append(cid)
+            if ':param TypeMessage message:' in doc:
+                _has_message.append(cid)
+
+    # Future-proof check: if the documentation format ever changes
+    # then we won't be able to pick the update types we are interested
+    # in, so we must make sure we have at least an update for each field
+    # which likely means we are doing it right.
+    if not all((_has_user_id, _has_chat_id, _has_channel_id,
+                _has_peer, _has_dialog_peer)):
+        raise RuntimeError('FIXME: Did the generated docs or updates change?')
+
+
+# We use a function to avoid cluttering the globals (with name/update/cid/doc)
+_fill()
+
 
 class EntityCache:
     """
@@ -46,3 +96,51 @@ class EntityCache:
                 return result
 
         raise KeyError('No cached entity for the given key')
+
+    def ensure_cached(
+            self,
+            update,
+            has_user_id=frozenset(_has_user_id),
+            has_channel_id=frozenset(_has_channel_id),
+            has_peer=frozenset(_has_peer + _has_dialog_peer),
+            has_message=frozenset(_has_message)
+    ):
+        """
+        Ensures that all the relevant entities in the given update are cached.
+        """
+        # This method is called pretty often and we want it to have the lowest
+        # overhead possible. For that, we avoid `isinstance` and constantly
+        # getting attributes out of `types.` by "caching" the constructor IDs
+        # in sets inside the arguments, and using local variables.
+        dct = self.__dict__
+        cid = update.CONSTRUCTOR_ID
+        if cid in has_user_id and \
+                update.user_id not in dct:
+            return False
+
+        if cid in _has_chat_id and \
+                utils.get_peer_id(types.PeerChat(update.chat_id)) not in dct:
+            return False
+
+        if cid in has_channel_id and \
+                utils.get_peer_id(types.PeerChannel(update.channel_id)) not in dct:
+            return False
+
+        if cid in has_peer and \
+                utils.get_peer_id(update.peer) not in dct:
+            return False
+
+        if cid in has_message:
+            x = update.message
+            y = getattr(x, 'to_id', None)  # handle MessageEmpty
+            if y and utils.get_peer_id(y) not in dct:
+                return False
+
+            y = getattr(x, 'from_id', None)
+            if y and y not in dct:
+                return False
+
+            # We don't quite worry about entities anywhere else.
+            # This is enough.
+
+        return True