Fix infinite loop when invoking on update handlers (fix #336)

Every update that hadn't been acknowledged on the main connection
yet would be resent on any new connection. These new connections
are made temporary when invoking anything from any thread that's
not the main thread. It would also process all the updates, hence,
Telegram would be resending these not-acknowledged updates to the
temporary connection, and the updates would be processed again,
then the update handler would react to the duplicated updates over
and over.

To fix this, simply don't process updates on the temporary thread
at all. With this reasoning, if we don't acknowledge updates on
the temporary connections, Telegram will resend them on the main
connection, so we should not lose any.

telethon/telegram_bare_client.py

@@ -432,9 +432,18 @@ class TelegramBareClient:
         on_main_thread = threading.get_ident() == self._main_thread_ident
         if on_main_thread or self._on_read_thread():
             sender = self._sender
+            update_state = self.updates
         else:
             sender = self._sender.clone()
             sender.connect()
+            # We're on another connection, Telegram will resend all the
+            # updates that we haven't acknowledged (potentially entering
+            # an infinite loop if we're calling this in response to an
+            # update event, as it would be received again and again). So
+            # to avoid this we will simply not process updates on these
+            # new temporary connections, as they will be sent and later
+            # acknowledged over the main connection.
+            update_state = None
 
         # We should call receive from this thread if there's no background
         # thread reading or if the server disconnected us and we're trying
@@ -447,7 +456,9 @@ class TelegramBareClient:
                 if self._background_error and on_main_thread:
                     raise self._background_error
 
-                result = self._invoke(sender, call_receive, *requests)
+                result = self._invoke(
+                    sender, call_receive, update_state, *requests
+                )
                 if result is not None:
                     return result
 
@@ -459,7 +470,7 @@ class TelegramBareClient:
     # Let people use client.invoke(SomeRequest()) instead client(...)
     invoke = __call__
 
-    def _invoke(self, sender, call_receive, *requests):
+    def _invoke(self, sender, call_receive, update_state, *requests):
         try:
             # Ensure that we start with no previous errors (i.e. resending)
             for x in requests:
@@ -479,7 +490,7 @@ class TelegramBareClient:
                     )
             else:
                 while not all(x.confirm_received.is_set() for x in requests):
-                    sender.receive(update_state=self.updates)
+                    sender.receive(update_state=update_state)
 
         except TimeoutError:
             pass  # We will just retry
@@ -526,7 +537,7 @@ class TelegramBareClient:
             # be on the very first connection (not authorized, not running),
             # but may be an issue for people who actually travel?
             self._reconnect(new_dc=e.new_dc)
-            return self._invoke(sender, call_receive, *requests)
+            return self._invoke(sender, call_receive, update_state, *requests)
 
         except ServerError as e:
             # Telegram is having some issues, just retry