Make MtProtoSender not thread-safe

Rationale: a new connection should be spawned if one desires to
send and receive requests in parallel, which would otherwise cause
one of either threads to lock.

telethon/network/mtproto_sender.py

@@ -1,7 +1,6 @@
 import gzip
 import logging
 import struct
-from threading import RLock
 
 from .. import helpers as utils
 from ..crypto import AES
@@ -20,7 +19,12 @@ logging.getLogger(__name__).addHandler(logging.NullHandler())
 
 class MtProtoSender:
     """MTProto Mobile Protocol sender
-       (https://core.telegram.org/mtproto/description)
+       (https://core.telegram.org/mtproto/description).
+
+       Note that this class is not thread-safe, and calling send/receive
+       from two or more threads at the same time is undefined behaviour.
+       Rationale: a new connection should be spawned to send/receive requests
+                  in parallel, so thread-safety (hence locking) isn't needed.
     """
 
     def __init__(self, session, connection):
@@ -37,11 +41,6 @@ class MtProtoSender:
         # Requests (as msg_id: Message) sent waiting to be received
         self._pending_receive = {}
 
-        # Sending and receiving are independent, but two threads cannot
-        # send or receive at the same time no matter what.
-        self._send_lock = RLock()
-        self._recv_lock = RLock()
-
     def connect(self):
         """Connects to the server"""
         self.connection.connect()
@@ -93,7 +92,6 @@ class MtProtoSender:
            Any unhandled object (likely updates) will be passed to
            update_state.process(TLObject).
         """
-        with self._recv_lock:
         try:
             body = self.connection.recv()
         except (BufferError, InvalidChecksumError):
@@ -128,7 +126,6 @@ class MtProtoSender:
         cipher_text = AES.encrypt_ige(plain_text, key, iv)
 
         result = key_id + msg_key + cipher_text
-        with self._send_lock:
         self.connection.send(result)
 
     def _decode_msg(self, body):