Prevent double-connect causing double-reads later

Which leads to "RuntimeError: readexactly() called while another
coroutine is already waiting for incoming data" being raised,
and causing everything to break or halt.

telethon/client/telegrambaseclient.py

@@ -405,14 +405,17 @@ class TelegramBaseClient(abc.ABC):
                 except OSError:
                     print('Failed to connect')
         """
-        await self._sender.connect(self._connection(
+        if not await self._sender.connect(self._connection(
             self.session.server_address,
             self.session.port,
             self.session.dc_id,
             loop=self._loop,
             loggers=self._log,
             proxy=self._proxy
-        ))
+        )):
+            # We don't want to init or modify anything if we were already connected
+            return
+
         self.session.auth_key = self._sender.auth_key
         self.session.save()
 

telethon/network/mtprotosender.py

@@ -54,6 +54,7 @@ class MTProtoSender:
         self._auth_key_callback = auth_key_callback
         self._update_callback = update_callback
         self._auto_reconnect_callback = auto_reconnect_callback
+        self._connect_lock = asyncio.Lock(loop=loop)
 
         # Whether the user has explicitly connected or disconnected.
         #
@@ -114,13 +115,15 @@ class MTProtoSender:
         """
         Connects to the specified given connection using the given auth key.
         """
+        async with self._connect_lock:
             if self._user_connected:
                 self._log.info('User is already connected!')
-            return
+                return False
 
             self._connection = connection
             await self._connect()
             self._user_connected = True
+            return True
 
     def is_connected(self):
         return self._user_connected