Use the idling state instead checking if read thread is present

This caused some multithreading bugs, for instance, when there was
no read thread and the main thread was idling, and there were some
update workers. Several threads would try to read from the socket
at the same time (since there's no lock for reading), causing
reads to be corrupted and receiving "invalid packet lengths"
from the network. Closes #538.

telethon/telegram_bare_client.py

@@ -163,6 +163,7 @@ class TelegramBareClient:
         # if the user has left enabled such option.
         self._spawn_read_thread = spawn_read_thread
         self._recv_thread = None
+        self._idling = threading.Event()
 
         # Default PingRequest delay
         self._last_ping = datetime.now()
@@ -438,8 +439,9 @@ class TelegramBareClient:
 
         # Determine the sender to be used (main or a new connection)
         __log__.debug('Invoking %s', which)
+        call_receive = \
+            not self._idling.is_set() or self._reconnect_lock.locked()
 
-        call_receive = self._recv_thread is None or self._reconnect_lock.locked()
         for retry in range(retries):
             result = self._invoke(call_receive, *requests)
             if result is not None:
@@ -829,6 +831,7 @@ class TelegramBareClient:
         if self._spawn_read_thread and not self._on_read_thread():
             raise RuntimeError('Can only idle if spawn_read_thread=False')
 
+        self._idling.set()
         for sig in stop_signals:
             signal(sig, self._signal_handler)
 
@@ -857,7 +860,11 @@ class TelegramBareClient:
                 with self._reconnect_lock:
                     while self._user_connected and not self._reconnect():
                         sleep(0.1)  # Retry forever, this is instant messaging
+            except:
+                self._idling.clear()
+                raise
 
+        self._idling.clear()
         __log__.info('Connection closed by the user, not reading anymore')
 
     # By using this approach, another thread will be