Fix race-condition that resulted in the REPL hanging

With two successive requests, for example a completion request and an
eval request fired from two different Python threads, the second response
could end up overwriting the first one before it got processed. The
initial design assumed that only a single Python thread would be making
these requests, but that changed when the completion was introduced. We
now fix this issue by using a queue and knowing that these requests are
processed and replied to in FIFO order.

src/frida/repl.py

@@ -3,6 +3,7 @@ from __future__ import unicode_literals, print_function
 
 def main():
     import codecs
+    from collections import deque
     from colorama import Fore, Style
     import frida
     from frida.application import ConsoleApplication
@@ -24,7 +25,7 @@ def main():
             self._seqno = 0
             self._ready = threading.Event()
             self._response_cond = threading.Condition()
-            self._response_data = None
+            self._response_queue = deque()
             self._completor_locals = []
             self._history = FileHistory(os.path.join(os.path.expanduser('~'), '.frida_history'))
             self._completer = FridaCompleter(self)
@@ -313,12 +314,11 @@ def main():
         def _evaluate(self, text):
             self._reactor.schedule(lambda: self._script.post_message({'name': '.evaluate', 'payload': {'expression': text}}))
             with self._response_cond:
-                while self._response_data is None:
+                while len(self._response_queue) == 0:
                     if not self._reactor.is_running():
                         raise frida.InvalidOperationError("Invalid operation while stopping")
                     self._response_cond.wait(0.5)
-                response = self._response_data
-                self._response_data = None
+                response = self._response_queue.popleft()
             stanza, data = response
             if data is not None:
                 return ('binary', data)
@@ -334,7 +334,8 @@ def main():
             if message_type == 'send':
                 stanza = message['payload']
                 with self._response_cond:
-                    self._response_data = (stanza, data)
+                    response = (stanza, data)
+                    self._response_queue.append(response)
                     self._response_cond.notify()
             elif message_type == 'error':
                 text = message.get('stack', message['description'])