fix #653

libmproxy/script.py

@@ -128,15 +128,17 @@ class Script:
 
 
 class ReplyProxy(object):
-    def __init__(self, original_reply):
-        self._ignore_calls = 1
-        self.lock = threading.Lock()
+    def __init__(self, original_reply, script_thread):
         self.original_reply = original_reply
+        self.script_thread = script_thread
+        self._ignore_call = True
+        self.lock = threading.Lock()
 
     def __call__(self, *args, **kwargs):
         with self.lock:
-            if self._ignore_calls > 0:
-                self._ignore_calls -= 1
+            if self._ignore_call:
+                self.script_thread.start()
+                self._ignore_call = False
                 return
         self.original_reply(*args, **kwargs)
 
@@ -145,16 +147,19 @@ class ReplyProxy(object):
 
 
 def _handle_concurrent_reply(fn, o, *args, **kwargs):
-    # Make first call to o.reply a no op
-
-    reply_proxy = ReplyProxy(o.reply)
-    o.reply = reply_proxy
+    # Make first call to o.reply a no op and start the script thread.
+    # We must not start the script thread before, as this may lead to a nasty race condition
+    # where the script thread replies a different response before the normal reply, which then gets swallowed.
 
     def run():
         fn(*args, **kwargs)
         # If the script did not call .reply(), we have to do it now.
         reply_proxy()
-    ScriptThread(target=run).start()
+
+    script_thread = ScriptThread(target=run)
+
+    reply_proxy = ReplyProxy(o.reply, script_thread)
+    o.reply = reply_proxy
 
 
 class ScriptThread(threading.Thread):