pypi cproto (chrome debugging client) hangs on exit

Question

Using https://pypi.org/project/cproto/, attached to Chrome running headless in a Docker container, I find that it gets stuck from time to time (the example quoted below is not reliable -- you may need to run it a few times):

$ python3
Python 3.6.8 (default, Jan 14 2019, 11:02:34) 
[GCC 8.0.1 20180414 (experimental) [trunk revision 259383]] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cproto
>>> cp = cproto.CProto()   # localhost:9222 == my Chrome container
>>> cp.close()
>>> exit()
^CException ignored in: <module 'threading' from '/usr/lib/python3.6/threading.py'>
Traceback (most recent call last):
  File "/usr/lib/python3.6/threading.py", line 1294, in _shutdown
    t.join()
  File "/usr/lib/python3.6/threading.py", line 1056, in join
    self._wait_for_tstate_lock()
  File "/usr/lib/python3.6/threading.py", line 1072, in _wait_for_tstate_lock
    elif lock.acquire(block, timeout):
KeyboardInterrupt
$

This also happens with Chrome running normally, with a UI, outside of Docker.

Irrespective of whatever nastiness Chrome might be dealing with, it seems a bit "off" that the cproto library should get itself in a knot like this.

Is there any way of forcibly quitting cproto? (Have I done something wrong above, or is this a bug?)

Answer 1

It looks like some non-daemon threads remain running after the main thread exits. This work around works for me (didn't find the root cause yet, need more investigation):

diff -ur cproto.orig/core/websocket.py cproto/core/websocket.py
--- cproto.orig/core/websocket.py       2021-01-04 01:24:53.000000000 +0000
+++ cproto/core/websocket.py    2021-03-10 04:10:32.228436532 +0000
@@ -38,8 +38,16 @@
     def connect(self, *args, **kwargs):
         super(self.__class__, self).connect(*args, **kwargs)
 
-        threading.Thread(target=self._read_messages).start()
-        threading.Thread(target=self._process_events).start()
+        # threading.Thread(target=self._read_messages).start()
+        # threading.Thread(target=self._process_events).start()
+
+        t1 = threading.Thread(target=self._read_messages)
+        t1.setDaemon(True)
+        t1.start()
+
+        t2 = threading.Thread(target=self._process_events)
+        t2.setDaemon(True)
+        t2.start()
 
     def close(self):
         # Terminates blocking `get` call on events processing thread