gevent error when using accross threads

[alex-v-dev]

Hi !

I am using mprpc in a large-ish code base, and it might happen that sometimes rpc calls are made from another thread than the main. Is there anything I can do about this ? This is giving the following error :

File "mprpc/client.pyx", line 138, in mprpc.client.RPCClient.call (mprpc/client.c:2545) data = self._socket.recv(SOCKET_RECV_SIZE) File "D:\_DEV_GIT\avPipe_GIT\avPipe\Dependencies\PyLibraries\MSVC14\gevent\_socket2.py", line 295, in recv self._wait(self._read_event) File "src/gevent/_hub_primitives.py", line 265, in gevent.__hub_primitives.wait_on_socket File "src/gevent/_hub_primitives.py", line 266, in gevent.__hub_primitives.wait_on_socket File "src/gevent/_hub_primitives.py", line 252, in gevent.__hub_primitives._primitive_wait File "src/gevent/_hub_primitives.py", line 46, in gevent.__hub_primitives.WaitOperationsGreenlet.wait File "src/gevent/_hub_primitives.py", line 46, in gevent.__hub_primitives.WaitOperationsGreenlet.wait File "src/gevent/_hub_primitives.py", line 55, in gevent.__hub_primitives.WaitOperationsGreenlet.wait File "src/gevent/_waiter.py", line 151, in gevent.__waiter.Waiter.get File "src/gevent/_greenlet_primitives.py", line 59, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch File "src/gevent/_greenlet_primitives.py", line 59, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch File "src/gevent/_greenlet_primitives.py", line 63, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch File "src/gevent/__greenlet_primitives.pxd", line 35, in gevent.__greenlet_primitives._greenlet_switch error: cannot switch to a different thread

Thanks !

[wosiu]

+1
My stack:

  File "mprpc/client.pyx", line 138, in mprpc.client.RPCClient.call
  File "/home/m/OCR/django-upload/env/lib/python3.5/site-packages/gevent/_socket3.py", line 382, in recv
    self._wait(self._read_event)
  File "src/gevent/_hub_primitives.py", line 265, in gevent.__hub_primitives.wait_on_socket
  File "src/gevent/_hub_primitives.py", line 266, in gevent.__hub_primitives.wait_on_socket
  File "src/gevent/_hub_primitives.py", line 261, in gevent.__hub_primitives._primitive_wait
  File "src/gevent/_hub_primitives.py", line 262, in gevent.__hub_primitives._primitive_wait
  File "src/gevent/_hub_primitives.py", line 46, in gevent.__hub_primitives.WaitOperationsGreenlet.wait
  File "src/gevent/_hub_primitives.py", line 46, in gevent.__hub_primitives.WaitOperationsGreenlet.wait
  File "src/gevent/_hub_primitives.py", line 55, in gevent.__hub_primitives.WaitOperationsGreenlet.wait
  File "src/gevent/_waiter.py", line 151, in gevent.__waiter.Waiter.get
  File "src/gevent/_greenlet_primitives.py", line 59, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_greenlet_primitives.py", line 59, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_greenlet_primitives.py", line 63, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/__greenlet_primitives.pxd", line 35, in gevent.__greenlet_primitives._greenlet_switch
greenlet.error: cannot switch to a different thread

Moreover, if I understand correctly, client part is not thread-safe at all:

mprpc/mprpc/client.pyx

Line 138 in d7674c0

data = self._socket.recv(SOCKET_RECV_SIZE)

It creates one socket per all threads. If few threads writes at the same time, a response might be read by wrong one.
Tell me please if I'm wrong.

[wbthomason]

I'm running into this error too. Is there any workaround other than keeping the client on the main thread and using message-passing between threads to coordinate calls (which seems like a huge pain)?

[wosiu]

@wbthomason
All in all I stopped using rpc for communication between my services (originally I was calling c++ rpc server from a django server). Now I run compiled c++ code directly from python using boost::python. But I guess your case is different.
Afair I ended up with creating separate client and connection before a call and closing it just after a call to ensure one thread at a time. Also, because of some reasons, I switched to msgpack-rpc-python at some point, but now I can't recall why. Anyway, it had its own issues as well.

[wbthomason]

@wosiu Got it, thanks! I'd incidentally also tried using msgpack-rpc-python and found that it worked for my use case (doesn't crash when used across threads and the worse performance isn't a huge problem for me).

And yeah, my case necessitates some sort of RPC, unfortunately.

Thanks for the info!

[ikuyamada]

I am not sure that I understand this issue correctly, but the current implementation of RPCClient is not thread-safe. You need to create a client per thread.