Unable to catch exceptions in remote due to possible bug in protocol._handle_ctxexit

[michael-a-schmidt]

Hello - first off I'd like to thank the developer(s) for creating such a great project, really impressive!

I've recently started using rpyc and I noticed something odd with how errors are handled on the server-side in the case of a remote object being used as a context manager on the client side.

Specifically, I have an application where I'd like to use a with statement in my client-side code on a remote object that has __enter__ and __exit__ functions defined. I am able to see the correct error type and message on the client side, but on the server side I get another exception:

caught an exception: type=<class 'TypeError'>, val=exceptions must derive from BaseException

I have included a minimal example below that shows the issue.

I have traced this with my debugger to rpyc.core.protocol, line 891, the function _handle_ctxexit. When my example server-side code raises a built-in error (ValueError in this case) I can see that the right exception is making its way into this function, but somehow the raise exc statement is failing, which is caught in the except Exception statement and a TypeError is returned:

    def _handle_ctxexit(self, obj, exc):  # request handler
        if exc:
            try:
                raise exc
            except Exception:
                exc, typ, tb = sys.exc_info()
        else:
            typ = tb = None
        return self._handle_getattr(obj, "__exit__")(exc, typ, tb)

My expectation is that both the client and the server should see the same type of exception, and that we should be able to filter and handle exceptions in the __exit__ statement on the remote object.

To reproduce this error, start the server, then run the client.

Server Output:

% python minimal_server.py 
in __enter__
in __exit__
caught an exception: type=<class 'TypeError'>, val=exceptions must derive from BaseException

Client output

% python minimal_client.py 
Traceback (most recent call last):
  File "/path/to/rpyc-example/minimal_client.py", line 6, in <module>
    conn.root.raise_remote()
  File "/path/to/rpyc-example/venv/lib/python3.11/site-packages/rpyc/core/netref.py", line 239, in __call__
    return syncreq(_self, consts.HANDLE_CALL, args, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/rpyc-example/venv/lib/python3.11/site-packages/rpyc/core/netref.py", line 63, in syncreq
    return conn.sync_request(handler, proxy, *args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/rpyc-example/venv/lib/python3.11/site-packages/rpyc/core/protocol.py", line 744, in sync_request
    return _async_res.value
           ^^^^^^^^^^^^^^^^
  File "/path/to/rpyc-example/venv/lib/python3.11/site-packages/rpyc/core/async_.py", line 111, in value
    raise self._obj
_get_exception_class.<locals>.Derived: Hello, World!

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/path/to/rpyc-example/venv/lib/python3.11/site-packages/rpyc/core/protocol.py", line 369, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/rpyc-example/venv/lib/python3.11/site-packages/rpyc/core/protocol.py", line 863, in _handle_call
    return obj(*args, **dict(kwargs))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/rpyc-example/minimal_server.py", line 14, in raise_remote
    raise ValueError("Hello, World!")
ValueError: Hello, World!

Environment

• rpyc version: 6.0.1
• python version: 3.11.3
• operating system: Darwin (Mac Ventura 13.6.9)

Minimal example

Server:

from rpyc import Service, ThreadedServer


class MyService(Service):
    """ example service showing TypeError in __exit__ """

    def on_connect(self, conn):
        pass

    def on_disconnect(self, conn):
        pass

    def raise_remote(self):
        raise ValueError("Hello, World!")

    def __enter__(self):
        print("in __enter__")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("in __exit__")
        if exc_type:
            print(f"caught an exception: type={exc_type}, val={exc_val}")
        return


if __name__ == "__main__":
    t = ThreadedServer(
        MyService(),
        port=18861,
        protocol_config={
            'allow_public_attrs': True
        }
    )
    t.start()

Client:

import rpyc

conn = rpyc.connect("localhost", port=18861)

with conn.root:
    conn.root.raise_remote()