Exposed descriptors raising AttributeErrors cause different remote tracebacks when accessing as exposed_foo vs. foo

[gschaffner]

An AttributeError raised in the __get__ of an exposed descriptor exposed_foo is only propagated correctly to the client if the client accesses the exposed descriptor as conn.root.exposed_foo rather than as conn.root.foo. See example below :)

Environment

• RPyC version: 5.1.0
• Python version: CPython 3.10.2
• operating system: Manjaro

Minimal example

Server:

import rpyc


class Foo:
    def __get__(self, owner, owner_type=None):
        raise AttributeError("This string doesn't always reach the client's traceback!")


class MWEService(rpyc.Service):
    exposed_foo = Foo()


if __name__ == "__main__":
    from rpyc.utils.server import ThreadedServer

    svr = ThreadedServer(MWEService, port=18861)
    svr.start()

Client:

import rpyc

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

# The following two lines will raise different exceptions!
conn.root.exposed_foo
# conn.root.foo

Tracebacks

When using conn.root.exposed_foo:

Traceback (most recent call last):
  File "/tmp/tmp.YPumucnVwE/mwe_c.py", line 6, in <module>
    conn.root.exposed_foo
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/netref.py", line 153, in __getattr__
    return syncreq(self, consts.HANDLE_GETATTR, name)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/netref.py", line 63, in syncreq
    return conn.sync_request(handler, proxy, *args)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 474, in sync_request
    return self.async_request(handler, *args, timeout=timeout).value
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/async_.py", line 103, in value
    raise self._obj
_get_exception_class.<locals>.Derived: This string doesn't always reach the client's traceback!

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 324, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 609, in _handle_getattr
    return self._access_attr(obj, name, (), "_rpyc_getattr", "allow_getattr", getattr)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 537, in _access_attr
    return accessor(obj, name, *args)
  File "/tmp/tmp.YPumucnVwE/mwe_s.py", line 6, in __get__
    raise AttributeError("This string doesn't always reach the client's traceback!")
AttributeError: This string doesn't always reach the client's traceback!

When using conn.root.foo:

Traceback (most recent call last):
  File "/tmp/tmp.YPumucnVwE/mwe_c.py", line 7, in <module>
    conn.root.foo
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/netref.py", line 153, in __getattr__
    return syncreq(self, consts.HANDLE_GETATTR, name)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/netref.py", line 63, in syncreq
    return conn.sync_request(handler, proxy, *args)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 474, in sync_request
    return self.async_request(handler, *args, timeout=timeout).value
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/async_.py", line 103, in value
    raise self._obj
_get_exception_class.<locals>.Derived: cannot access 'foo'

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 324, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 609, in _handle_getattr
    return self._access_attr(obj, name, (), "_rpyc_getattr", "allow_getattr", getattr)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 536, in _access_attr
    name = self._check_attr(obj, name, param)
  File "/tmp/tmp.YPumucnVwE/.venv/lib/python3.10/site-packages/rpyc/core/protocol.py", line 526, in _check_attr
    raise AttributeError(f"cannot access {name!r}")
AttributeError: cannot access 'foo'

Debugging

This appears to occur only when __get__ raises an AttributeError in particular, which makes sense. If __get__ raises an exception of a different type, it is raised by hasattr here:

rpyc/rpyc/core/protocol.py

Line 518 in ce6f432

has_exposed = prefix and hasattr(obj, prefix + name)

But if __get__ raises an AttributeError, this hasattr returns False instead of raising, and so the arguments of the AttributeError are lost.