handler functions should not raise python exceptions

[ThomasWaldmann]

Copied from a comment of @Nikratio in #81 :

The cdef void functions in handlers.pxi that are assigned to fuse_lowlevel_ops should never raise exceptions. Where do you see them use raise?

Nevermind, found it. I believe the affected methods are fuse_write and fuse_setxattr (which raise OverflowError if the data size doesn't fit into ssize_t (pretty unlikely) and fuse_init (which raises a RuntimeError if the kernel is too old).

Neither of these checks can easily be moved elsewhere. For the first two, I would suggest to instead return EINVAL through fuse_reply_err() and log the error (using Python's logging module). For the third, the best I can think of is to set some sort of global flag, check it in init() (after the call to fuse_session_new) and raise the exception there if set.

[Nikratio]

Here's an alternate patch to fix the issue: https://github.com/libfuse/pyfuse3/tree/error_handling

This tells Cython that callbacks may raise exceptions, and that exception status should be checked on return from fuse_session_process_buf. It eliminates the warnings and seems to work:

python-venv examples/hello.py ~/tmp/mnt
Traceback (most recent call last):
  File "/home/nikratio/in-progress/pyfuse3/examples/hello.py", line 157, in <module>
    main()
  File "/home/nikratio/in-progress/pyfuse3/examples/hello.py", line 148, in main
    trio.run(pyfuse3.main)
  File "/usr/lib/python3/dist-packages/trio/_core/_run.py", line 2010, in run
    raise runner.main_task_outcome.error
  File "/home/nikratio/in-progress/pyfuse3/src/_pyfuse3.py", line 43, in wrapper
    await fn(*args, **kwargs)
  File "src/pyfuse3.pyx", line 781, in main
    async with trio.open_nursery() as nursery:
  File "/usr/lib/python3/dist-packages/trio/_core/_run.py", line 850, in __aexit__
    raise combined_error_from_nursery
  File "/home/nikratio/in-progress/pyfuse3/src/_pyfuse3.py", line 43, in wrapper
    await fn(*args, **kwargs)
  File "src/internal.pxi", line 270, in _session_loop
    fuse_session_process_buf(session, &buf)
  File "src/handlers.pxi", line 53, in pyfuse3.fuse_init
    raise RuntimeError('testing error handling')
RuntimeError: testing error handling

[ThomasWaldmann]

closing, #86 with that alternative solution was just merged.