nogil and custom exception handler

[houbaastef .]

Hello,

i'm wrapping a C++ API.

The C++ calls can block for several seconds, so it's a good opportunity for nogil. Thus I declare the API with :

cdef extern from "blabla.h" namespace "NS" nogil

The API uses a bunch of specific C++ exceptions that inherit from std::runtime_error.

So i mapped the C++ exceptions to Python exceptions using a custom handler:

-----------

custom_exception_handler.cpp:

extern PyObject *py_option_not_found;

void custom_exception_handler() {
    // Catch a handful of different errors here and turn them into the
    // equivalent Python errors.
    try {
        if (PyErr_Occurred())
            ; // let the latest Python exn pass through and ignore the current one
        else
            throw;
    } catch (const NS::option_not_found& exn) {
        PyErr_SetString(py_option_not_found, exn.what());
...

--------------

cython exceptions in pxd/pyx:

cdef public PyObject* py_option_not_found   (pxd)

class OptionNotFound(ValueError):           (pyx)
    """
    Exception thrown when an option is not found.
    """
cdef public PyObject* py_option_not_found = <PyObject*>OptionNotFound

--------------------

The handler is declared like in cython:

cdef extern from "custom_exception_handler.h" namespace "NS":
    cdef void custom_exception_handler()

--------------------
and used like in cython:

cdef extern from "blabla.h" namespace "NS" nogil:
    cdef cppclass RAW:
        RAW(const string &data) except +custom_exception_handler

All is fine... But i wondered if the nogil statement will get along nicely with the "except+". custom_exception_handler clearly uses Python objects. So shoud I acquire the GIL back in custom_exception_handler ? Or cython just gently takes care of it ?

Thanks,

Stephane

[Ian Henriksen]

Good question. This was fixed some time ago in https://github.com/cython/cython/commit/6795a2ba4761110c64c7a36401cb676614ba9f79.

If it needs to, Cython acquires the gil in the catch statement that calls the exception handling function.

Cheers,

-Ian Henriksen

[houbaastef .]

Good to know :) Thanks

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