[BUG]throw in def_buffer cause terminate

[caishanli]

cpp

.def_buffer([](Matrix &m) -> py::buffer_info {
    throw std::runtime_error("Incompatible buffer format!");
    return py::buffer_info(
        m.data(),                                /* Pointer to buffer */
        sizeof(Scalar),                          /* Size of one scalar */
        py::format_descriptor<Scalar>::format(), /* Python struct-style format descriptor */
        2,                                       /* Number of dimensions */
        { m.rows(), m.cols() },                  /* Buffer dimensions */
        { sizeof(Scalar) * (rowMajor ? m.cols() : 1),
          sizeof(Scalar) * (rowMajor ? 1 : m.rows()) }
                                                 /* Strides (in bytes) for each index */
    );
 })

and in python

m = Matrix(3, 2)
np.array(m)

cause python terminate, output is

terminate called after throwing an instance of 'std::runtime_error'
  what():  Incompatible buffer format!
Aborted (core dumped)

expected output is runtime error, not terminate

RuntimeError: Incompatible buffer format!

[YannickJadoul]

@caishanli, can you specify why you want to throw from def_buffer, though? This should be fixable, but I'd figure out if we should allow this. "Incompatible buffer format!" is not up to you to throw; def_buffer should specify what's offered and the caller will see if it can handle the buffer format offered.

[caishanli]

@YannickJadoul Hi, I use pybind11 to bind Tencent/ncnn. And in ncnn::Mat, the element size(in byte) can be 1, 2, 3, 4 or any other positive integers, but as my best knowledge, python have no data type of 3 bytes, 5 bytes and so on, so I need to throw a runtime error, and the same as element pack.

[YannickJadoul]

@caishanli Ah, I see. And that's only known at runtime? :-(

But OK, then we mainly need a way to indicate failure from a buffer, rather than a "catch all exceptions"? The latter wouldn't be wrong to do either, though.

[caishanli]

@YannickJadoul I think just catch all exceptions is ok as we have error message to indicate what's wrong.