Please document the callback GIL semantics

[seerdecker]

The doc, the functional.h file, and the examples do not specify the behavior of callbacks with respect to the GIL.

I have a C++ class that calls a std::function callback bound to a Python function from an arbitrary C++ thread asynchronously (e.g. from a C++ thread pool). The main Python thread can be anywhere during that time, e.g. it may be sleeping, it may be calling the C++ class methods with or without py::call_guardpy::gil_scoped_release(), etc.

What happens under the hood during the callback call with respect to the GIL? How must I write the code to ensure that it's safe to call the callback? Right now I'm assuming that Pybind11 will somehow figure out whether it needs to re-acquire the GIL by magic.

Thank you

[AlexanderSerov]

Looks like satisfied here and here