Issue description

The exact setup for this to happen seems really bizarre to me, but here it goes: I get a deadlock when using Flask with its default debug app reloader with my pybind11 module. I've reduced the example down to the smallest thing I can think of that still replicates.

The ingredients:

1. #include <pybind11/functional.h>
2. A pybind11-bound function that takes a std::function
3. A class defined in the module
4. A Flask app running with app.run(debug=True)
5. A route in that Flask app that calls the function from 2.

What really confuses me is that if I remove the class in step 3, the deadlock goes away. Similarly, if the function in step 2 doesn't take something complicated (like just an int), the deadlock goes away. If the call to run the app is instead just app.run(), the deadlock goes away (similarly if use_reloader=False is passed). Something in the combination of all three of these things causes the deadlock when the passed-in function is invoked. Using a default std::function parameter doesn't seem to change anything.

Versions:

$ python --version
Python 3.6.4

$ gcc --version
gcc (GCC) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

pybind11 stable branch (see submodule in reproduction repo)

Reproducible example code

https://github.com/skystrife/pybind11-bug