Blocking calls slow down multi-threaded apps

[MattClarkson]

I was debugging a multi-threaded app.

Basically, ndicapi is blocking when doing I/O, and we could do with something like the following conceptual code:

static PyObject* Py_ndiCommand(PyObject* self, PyObject* args) {
      // ... parse args (needs GIL) ...
      Py_BEGIN_ALLOW_THREADS
      ndiCommand(device, command); // blocking serial I/O — safe without GIL
      Py_END_ALLOW_THREADS
      // ... build return value (needs GIL) ...
      return result;
}

i.e. allow Python to release the GIL, which would help multi-threading, as it currently blocks.