Rare segfault on termination

[tmplt]

A rare race condition is encountered with Python threads upon termination:

Python thread backtrace (segfault)

#0  0x00007f62af378be0 in raise () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libc.so.6
#0  0x00007f62af378be0 in raise () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libc.so.6
#1  0x00007f62af379dc1 in abort () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libc.so.6
#2  0x00007f62af3ba2ac in __libc_message () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libc.so.6
#3  0x00007f62af3ba2ce in __libc_fatal () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libc.so.6
#4  0x00007f62af708630 in unwind_cleanup () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libpthread.so.0
#5  0x00007f62a4e530d8 in pybind11::cpp_function::dispatcher (self=0x7f62a53cc0c0, args_in=0x7f62a05ef3c8, kwargs_in=0x0)
    at /home/tmplt/code/bookwyrm/lib/pybind11/include/pybind11/pybind11.h:666
#6  0x00007f62b07ad8ba in _PyCFunction_FastCallDict () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#7  0x00007f62b083bb99 in call_function () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#8  0x00007f62b0840ef1 in _PyEval_EvalFrameDefault () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#9  0x00007f62b083c039 in call_function () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#10 0x00007f62b0840ef1 in _PyEval_EvalFrameDefault () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#11 0x00007f62b083c039 in call_function () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#12 0x00007f62b0840ef1 in _PyEval_EvalFrameDefault () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#13 0x00007f62b083c039 in call_function () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#14 0x00007f62b0840ef1 in _PyEval_EvalFrameDefault () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#15 0x00007f62b083ba3a in _PyEval_EvalCodeWithName () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#16 0x00007f62b083c09e in PyEval_EvalCodeEx () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#17 0x00007f62b0776475 in function_call () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#18 0x00007f62b0743158 in PyObject_Call () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#19 0x00000000004b6bca in bookwyrm::core::plugin_handler::python_module_runner (this=0x14b8d80, module=...)
    at /home/tmplt/code/bookwyrm/src/core/plugin_handler.cpp:186
#20 0x00000000004c4c77 in std::__invoke_impl<void, void (bookwyrm::core::plugin_handler::*)(pybind11::module), bookwyrm::core::plugin_handler*, pybind11::module> (__f=
    @0x1b63848: (void (bookwyrm::core::plugin_handler::*)(bookwyrm::core::plugin_handler * const, pybind11::module)) 0x4b6a0e <bookwyrm::core::plugin_handler::python_module_runner(pybind11::module)>, __t=@0x1b63840: 0x14b8d80, __args#0=...)
    at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/invoke.h:73
#21 0x00000000004c35f9 in std::__invoke<void (bookwyrm::core::plugin_handler::*)(pybind11::module), bookwyrm::core::plugin_handler*, pybind11::module> (
    __fn=
    @0x1b63848: (void (bookwyrm::core::plugin_handler::*)(bookwyrm::core::plugin_handler * const, pybind11::module)) 0x4b6a0e <bookwyrm::core::plugin_handler::python_module_runner(pybind11::module)>, __args#0=@0x1b63840: 0x14b8d80, __args#1=...)
    at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/invoke.h:95
#22 0x00000000004c670d in std::thread::_Invoker<std::tuple<void (bookwyrm::core::plugin_handler::*)(pybind11::module), bookwyrm::core::plugin_handler*, pybind11::module> >::_M_invoke<0ul, 1ul, 2ul> (this=0x1b63838) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/thread:234
#23 0x00000000004c66a7 in std::thread::_Invoker<std::tuple<void (bookwyrm::core::plugin_handler::*)(pybind11::module), bookwyrm::core::plugin_handler*, pybind11::module> >::operator() (this=0x1b63838) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/thread:243
#24 0x00000000004c6686 in std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (bookwyrm::core::plugin_handler::*)(pybind11::module), bookwyrm::core::plugin_handler*, pybind11::module> > >::_M_run (this=0x1b63830) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/thread:186
#25 0x00007f62aff83edf in ?? () from /nix/store/zk5zj2307zxaq7dx585yia3dn5k4qlsl-gcc-7.3.0-lib/lib/libstdc++.so.6
#26 0x00007f62af6ff5a7 in start_thread () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libpthread.so.0
#27 0x00007f62af43622f in clone () from /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27/lib/libc.so.6

Main thread backtrace

#0  0x00007f62b085d63c in PyImport_Cleanup () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#0  0x00007f62b085d63c in PyImport_Cleanup () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#1  0x00007f62b086d480 in Py_FinalizeEx () from /nix/store/hy65mn4wjswqih75gfr6g4q3xgqdm325-python3-3.6.6/lib/libpython3.6m.so.1.0
#2  0x00000000004ba08e in pybind11::finalize_interpreter () at /home/tmplt/code/bookwyrm/lib/pybind11/include/pybind11/embed.h:159
#3  0x00000000004ba192 in pybind11::scoped_interpreter::~scoped_interpreter (this=0x14b8fd0, __in_chrg=<optimized out>)
    at /home/tmplt/code/bookwyrm/lib/pybind11/include/pybind11/embed.h:193
#4  0x00000000004b65a4 in bookwyrm::core::plugin_handler::~plugin_handler (this=0x14b8d80, __in_chrg=<optimized out>)
    at /home/tmplt/code/bookwyrm/src/core/plugin_handler.cpp:114
#5  0x000000000045bc19 in __gnu_cxx::new_allocator<bookwyrm::core::plugin_handler>::destroy<bookwyrm::core::plugin_handler> (this=0x14b8d80, 
    __p=0x14b8d80) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/ext/new_allocator.h:140
#6  0x000000000045bb5b in std::allocator_traits<std::allocator<bookwyrm::core::plugin_handler> >::destroy<bookwyrm::core::plugin_handler> (__a=..., 
    __p=0x14b8d80) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/alloc_traits.h:487
#7  0x000000000045b913 in std::_Sp_counted_ptr_inplace<bookwyrm::core::plugin_handler, std::allocator<bookwyrm::core::plugin_handler>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=0x14b8d70) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/shared_ptr_base.h:535
#8  0x0000000000426c8e in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x14b8d70)
    at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/shared_ptr_base.h:154
#9  0x0000000000420d71 in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=0x7ffc4d9f9758, __in_chrg=<optimized out>)
    at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/shared_ptr_base.h:684
#10 0x000000000041f3a8 in std::__shared_ptr<bookwyrm::core::plugin_handler, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=0x7ffc4d9f9750, 
    __in_chrg=<optimized out>) at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/shared_ptr_base.h:1123
#11 0x000000000041f3c4 in std::shared_ptr<bookwyrm::core::plugin_handler>::~shared_ptr (this=0x7ffc4d9f9750, __in_chrg=<optimized out>)
    at /nix/store/vv4r320p5yd1k01kld62q1lppjxcswhb-gcc-7.3.0/include/c++/7.3.0/bits/shared_ptr.h:93
#12 0x0000000000410de0 in main (argc=3, argv=0x7ffc4d9f9ee8) at /home/tmplt/code/bookwyrm/src/main.cpp:220