Jep should throw exception when closing a Jep sub-interpreter instance that has threads still running

[ndjensen]

See https://groups.google.com/forum/#!topic/jep-project/9qnpnNBipXA

If a Jep instance is closed while a thread is still running inside that sub-interpreter, the process/application will exit and shut down.

Example:

try(Jep jep = new Jep()) {
    jep.runScript("./src/main/python/thread_test.py");
} catch (JepException e) {
    e.printStackTrace();
}

import threading
import time
import sys
# import thread
def f1():
    for i in range(5):
        print ("f1...\n")
        sys.stdout.flush()
        time.sleep(1)


if __name__ == "__main__":

    t = threading.Thread(target=f1, args=())
    t.setDaemon(True)
    t.start()

    for t in threading.enumerate():
        print (t.getName())
        #if t.getName() != "MainThread":
            #t._Thread__stop()
            #t._stop()
    print (threading.enumerate())

Current C code in pyembed.c's pyembed_thread_close():

if (jepThread->tstate->interp == mainThreadState->interp) {
    PyThreadState_Clear(jepThread->tstate);
    PyThreadState_Swap(NULL);
    PyThreadState_Delete(jepThread->tstate);
} else {
    Py_EndInterpreter(jepThread->tstate);
}

When Jep.close() is called, there is a fatal error:
Fatal Python error: Py_EndInterpreter: not the last thread. Py_EndInterpreter(jepThread->tstate); eventually uses Py_FatalError which will end the process. https://github.com/python/cpython/blob/46dc4e34ed8005a688d7f3512844ef227a3465f4/Python/pylifecycle.c#L1506

Jep should attempt to detect this situation and throw an exception indicating the developer needs to manage their threads, preventing an exit and shutdown of the process.

[bsteffensmeier]

This is fixed in dev_3.9 with d36a4b9.

[bsteffensmeier]

Jep 3.9.0 has been released