$ cat > /tmp/bug-interp-not-interrupted.py << EOF
import threading
import time
from test.support import interpreters

done = False

# A subthread running the main interpreter does not get interrupted.
def task():
    while not done:
        time.sleep(0.1)
t = threading.Thread(target=task)
t.start()

interp = interpreters.create()

start = time.time()
try:
    interp.exec_sync("""if True:
        import time
        print('Hit Ctrl-C now!')
        for i in range(1, 6):
            time.sleep(1)
            print(i)
    """)
    # Ctrl-C isn't handled until the subinterpreter finishes.
finally:
    done = True

EOF
$ ./python /tmp/bug-interp-not-interrupted.py
1
^C^C2
3
4
^C5
Traceback (most recent call last):
  File "/tmp/bug-interp-not-interruptedpy", line 17, in <module>
    interp.exec_sync("""if True:
    ...
  File "./cpython/Lib/test/support/interpreters/__init__.py", line 178, in exec_sync
    excinfo = _interpreters.exec(self._id, code)
              ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
KeyboardInterrupt