Eventfd with epoll BlockingIOError

[jenkris at tutanota.com]

I have a C program that uses fork-execv to run Python 3.10 in a child process, and I am using eventfd with epoll for IPC between them.  The eventfd file descriptor is created in C and passed to Python through execv.  Once the Python child process starts I print the file descriptor to verify that it is correct (it is).  

In this scenario C will write to the eventfd at intervals and Python will read the eventfd and take action based on the value in the eventfd.  But in the Python while True loop I get "BlockingIOError: [Errno 11] Resource temporarily unavailable" then with each new read it prints "Failed epoll_wait Bad file descriptor." 

This is the Python code:

#!/usr/bin/python3
import sys
import os
import select

print("Inside Python")

event_fd = int(sys.argv[3])

print("Eventfd received by Python")
print(event_fd)

ep = select.epoll(-1)
ep.register(event_fd, select.EPOLLIN | select.EPOLLOUT)

event_write_value = 100

while True:

    print("Waiting in Python for event")
    ep.poll(timeout=None, maxevents=- 1)
    v = os.eventfd_read(event_fd)

    if v != 99:
        print("found")
        print(v)
        os.eventfd_write(event_fd, event_write_value)

    if v == 99:
        os.close(event_fd)

This is the C code that writes to Python, then waits for Python to write back:

ssize_t epoll_write(int event_fd, int epoll_fd, struct epoll_event * event_struc, int action_code)
{
   int64_t ewbuf[1];
   ewbuf[0] = (int64_t)action_code;
   int maxevents = 1;
   int timeout = -1;

   fprintf(stdout, " Writing to Python \n%d", event_fd);

   write(event_fd, &ewbuf, 8);

    if (epoll_wait(epoll_fd, event_struc, maxevents, timeout) == -1)
    {
        fprintf(stderr, "Failed epoll_wait %s\n", strerror(errno));
    }

    ssize_t rdval = read(event_fd, &ewbuf, 8);   

    fprintf(stdout, " Received from Python \n%ld", rdval);

    return 0;
}

This is the screen output when I run with gdb:

          Inside Python
Eventfd received by Python
5
Waiting in Python for event
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/runpy.py", line 196, in 	_run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/opt/P01_SH/NPC_CPython.py", line 36, in <module>
    v = os.eventfd_read(event_fd)
BlockingIOError: [Errno 11] Resource temporarily unavailable
Writing to Python
5 Received from Python
8 Writing to Python
Failed epoll_wait Bad file descriptor
5 Received from Python
8 Writing to Python
Failed epoll_wait Bad file descriptor
5 Received from Python
-1time taken 0.000548
Failed to close epoll file descriptor
Unlink_shm status: Bad file descriptor
fn() took 0.000648 seconds to execute
[Inferior 1 (process 12618) exited normally]
(gdb)

So my question is why do I get "BlockingIOError: [Errno 11] Resource temporarily unavailable" and "Failed epoll_wait Bad file descriptor" from Python? 

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