Python Selector with FIFO running to infinite loop - python-3.x

Question

I am trying to write some non-blocking FIFO code with kqueue on my BSD machine. Here's the small server code: server.py
import os
import selectors
sel = selectors.KqueueSelector()
TMP_PATH="/tmp/myfifo"
def fifo_read(fd, mask):
data = os.read(fd, 8)
print("fd:{} gives:{} \n", fd, data)
sel.unregister(fd)
print("unregistered")
def fifo_accept(listen_fd, mask):
print("accepted {}".format(listen_fd))
fd = os.dup(listen_fd)
print("duped to {}".format(fd))
sel.register(fd, selectors.EVENT_READ, fifo_read)
if __name__ == "__main__":
try:
os.unlink(TMP_PATH)
except:
pass
os.mkfifo(TMP_PATH)
listen_fd = os.open(TMP_PATH, os.O_RDONLY, mode=0o600)
sel.register(listen_fd, selectors.EVENT_READ, fifo_accept)
while True:
events = sel.select()
for key, mask in events:
cb = key.data
cb(key.fileobj, mask)
sel.close()
Now, when I run a client.py as:
import os
TMP_PATH="/tmp/myfifo"
fd = os.open(TMP_PATH, os.O_WRONLY, mode=0o600)
res = os.write(fd, b"1234567")
print("sent {}".format(res))
When I run the client, I get:
sent 7
But on server, it runs to inifinite loop. Now I understand why the infinite loop is happening. I actually tried mimicking the socket way of using selectors in this Python Docs example.
Here's what I have tried:
I did try the code without duplicating the fd, but it's still in infinite loop.
I tried calling sel.unregister on the original listen_fd, but in this case, running the client the second time doesn't work (which is expected).
Can anyone please let me know if I'm missing something?

Answer 1

So I found one solution to this problem. With sockets, we get a new socket object on accept. So we need to emulate that behaviour by calling unregister on the original fileobj, open again and call register on that.
Fixed code:
import os
import selectors
sel = selectors.KqueueSelector()
try:
os.unlink("./myfifo")
except:
pass
os.mkfifo("./myfifo", 0o600)
def cb(fp):
sel.unregister(fp)
print(f"got {fp.read()}")
fp.close()
fp2 = open("./myfifo", "rb")
sel.register(fp2, selectors.EVENT_READ, cb)
if __name__ == "__main__":
orig_fp = open("./myfifo", "rb")
print("open done")
ev = sel.register(orig_fp, selectors.EVENT_READ, cb)
print(f"registration done for {ev}")
while True:
events = sel.select()
print(events)
for key, mask in events:
key.data(key.fileobj)