fake systemd notify socket listener with proper barrier support

sd-notify-listener.py

#!/usr/bin/env python3
# sd-notify-listener.py - fake systemd notify socket listener with proper barrier support
# this represents the outer process waiting for the ready signal, usually this role is played by systemd
# but if you are trying to manage a process that uses sd_notify/systemd-notify and you aren't using systemd... well here you go

import socket
import os
import struct
import array

sock_path = "/tmp/fake-notify.sock"

# Clean up old socket
try:
    os.unlink(sock_path)
except:
    pass

# Create datagram socket
sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
sock.bind(sock_path)

print(f"Listening on {sock_path}...")

while True:
    # Use recvmsg to receive both message data and ancillary data (file descriptors)
    # Buffer for the message data
    bufsize = 4096
    # Buffer for ancillary data (file descriptors)
    # We expect at most one file descriptor
    ancbufsize = socket.CMSG_SPACE(struct.calcsize("i"))
    
    try:
        msg, ancdata, flags, addr = sock.recvmsg(bufsize, ancbufsize)
    except Exception as e:
        print(f"Error receiving message: {e}")
        continue
    
    msg_str = msg.decode('utf-8')
    print(f"Received: {msg_str.strip()}")
    
    # Check for ready signal
    if "READY=1" in msg_str:
        pass # do something
    
    # Handle BARRIER=1 properly according to sd_notify protocol
    if "BARRIER=1" in msg_str:
        # Extract file descriptor from ancillary data
        for cmsg_level, cmsg_type, cmsg_data in ancdata:
            if cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS:
                # Extract the file descriptor
                fd_array = array.array("i")
                fd_array.frombytes(cmsg_data)
                for fd in fd_array:
                    print(f"Received file descriptor {fd} with BARRIER=1, closing it to acknowledge")
                    # Close the fd immediately to signal barrier acknowledgment
                    os.close(fd)
                    print("Barrier acknowledged by closing fd")