The daemon post

daemons.py

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+#!/usr/bin/env python3
+## This Blog post is executable python code, it requires the gevent modules to run
+# pip as gevent, Debian as python3-gevent.
+
+import os
+import sys
+import gevent
+import gevent.socket as socket
+import signal
+
+
+# A long time ago circa 2010ish when i was taking the network programing class in
+# college there was a blog post that explained in great detail,
+# how to "fork into the background" e.g make a daemon on a Unix
+# system. It was called Demystifing Daemons. By some guy on Blogspot,
+# I think they went by Tom. I can no longer find this post.
+# I get back more and more AI slop every time I look.
+# This trick is not something i see often anymore.
+# Therefore let's recreate a classic.
+# Daemon's Demystified 2.0 starts right now
+
+# Sometimes programs need to run unattended, without tying up
+# a terminal session. This is less of a problem now today
+# then 30 years ago with our new fangled terminal multiplexers
+# and the like. But i find that there's value  in learning archane
+# knowledge. Unix has had the concept of daemons since AT&T version 7
+# I think, but I'm finding that a decreasing  number of people are
+# familiar with this once basic knowledge.
+
+# To that end this post will show how to create a Disk and Execution Monitor
+# a daemon, also called a service by the systemd kids.
+
+# In order to have a service we must first have something to serve.
+# Here's one of my favorite prank services. RickRoll over http
+
+# you know it, you love it, It's a valid HTTP response
+
+RICKROLL_LYRICS = """
+HTTP/1.1 200 OK
+Content-Type: text/plain; charset=UTF-8
+Last-Modified: Mon, 27 July 1987 00:00 GMT
+Content-Length: 982
+
+
+We're no strangers to love
+You know the rules and so do I
+A full commitment's what I'm thinkin' of
+You wouldn't get this from any other guy
+
+I just wanna tell you how I'm feeling
+Gotta make you understand
+
+Never gonna give you up, never gonna let you down
+Never gonna run around and desert you
+Never gonna make you cry, never gonna say goodbye
+Never gonna tell a lie and hurt you
+
+We've known each other for so long
+Your heart's been aching, but you're too shy to say it
+Inside, we both know what's been going on
+We know the game and we're gonna play it
+
+And if you ask me how I'm feeling
+Don't tell me you're too blind to see
+
+Never gonna give you up, never gonna let you down
+Never gonna run around and desert you
+Never gonna make you cry, never gonna say goodbye
+Never gonna tell a lie and hurt you
+
+Never gonna give you up, never gonna let you down
+Never gonna run around and desert you
+Never gonna make you cry, never gonna say goodbye
+Never gonna tell a lie and hurt you
+
+"""
+
+client_procs = []
+
+svr_proc = None
+
+
+# more on this later
+class NullDevice:
+    def write(self, s):
+        pass
+
+
+# See ln 104 and following
+def hup_handle(sig, fr):
+    sys.exit()
+
+
+# just standard Unix Network Programing Stuff
+# Only interesting bit is if using INET6 in python at least
+# IPV4 comes for free so no need to use AF_UNSPEC.
+# See Refs 1 and 2 for more info
+
+# By the way i hate python 3's concurrency stuff so we'll be using
+# gevent.
+
+
+def server_handler():
+    serversock = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
+    serversock.bind(("", 1337))
+    serversock.listen(10)
+    while True:
+        client, addr = serversock.accept()
+        print(addr)
+        client_procs.append(gevent.spawn(client_handler, client))
+        gevent.sleep(0)
+
+    serversock.close()
+    return
+
+
+# This is a simple infinite rickroll http server
+# it only responds to GET requests. and no matter what
+# will give you a rickroll back.
+#
+# This is somewhat useful in elfing with ai scraper bots.
+# There are better methods if you wanna try that
+def client_handler(sock):
+    print("Client handler spawn")
+    while True:
+        data = sock.recv(4096)
+        dstring = data.decode("UTF-8")
+        if dstring.startswith("GET"):
+            break
+        gevent.sleep(0)
+
+    sock.send(RICKROLL_LYRICS.encode("utf-8"))
+    sock.close()
+    return
+
+
+def daemon_main():
+    svr_proc = gevent.spawn(server_handler)
+    client_procs.append(svr_proc)
+    gevent.joinall(client_procs)
+    sys.exit(0)
+
+
+pid = os.fork()  # Hmmm this looks an awful lot like... C
+# Yes it does per C fork(3) creates a nearly identical copy of the
+# calling process as a child of the calling process
+# returning it's pid to the caller. Asexual reproduction at it's finest
+
+if pid:
+    os._exit(0)
+    # exit without running exit handlers, that might cause race condition
+    # in the child
+
+
+# Per the fork manual the child begins execution at the point where fork
+# is called, as if the child had called it. The only difference being
+# is the child process gets a zero as return value, and so the else branch
+# of this if is followed.
+else:
+    # It turns out my CS prof lied about the purpose of these two calls
+    # the child process needs to be the process group leader, when parent
+    # exits or it gets reaped by the init system
+    os.setpgrp()
+    os.umask(0)
+
+print(os.getpid())  # to aid in stopping the server
+# We want to close our connection to the controlling terminal
+# to avoid accedentially spamming the use. And causing interactive processes
+# to be SIGSTOP'ed. I do this with a Null Device class.
+# You could just as easily do some sort of logging thing.
+sys.stdin.close()
+sys.stdout = NullDevice()
+sys.stderr = NullDevice()
+
+# The last thing we do before handing things off to the daemon's main
+# function is set up the daemon's signal table how we want it
+# fork, may have initialized it with the default handlers
+# depending on implementation
+signal.signal(signal.SIGHUP, hup_handle)
+signal.signal(signal.SIGTERM, hup_handle)
+daemon_main()
+
+
+# References
+# 1. Beej's guide to Network Programing https://beej.us/guide/bgnet/
+# 2. Foundations of Python Network Programming 2ed Rhodes and Goerzen