Add support for socket groups, so different sockets can be connected to different watchers

[tjikkun]

This is useful if you use circus for socket activation of multiple daemons,
connected to different sockets.

[coveralls]

Coverage decreased (-0.06%) to 62.884% when pulling 7fca067 on AntagonistHQ:sock_act_groups into 6ca0d18 on circus-tent:master.

[tjikkun]

I hope I can get some feedback on this pull request. If the general idea is fine, I will work on test coverage and failing tests.

[k4nar]

Thanks for the PR!

I'm not sure to understand the purpose. Could you show an example of configuration using it?

[tjikkun]

@k4nar Suppose you have the following config:
/tmp/testsock1.py:

#!/usr/bin/env python3

from socketserver import UnixStreamServer, StreamRequestHandler
import socket
import logging
import sys

class Handler(StreamRequestHandler):
    def handle(self):
        self.data = self.rfile.readline().strip()
        logging.info("From <%s>: %s" % (self.client_address, self.data))
        self.wfile.write(self.data.upper() + "\r\n".encode("utf-8"))

class Server(UnixStreamServer):
    
    def __init__(self, server_address, handler_cls):
        # Invoke base but omit bind/listen steps (performed by systemd activation!)
        UnixStreamServer.__init__(
            self, server_address, handler_cls, bind_and_activate=False)
        # Override socket
        self.socket = socket.socket(fileno=int(sys.argv[1]))

if __name__ == "__main__":
    logging.basicConfig(level=logging.INFO)
    PATH = None # not really needed here
    server = Server(PATH, Handler)
    server.serve_forever()

/tmp/testsock2.py

#!/usr/bin/env python3

from socketserver import UnixStreamServer, StreamRequestHandler
import socket
import logging
import sys

class Handler(StreamRequestHandler):
    def handle(self):
        self.data = self.rfile.readline().strip()
        logging.info("From <%s>: %s" % (self.client_address, self.data))
        self.wfile.write(self.data.lower() + "\r\n".encode("utf-8"))

class Server(UnixStreamServer):
    
    def __init__(self, server_address, handler_cls):
        # Invoke base but omit bind/listen steps (performed by systemd activation!)
        UnixStreamServer.__init__(
            self, server_address, handler_cls, bind_and_activate=False)
        # Override socket
        self.socket = socket.socket(fileno=int(sys.argv[1]))

if __name__ == "__main__":
    logging.basicConfig(level=logging.INFO)
    PATH = None # not really needed here
    server = Server(PATH, Handler)
    server.serve_forever()

[watcher:watcher1]
cmd = /tmp/testsock1.py $(circus.sockets.socket1)
uid = root
gid = root
use_sockets = True
on_demand = True
singleton = True
socket_group = sockettest1

[socket:socket1]
path = /tmp/upsock
umask = 077
replace = True
group = sockettest1

[watcher:watcher2]
cmd = /tmp/testsock2.py $(circus.sockets.socket2)
uid = root
gid = root
use_sockets = True
on_demand = True
singleton = True
socket_group = sockettest2

[socket:socket2]
path = /tmp/downsock
umask = 077
replace = True
group = sockettest2

Without this patch and socket_group and group, if you connect to either socket, only watcher1 is started.
This means /tmp/upsock works fine, but /tmp/downsock doesn't.

With the patch, but without socket_group and group, if you connect to the socket both watchers are started, and you are connected to the right watcher.

With the patch, and with socket_group and group, if you connect to the socket only the corresponding watcher is started and you are connected to the right watcher