feat: Add socket-activated systemd service for discovery

Introduce a system-wide daemon listening on /run/wsdd.socket,
which is managed by systemd.
When a client first connects to the socket the daemon is started.
Multiple clients can connect and safely use the API simultaneously.

The service runs in discovery-only mode.

Uses multicast source-port 37020.

etc/systemd/wsdd-discovery.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Web Services Dynamic Discovery service
+Documentation=man:wsdd(8)
+Requires=wsdd-discovery.socket
+RefuseManualStart=true
+
+[Service]
+Type=simple
+; The service is put into an empty runtime directory chroot,
+; i.e. the runtime directory which usually resides under /run
+ExecStart=/usr/bin/wsdd --shortlog --chroot=/run/wsdd --source-port=37020 --no-host --discovery
+DynamicUser=yes
+User=wsdd
+Group=wsdd
+RuntimeDirectory=wsdd
+AmbientCapabilities=CAP_SYS_CHROOT

etc/systemd/wsdd-discovery.socket

@@ -0,0 +1,9 @@
+[Unit]
+Description=Web Services Dynamic Discovery API socket
+Documentation=man:wsdd(8)
+
+[Socket]
+ListenStream=%t/wsdd.socket
+
+[Install]
+WantedBy=sockets.target

src/wsdd.py

@@ -42,6 +42,11 @@
 except ModuleNotFoundError:
     from xml.etree.ElementTree import fromstring as ETfromString
 
+try:
+    import systemd.daemon
+except Exception:
+    # Non-systemd host
+    pass
 
 WSDD_VERSION: str = '0.8'
 
@@ -1137,7 +1142,10 @@ async def create_server(self, aio_loop: asyncio.AbstractEventLoop, listen_addres
         # It appears mypy is not able to check the argument to create_task and the return value of start_server
         # correctly. The docs say start_server returns a coroutine and the create_task takes a coro. And: It works.
         # Thus, we ignore type errors here.
-        if isinstance(listen_address, int) or listen_address.isnumeric():
+        if isinstance(listen_address, socket.SocketType):
+            self.server = await aio_loop.create_task(asyncio.start_unix_server(  # type: ignore
+                self.on_connect, sock=listen_address))
+        elif isinstance(listen_address, int) or listen_address.isnumeric():
             self.server = await aio_loop.create_task(asyncio.start_server(  # type: ignore
                 self.on_connect, host='localhost', port=int(listen_address), reuse_address=True,
                 reuse_port=True))
@@ -2010,6 +2018,14 @@ def main() -> int:
     api_server = None
     if args.listen:
         api_server = ApiServer(aio_loop, args.listen, nm)
+    else:
+        try:
+            fds = systemd.daemon.listen_fds()
+            if fds:
+                api_server = ApiServer(aio_loop, socket.socket(fileno=fds[0]), nm)
+        except Exception:
+            # Non-systemd host
+            pass
 
     # get uid:gid before potential chroot'ing
     if args.user is not None: