Add support for systemd socket activation

Instead of enabling lorax-composer.service enable lorax-composer.socket
and it will start lorax-composer on first access to
/run/weldr/api.socket

lorax.spec

@@ -131,12 +131,15 @@ getent passwd weldr >/dev/null 2>&1 || useradd -r -g weldr -d / -s /sbin/nologin
 
 %post composer
 %systemd_post lorax-composer.service
+%systemd_post lorax-composer.socket
 
 %preun composer
 %systemd_preun lorax-composer.service
+%systemd_preun lorax-composer.socket
 
 %postun composer
 %systemd_postun_with_restart lorax-composer.service
+%systemd_postun_with_restart lorax-composer.socket
 
 %files
 %defattr(-,root,root,-)
@@ -162,6 +165,8 @@ getent passwd weldr >/dev/null 2>&1 || useradd -r -g weldr -d / -s /sbin/nologin
 %{python_sitelib}/pylorax/api/*
 %{_sbindir}/lorax-composer
 %{_unitdir}/lorax-composer.service
+%{_unitdir}/lorax-composer.socket
+%{_tmpfilesdir}/lorax-composer.conf
 
 %files -n composer-cli
 %{_bindir}/composer-cli

setup.py

@@ -8,7 +8,9 @@ import sys
 # config file
 data_files = [("/etc/lorax", ["etc/lorax.conf"]),
               ("/etc/lorax", ["etc/composer.conf"]),
-              ("/usr/lib/systemd/system", ["systemd/lorax-composer.service"])]
+              ("/usr/lib/systemd/system", ["systemd/lorax-composer.service",
+                                           "systemd/lorax-composer.socket"]),
+              ("/usr/lib/tmpfiles.d/", ["systemd/lorax-composer.conf"])]
 
 # shared files
 for root, dnames, fnames in os.walk("share"):

src/sbin/lorax-composer

@@ -228,6 +228,19 @@ if __name__ == '__main__':
             log.error(e)
         sys.exit(1)
 
+    # Did systemd pass any extra fds (for socket activation)?
+    try:
+        fds = int(os.environ['LISTEN_FDS'])
+    except (ValueError, KeyError):
+        fds = 0
+
+    if fds == 1:
+        # Inherit the fd passed by systemd
+        listener = socket.fromfd(3, socket.AF_UNIX, socket.SOCK_STREAM)
+    elif fds > 1:
+        log.error("lorax-composer only supports inheriting 1 fd from systemd.")
+        sys.exit(1)
+    else:
         # Setup the Unix Domain Socket, remove old one, set ownership and permissions
         if os.path.exists(opts.socket):
             os.unlink(opts.socket)

systemd/lorax-composer.conf

@@ -0,0 +1 @@
+d /run/weldr 750 root weldr

systemd/lorax-composer.socket

@@ -0,0 +1,12 @@
+[Unit]
+Description=lorax-composer socket activation
+
+[Socket]
+ListenStream=/run/weldr/api.socket
+SocketUser=root
+SocketGroup=weldr
+SocketMode=0660
+DirectoryMode=0750
+
+[Install]
+WantedBy=sockets.target