service: Notify systemd when service starts up

- Run as a Type=notify service with systemd service.

- Notify systemd just before blocking in the main thread.

- This allows systemd to catch any errors with startup of the service and log
appropriately. This also allows clients depending on making DBus calls etc. to
know that service is ready to serve requests.

- This will increase the boot time slightly as systemd will wait until
FreedomBox service to become active.

Tests:

- Raise an exception in main() during startup. Run 'systemctl start plinth'. No
error is thrown without this patch. With the patch, an error is shown.

- After 'systemctl start plinth', service shows in 'active' state.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

data/usr/lib/systemd/system/plinth.service

@@ -7,6 +7,7 @@ After=network.target
 StartLimitIntervalSec=0
 
 [Service]
+Type=notify
 ExecStart=/usr/bin/plinth
 Restart=on-failure
 RestartSec=5
@@ -15,6 +16,7 @@ User=plinth
 Group=plinth
 StandardOutput=null
 StandardError=null
+NotifyAccess=main
 PrivateTmp=yes
 
 [Install]

plinth/__main__.py

@@ -6,6 +6,8 @@ import logging
 import sys
 import threading
 
+import systemd.daemon
+
 from . import __version__
 from . import app as app_module
 from . import (cfg, frontpage, glib, log, menu, module_loader, setup,
@@ -158,6 +160,12 @@ def main():
     web_server.init()
     web_server.run(on_web_server_stop)
 
+    # systemd will wait until notification to proceed with other processes. We
+    # have service Type=notify.
+    systemd.daemon.notify('READY=1')
+
+    web_server.block()
+
 
 if __name__ == '__main__':
     main()

plinth/web_server.py

@@ -122,6 +122,10 @@ def run(on_web_server_stop):
         cherrypy.engine.start()
 
     cherrypy.engine.subscribe('stop', on_web_server_stop)
+
+
+def block():
+    """Block the calling thread until web server exits."""
     cherrypy.engine.block()