Open shm only if server is working & signal to systemd

[matteodelabre]

This changeset is aimed at giving more information to external processes as to whether the rm2fb server is running or not, in two ways:

• The shared memory segment will only be instantiated after the server has confirmed it has all the offsets it needs to work. Otherwise, the server process will exit with code 255 (as before), without creating the shm. This enables other processes to check whether the server has been started by testing for the existence of /dev/shm/swtfb.01 (this fixes #76).
• The server will now send a signal to systemd when it is done initializing, so that boot-time ordering of dependencies can work properly. This requires changing the rm2fb.service type to notify:

 [Unit]
 Description=reMarkable 2 Framebuffer Server
 Before=xochitl.service launcher.service remarkable-reboot.service remarkable-shutdown.service
 After=opt.mount
 StartLimitInterval=30
 StartLimitBurst=5
 Conflicts=
 ConditionFileNotEmpty=/opt/lib/librm2fb_server.so.1

 [Service]
+Type=notify
 ExecStart=/usr/bin/xochitl
 Restart=on-failure
 RestartSec=5
 Environment="HOME=/home/root"
 Environment="LD_PRELOAD=/home/root/librm2fb_server.so.1.0.1"

 [Install]
 WantedBy=multi-user.target

To test, replace both librm2fb_server and librm2fb_client, update the rm2fb service definition as above, then:

• Enable rm2fb.service and xochitl.service, disable any launcher, and reboot. After reboot, check the logs of xochitl: they should start with lines saying Sourcing /opt/etc/xochitl.env.d/rm2fb-preload.env, Replacing 'update' (at 0x3a9cdc): OK, etc. You should NOT see a line saying rm2fb server is not running: starting without rm2fb client. Take note of the timestamp of xochitl’s first log line. Check the logs of rm2fb, make sure that the timestamp of the log line saying WAITING FOR SEND UPDATE ON MSG Q is lower than or equal to the timestamp of the first xochitl log line.
• Edit /etc/version to simulate running an unsupported OS release. Reboot. Check the status of rm2fb.server, it should have failed with exit code 255. Check that /dev/shm/swtfb.01 does not exist. Xochitl should start anyway. Check the logs of xochitl: you should now see the line saying rm2fb server is not running: starting without rm2fb client in place of the Replacing... lines.

[matteodelabre]

Thank you for testing!