Add service to persist /etc/fake-hwclock.data past /etc-overlay unmount (#542)

* Add service to persist `/etc/fake-hwclock.data` past etc overlay unmount

* Update `CHANGELOG.md`

* Properly enable the new systemd service

* Try to fix incorrect implementation of the systemd service

* Try to adjust the system service's dependencies

Author: ethanjli

software/CHANGELOG.md

@@ -37,6 +37,7 @@ All dates in this file are given in the [UTC time zone](https://en.wikipedia.org
 - (System: networking) `planktoscope.local` and `pkscope.local` should now work on local area networks (i.e. when the PlanktoScope is connected to a router) and not just on direct connections.
 - (Application: GUI) The Node-RED dashboard's sample page's "Dilution Factor" input field has been renamed to "Concentration Factor", which is a less misleading name for what that input field actually represents.
 - (Application) The `/home/pi/data` and `/home/pi/device-backend-logs` are now created with non-`root` user ownership, so that their contents can be managed via an SFTP/SCP connection as the `pi` user. This fixes a regression introduced with v2023.9.0-beta.0.
+- (System) The system time is now correctly persisted on the filesystem (in `/etc/fake-hwclock.data`) in a way that the system time should no longer reset back to a previous time in the past between reboots.
 - (System) Machine name generation now falls back to the `en_US.UTF-8`-based naming scheme when the OS is set to a non-default locale (i.e. anything other than `en_US.UTF-8`), instead of failing and falling back to `unknown`.
 
 ## v2024.0.0 - 2024-12-25

software/distro/setup/base-os/forklift/install.sh

@@ -101,4 +101,5 @@ sudo systemctl preset \
   bindro-run-forklift-stages-current.service \
   overlay-usr.service \
   overlay-etc.service \
-  start-overlaid-units.service
+  start-overlaid-units.service \
+  fake-hwclock-overlay-support.service

software/distro/setup/base-os/forklift/usr/lib/systemd/system-preset/20-forklift.preset

@@ -4,3 +4,4 @@ enable overlay-sysroot.service
 enable bindro-run-forklift-stages-current.service
 enable overlay-usr.service
 enable overlay-etc.service
+enable fake-hwclock-overlay-support.service

software/distro/setup/base-os/forklift/usr/lib/systemd/system/fake-hwclock-overlay-support.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Copy the fake-hwclock file from the overlay override during shutdown
+DefaultDependencies=no
+# This ensures that the shutdown part of this service will run after the `/etc` overlay is unmounted:
+Before=overlay-etc.service
+Conflicts=shutdown.target
+Before=shutdown.target
+ConditionPathExists=/var/lib/overlays/overrides/etc/fake-hwclock.data
+
+[Service]
+Type=oneshot
+RemainAfterExit=true
+ExecStart=sleep 1
+ExecStopPost=cp /var/lib/overlays/overrides/etc/fake-hwclock.data /etc/fake-hwclock.data
+
+[Install]
+WantedBy=overlay-etc.service