Timezone is no longer persisted due to read-only root file system

[zehnm]

Description

After introducing the read-only root file system, the timezone can no longer be set with timedatectl set-timezone.

How to Reproduce

Steps to reproduce the behavior:

1. Log in by ssh
2. Set a new timezone, e.g.: timedatectl set-timezone UTC
3. Reboot remote
4. Check timezone with date: back to CEST

Expected behavior

Timezone can be set and is persisted during reboots.

Your Environment

• Version used: latest master
• Running on:
  • YIO Remote
  • Standalone Raspberry Pi

Additional context

The /etc file system overlay is only an in-memory overlay which is not persisted during reboots.

[zehnm]

My current understanding: timedatectl is a systemd utility, which requires /etc/localtime to be a symlink to the timezone information. The /etc/timezone file is mostly a "Debian thing". Setting a new timezone with timedatectl only sets a new symlink and leaves /etc/localtime untouched.
More information:

• https://unix.stackexchange.com/a/451714
• https://unix.stackexchange.com/a/452566

Options:

• Use TZ environment variable
  • Easy for shell & system stuff
  • Awkward to integrate into app

# date
Tue Feb  2 18:00:51 CET 2021
# export TZ=UTC
# date
Tue Feb  2 17:01:04 UTC 2021
# export TZ=US/Hawaii
# date
Tue Feb  2 07:01:38 HST 2021

• Just use UTC as system time and handle the timezone offset in the app.
  • Needs special handling whenever dealing with times
  • Logfiles and file timestamps will be UTC
• Persist symlink so that timedatectl can be used. Options:
  • bind-mount doesn't seem possible
  • use a persistent overlay for /etc (as for /var). Easiest solution but /etc was intended to be read-only with special exceptions.
  • Individual files are synced between the in-memory overlay and the data folder.

[zehnm]

Setting the TZ variable in /opt/yio/app-launch.sh works fine. I think this is the easiest way to make the timezone user configurable with a read-only root filesystem and leaving systemd untouched.

• Set system timezone to UTC
• Store app timezone in user data file within the yio directory
• Read the user data file in the app start up script
• Write the user data file from within the initial setup / app configuration screen.

Alternatively a profile.d entry with a bind-mount could be used.