systemd-247 provides a mechanism called LoadCredential for secrets and
it is better than environment file. See the section of Environment=
in the manual of systemd.exec for more information.
Some options in config.yaml need values to be strings, which currently
can be used with environmentFile but not loadCredential. But it's
possible to use loadCredential for those options, e.g. we can
substitute their values in ExecStart, but not in ExecStartPre due to
[1].
[1]: https://github.com/systemd/systemd/issues/19604
Before this patch, services.dendrite.environmentFile is used for
secrets and environment variable substitution only happens when this
option is used.
systemd-247 provides a mechanism called LoadCredential for secrets and
it is better than environment file. See the section of Environment=
in the manual of systemd.exec for more information.
This patch always substitute environment variables, which enables the
usage of systemd LoadCredential.
