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When combined with a tmpfs on /run or /var/lib, allows to create arbitrary and ephemeral symlinks for StateDirectory or RuntimeDirectory. This is especially useful when sharing these directories between different services, to make the same state/runtime directory 'backend' appear as different names to each service, so that they can be added/removed to a sharing agreement transparently, without code changes. An example (simplified, but real) use case: foo.service: StateDirectory=foo bar.service: StateDirectory=bar foo.service.d/shared.conf: StateDirectory= StateDirectory=shared:foo bar.service.d/shared.conf: StateDirectory= StateDirectory=shared:bar foo and bar use respectively /var/lib/foo and /var/lib/bar. Then the orchestration layer decides to stop this sharing, the drop-in can be removed. The services won't need any update and will keep working and being able to store state, transparently. To keep backward compatibility, new DBUS messages are added.
System and Service Manager
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