Systemd.unit - Add option for instance translation script.

Submission type

• Request for enhancement (RFE)

systemd version the issue has been seen with

237.31-1

Used distribution

ArchLinux

Situation / use case

I have a systemd service template homie@.service that is being isntantiated using a unique customer ID.
Thus I can view the respective customer's service's logs using journalctl -au homie@<customer_id>.service.

The service unit may be triggered periodically using a systemd.timer named homie@.timer, that is instantiated in the same fashion (customer ID).

However, the service needs also be capable to be triggered by file uploads. Hence I created a systemd.path named homie@.path.
Since the paths to be monitored vary for each customer (but are unique for each customer), I instantiate the respective path unit with the path to monitor:

[Unit]
Description=Inotify trigger for HOMIE service for path %I

[Path]
PathChanged=%I

[Install]
WantedBy=multi-user.target

Obviously the respective service will now be passed the path, instead of the customer ID.
While I could just accept this behaviour and translate the path to the respective ID within the binary called in homie@.service's ExecStart=…, I then will have to check two logs for each customer: homie@<customer_id>.service and homie@<upload_path>.service.

Hence, it would be nice to add a feature to translate instance IDs within units by e.g. allowing to invoke a script InstanceTranslator=….
The provided script / binary would be expected to receive the unescaped instance as an argument and return the translated instance on STDOUT.
A non-zero exit code could indicate the failure of ID translation and cause the unit to fail.
On success, all %T (Translated instance, new) placeholders should be replaced by the translated ID:

[Unit]
Description=Inotify trigger for HOMIE service for path %I

[Path]
PathChanged=%I
InstanceTranslator=/usr/local/bin/path_to_id.sh
Unit=homie@%T.service

[Install]
WantedBy=multi-user.target

Start by reviewing the systemd.unit and systemd.path instance-handling areas described in the request, including how %I and Unit= are currently processed. Determine the design and tests needed for InstanceTranslator, translated %T expansion, and non-zero translation failures; done means the proposed path-triggered unit resolves to the customer-specific service.