Enable a command to be run on machine shutdown

[Dan Watkins]

Google Compute Engine support running scripts as an instance is being shutdown, which is currently implemented using a oneshot service that has RemainAfterExit=true and an ExecStop command set[0].

We will need to be able to specify commands to run on shutdown of a machine in order to snap up GCE's agent packages.

[0] https://github.com/GoogleCloudPlatform/compute-image-packages/blob/master/google_compute_engine_init/systemd/google-shutdown-scripts.service

[Zygmunt Krynicki (zyga)]

Dear reporter, the link in the bug report does not work. Could you please revise the description of the bug to update the link. How is this handled outside of the context of snaps? That is, how does one otherwise run a service on system shutdown?

[Ian Johnson (anonymouse67)]

The correct link seems to be https://github.com/GoogleCloudPlatform/compute-image-packages/blob/fa80cdb23df0a7e190e228a9d9d379922d1f0764/packages/google-compute-engine/src/lib/systemd/system/google-shutdown-scripts.service

AFAICT, the way most services do this is to have the command you want to run in ExecStop, then have RemainAfterExit=true which means that the service is always considered active and thus ExecStop only gets run when the system is shut down.

This seems a reasonable feature request for snapd, but admittedly is probably low priority right now.