[WIP] systemd lifecycle integration + tests #9
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The lifecycle is only relevant within the restart task returned by `try_into_restart_task`. It is not actually required in the config (e.g. for the shutdown coordination signal). This lets us defer creating the object used to handle lifecycle events later within the code.
Instead of forking the process and communicating with the child, shellflip can communicate with systemd to send data in the 'old' process and receive it in the 'new' process. The restart can either be initiated through a Unix socket - in which case the process and systemd unit file should agree on the exit code used to restart the process - or by `systemctl restart`. The process lifecycle is different when using systemd and its file descriptor store. Systemd stops the process completely then spawns a new one; old code does not stay running, unless you use the deprecated and hated `KillMode=none` option.
This ensures that various kinds of restart and handover work as intended. The test binary does very little except to check that data and errors are passed as needed, and error conditions can be detected through the notification socket.
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This allows shellflip to be used directly in systemd's process lifecycle, instead of forking a process.
The lifecycle handler is the same as with the forking model, with two significant differences because systemd must stop the process before starting a new one:
Mitigations for these two limitations will be presented later. If these are not significant limitations for your usecase, e.g. because you can shutdown a process cleanly within a few seconds, or you test serialisation between versions thoroughly, then you can use this new method as-is.
In addition, we add long-awaited tests to assert the behaviour of the current and new operating modes.
WIP due to needing a change in sd-notify to support the systemd file descriptor store.