There is a race upon reload, where it can happen that:
1. systemd forks off /bin/kill -HUP $MAINPID
2. Current instance forks off new one and notifies systemd with the
new MAINPID.
3. systemd sets new MAINPID.
4. systemd receives SIGCHLD for the kill process (which is the current
control process for the service) and reads the PID of the old
instance from the PID file, resetting MAINPID to the PID of the old
instance.
5. Old instance exits.
6. systemd receives SIGCHLD for the old instance, reads the PID of the
old instance from the PID file once more. systemd sees that the
MAINPID matches the child PID and considers the service exited.
7. systemd receivese notification from the new PID and is confused.
The service won't get active, because the notification wasn't
handled.
To fix it, update the PID file before sending the MAINPID
notification, similar to what a comment in systemd's
src/core/service.c suggests:
> /* Forking services may occasionally move to a new PID.
> * As long as they update the PID file before exiting the old
> * PID, they're fine. */
but for our Type=notify "before sending the notification" rather than
"before exiting", because otherwise, the mix-up in 4. could still
happen (although it might not actually be problematic without the
mix-up in 6., it still seems better to avoid).
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
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