Use simple type and -nofork in nbd@ systemd service - #200

@yeo should nbd@.service.sh.in or nbd@.service.tmpl be updated?

And can we delete the one that's not used?

As of e906c55 the duplicate file is now removed.

This was there because at once upon a time the sh.in file was generated from the .tmpl file, but that is no longer the case and having both there is turning out to be a bad idea :)

So I'm not sure I 100% understand the proposed change.

Can you explain how the proposed change will work in both persisting and not-persisting mode, and why I should merge it? :)

Can you explain how the proposed change will work in both persisting and not-persisting mode

Thinking about this more, I don't think this is the right solution to the problem I'm attempting to solve.

The problem is that when using persist mode, the systemd service hangs when starting, stuck in the "activating" state until it times out:

# systemctl status nbd@nbd0
● nbd@nbd0.service - NBD client connection for nbd0
     Loaded: loaded (/usr/lib/systemd/system/nbd@.service; enabled; preset: disabled)
    Drop-In: /etc/systemd/system/nbd@nbd0.service.d
             └─override.conf
     Active: activating (start) since Mon 2026-09-14 13:55:52 EDT; 2s ago
        Job: 3078978
 Invocation: 36a93b057def442d9caf99e9b60f2bfb
       Docs: man:nbd-client
   Main PID: 669130 (nbd-client)
      Tasks: 1 (limit: 19080)
     Memory: 1.3M (peak: 2.2M)
        CPU: 3ms
     CGroup: /system.slice/system-nbd.slice/nbd@nbd0.service
             └─669130 /usr/sbin/nbd-client nbd0

Sep 14 13:55:52 irrational systemd[1]: Starting NBD client connection for nbd0...
Sep 14 13:55:53 irrational nbd-client[669130]: Negotiation: ..size = 1907695MB
Sep 14 13:55:53 irrational nbd-client[669130]: Connected /dev/nbd0
Sep 14 13:55:53 irrational nbd-client[669130]: Persist mode active, monitoring device 0

I think the root cause is the persist mode changes in 5b2d384.

Perhaps the solution is to fork, even in persist mode.

Also, -nofork doesn't seem to work at all. When persist mode is not set, nbd-client -nofork nbd0 immediately exits when it should not exit

# nbd-client -nofork nbd0
Negotiation: ..size = 1907695MB
Error: Failed to setup device, check dmesg

Exiting.

Can you explain how the proposed change will work in both persisting and not-persisting mode

Thinking about this more, I don't think this is the right solution to the problem I'm attempting to solve.

Right, that's what I was afraid of, but I assumed I must have been wrong :)

[...]

Perhaps the solution is to fork, even in persist mode.

We could do that, but then we should probably have a "force fork" option, so that we are guaranteed to be forking.

Also, -nofork doesn't seem to work at all. When persist mode is not set, nbd-client -nofork nbd0 immediately exits when it should not exit

# nbd-client -nofork nbd0
Negotiation: ..size = 1907695MB
Error: Failed to setup device, check dmesg

Exiting.

I created #201 for that.