`nbd-server` immediately exits as systemd service

[eworm-de]

Currently packaging new nbd version 3.27.0, and suffering a severe issue.

I start the systemd service and it exits immediately, successfully:

root@leda ~ # systemctl status nbd.service                                                                                                                                                                                       
○ nbd.service - Network Block Device Server                                                                                                                                                                                      
     Loaded: loaded (/usr/lib/systemd/system/nbd.service; enabled; preset: disabled)                                                                                                                                             
    Drop-In: /etc/systemd/system/nbd.service.d                                                                                                                                                                                   
             └─keepalive.conf                                                                                                                                                                                                    
     Active: inactive (dead) since Fri 2026-03-20 13:02:32 CET; 27s ago                                                                                                                                                          
   Duration: 1ms                                                                                                                                                                                                                 
 Invocation: e331ebbae5a143c183d3b8f1f8bb2857                                                                                                                                                                                    
    Process: 1552 ExecStart=/usr/bin/nbd-server (code=exited, status=0/SUCCESS)                                                                                                                                                  
   Main PID: 1582 (code=exited, status=0/SUCCESS)                                                                                                                                                                                
   Mem peak: 3.1M                                                                                                                                                                                                                
        CPU: 810ms                                                                                                                                                                                                               
                                                                                                                                                                                                                                 
Mar 20 13:02:31 leda.eworm.net systemd[1]: Starting Network Block Device Server...                                                                                                                                               
Mar 20 13:02:31 leda.eworm.net systemd[1]: Started Network Block Device Server.                                                                                                                                                  
Mar 20 13:02:32 leda.eworm.net systemd[1]: nbd.service: Deactivated successfully.

Reverted da5e07c and things work again as expected.

Also just running nbd-server from command line works... Any idea?

[eworm-de]

Oh, wait... It does a double fork? Guess that's why.

The systemd service is of type forking. It expects the process to fork, and the forked process is the main and controlling one. If that forks again and the controlling process exits the service is stopped.

[eworm-de]

Ups, wrong button... 🫣 Did Github move things around?

And I do not have the permission to reopen. 🤨 Anybody please do.

[eworm-de]

Is there any reason we want or need two forks?

[yoe]

As explained in the pull request: it's possible to make this work with the correct systemd unit file.

If you can confirm that the suggested unit file fixes the issue for you, then I'll close this issue by shipping the unit file upstream rather than "just" in Debian.