[pve-devel] [PATCH common 04/14] tools: add run_fork_detached() for spawning daemons

[Christoph Heiss]

On Tue Mar 18, 2025 at 11:28 AM CET, Wolfgang Bumiller wrote:
> On Mon, Mar 17, 2025 at 03:11:41PM +0100, Christoph Heiss wrote:
>> This essentially just does a fork() + setsid().
>> Needed to e.g. properly spawn background processes.
>>
>> Signed-off-by: Christoph Heiss <c.heiss at proxmox.com>
>> ---
>> Something similar is already used in e.g. pve-storage to spawn fuse
>> mounts. If and when this is applied, I'd migrate these sites to this sub
>> too.
>
> IIRC there are still outstanding issues with it creating zombie
> processes because it only does a single fork instead of properly
> daemonizing with a double-fork.

Good to know, thanks!
I'd be open to fixing/implementing this properly, along with this
series.

The "simple" fix here though would probably be to fork+exec in perl
(like this patch already does) and additionally implement a
`--daemonize` flag on the "other" side, which forks again inside the
exec'd executable?

>
> While an alternative to double-forking would be to see if we can add a
> general child-reaping mechanic to our daemons (either via a proper
> SIGCLD handler, or a signalfd if AnyEvent supports that?), it is
> situation dependent on whether the process should be a child process or
> a "detached" process as the sub below implies

Yeah, in the context of this series the daemon should not be a
child process, at least not of pvedaemon. Its usefulness/lifetime is
basically tied to that of the associated VM, much like e.g. swtpm.

> - the question there being whether the child should be killed if the
> parent dies.