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From: Wolfgang Bumiller <w.bumiller@proxmox.com>
To: Christoph Heiss <c.heiss@proxmox.com>
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Subject: Re: [pve-devel] [PATCH common 04/14] tools: add run_fork_detached()
for spawning daemons
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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@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.
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 - the question there being
whether the child should be killed if the parent dies.
>
> src/PVE/Tools.pm | 30 ++++++++++++++++++++++++++++++
> 1 file changed, 30 insertions(+)
>
> diff --git a/src/PVE/Tools.pm b/src/PVE/Tools.pm
> index 0325f53..f5bf24a 100644
> --- a/src/PVE/Tools.pm
> +++ b/src/PVE/Tools.pm
> @@ -1117,6 +1117,36 @@ sub run_fork {
> return run_fork_with_timeout(undef, $code, $opts);
> }
>
> +sub run_fork_detached {
> + my ($fn) = @_;
> +
> + pipe(my $rd, my $wr) or die "failed to create pipe: $!\n";
> +
> + my $pid = fork();
> + die "fork failed: $!\n" if !defined($pid);
> +
> + if (!$pid) {
> + undef $rd;
> + POSIX::setsid();
> +
> + eval { $fn->(); };
> + if (my $err = $@) {
> + print {$wr} "ERROR: $err";
> + }
> + POSIX::_exit(1);
> + };
> + undef $wr;
> +
> + my $result = do { local $/ = undef; <$rd> };
> + if ($result =~ /^ERROR: (.*)$/) {
> + die "$1\n";
> + }
> +
> + if (waitpid($pid, POSIX::WNOHANG) == $pid) {
> + die "failed to spawn process, process exited with status $?\n";
> + }
> +}
> +
> # NOTE: NFS syscall can't be interrupted, so alarm does
> # not work to provide timeouts.
> # from 'man nfs': "Only SIGKILL can interrupt a pending NFS operation"
> --
> 2.48.1
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