Pipelines somewhat rely on the kernel delivering SIGPIPE to the writer as soon as the read end is closed. So if you have `foo | head -1`, then as soon as head reads enough and exits, foo gets killed via SIGPIPE. But as most systemd-managed services aren't shell interpreters, systemd marks SIGPIPE as "ignored" when starting the service process, so that if the service is somehow tricked into opening a pipe that a user has mkfifo'd, at least the kernel can't be tricked into killing the service. You can opt out of this using IgnoreSIGPIPE=.
(Though even if there's no signal, I believe the writer should also get an -EPIPE out of every write attempt, but not all tools pay attention to it – some just completely ignore the write() result, like apparently `fold` does in your case...) On Mon, Sep 19, 2022, 20:18 Brian Reichert <reich...@numachi.com> wrote: > I apologize for the vague subject. > > The background: I've inherited some legacy software to manage. > > This is on SLES12 SP5, running: > > systemd-228-157.40.1.x86_64 > > One element is a systemd-managed service, written in Perl, that in > turn, is using bash to generate random numbers (don't ask me why > this tactic was adopted). > > Here's an isolation of that logic: > > pheonix:~ # cat /root/random_str.pl > #!/usr/bin/perl > print "$0 start ".time."\n"; > my $randStr = `cat /dev/urandom|tr -dc "a-zA-Z0-9"|fold -w 64|head -1`; > print "$0 end ".time."\n"; > > You can run this from the command-line, to see how quickly it > nominally operates. > > What I can reproduce in my environment, very reliably, is that when > this is invoked as a service: > > - the 'head' command exits very quickly (to be expected) > - the shell does not exit (maybe missed a SIGCHILD?) > - 'fold' chews a CPU core > - A kernel trace shows that 'fold' is spinning on SIGPIPEs, as it's > STDOUT is no longer connected to another process. > > My service unit: > > pheonix:~ # cat /etc/systemd/system/random_str.service > [Unit] > Description=gernate random number > After=network.target local-fs.target > > [Service] > Type=oneshot > RemainAfterExit=yes > ExecStart=/root/random_str.pl > ExecStop=/usr/bin/true > #TimeoutSec=infinity > TimeoutSec=900 > > [Install] > WantedBy=multi-user.target > > Easy to repro; this hangs forever, instead of exiting quickly. > > pheonix:~ # systemctl daemon-reload > pheonix:~ # systemctl start random_str > > Let me know if there are any other details of my environment that > would be helpful here. > > -- > Brian Reichert <reich...@numachi.com> > BSD admin/developer at large >
