[systemd-devel] option to wait for pid file to appear

[Igor Bukanov]

On 17 May 2018 at 11:58, Mantas Mikulėnas <grawity at gmail.com> wrote:
> this would be anyways broken because systemd reads the PIDFile once at
> start to dtermine MAINPID and if MAINPID goes away the service fails

This is not true as with PIDFile systemd does pick up the new process
as a new main without restarting the unit. This is an example from my
development VM:

systemctl status nginx
...
   Active: active (running) since Thu 2018-05-17 11:02:57 UTC; 3min 18s ago
     Docs: man:nginx(8)
  Process: 2407 ExecStart=/usr/sbin/nginx ...
 Main PID: 2408 (nginx)
   CGroup: /system.slice/nginx.service
           ├─2408 nginx: master process ...
           ├─2409 nginx: worker process
           └─2410 nginx: worker process

Initiate a graceful restart of nginx:

kill -s USR2 2408
systemctl status nginx
...
   Active: active (running) since Thu 2018-05-17 11:02:57 UTC; 5min ago
     Docs: man:nginx(8)
  Process: 2407 ExecStart=/usr/sbin/nginx ...
 Main PID: 2408 (nginx)
   CGroup: /system.slice/nginx.service
           ├─2408 nginx: master process /usr/sbin/nginx ...
           ├─2409 nginx: worker process
           ├─2410 nginx: worker process
           ├─2418 nginx: master process /usr/sbin/nginx ...
           ├─2419 nginx: worker process
           └─2420 nginx: worker process

Notice that there are 2 master processes, the old one with pid 2408
and the one 2418. Initiate the shutdown of the initial master:

kill -s QUIT 2408
systemctl status nginx
   Active: active (running) since Thu 2018-05-17 11:02:57 UTC; 7min ago
     Docs: man:nginx(8)
  Process: 2407 ExecStart=/usr/sbin/nginx -c
/vol/opt/act/webapp/nginx/nginx.conf -g daemon on; master_process on;
worker_processes auto; pid /run/nginx/nginx.pid; error_log sys
 Main PID: 2418 (nginx)
   CGroup: /system.slice/nginx.service
           ├─2418 nginx: master process /usr/sbin/nginx -c
/vol/opt/act/webapp/nginx/nginx.conf -g daemon on; master_process on;
worker_processes auto; pid /run/nginx/nginx.pid;
           ├─2419 nginx: worker process
           └─2420 nginx: worker process

Here systemd correctly recognized 2418 as the new master without
restarting the unit as seen by active time.