[systemd-devel] MAINPID changing

[Christian Parpart]

Hi all,

back in January there was a very big discussion on the whole MAINPID changing 
issue related to lighttpd. Well, now I've ran into this myself, and wonder 
whether systemd now supports changing the MAINPID.

So what is the MAINPID in my case anyway?

I start my (HTTP) server without backgrounding, in favor of systemd, and even 
use sd_notify()s to even inform PID 1 about more details.
This process is the MAINPID for sure, while having CGIs forking off from time 
to time, that's all fine.
But now, I've implemented graceful executable/config upgrade as follows:

1.) worker threads suspend
2.) all listeners stop accepting new connections
3.) all listeners get the FD_CLOEXEC flag enabled
4.) fork a new child
5.) exec new binary with new config
6.) watch for child exit (e.g. invalid config file / bad executable)
7.) clear FD_CLOEXEC from all listeners
6.) resume worker threads to continue processing currently active connections

the child process then either exits, which will lead into the still-current 
MAINPID to accept() new connections, again, as if nothing happened.

if the child process fully initialized, including inherited listener sockets, 
it sends a SIGQUIT to the parent (still the MAINPID) to *gracefully* shutdown,
which might be this instant or take a few minutes, just until all current 
active requests have been fully processed.
The still-MAINPID has exited now, and the child, our new MAINPID is now meant 
to be the MAINPID. but how to dell systemd about that?

*** I'd whish some kind of sd_notify("MAINPID=%d", getpid()) here, if ***

I hope I have described my situation in a depth, that everybody knows what 
exactly is going on during the MAINPID change, which is really needed, because 
I can't just reexec into the current process image, because I need the current 
connections to be completed cleanly.

Best regards,
Christian Parpart.