Restarting a Server on Demand
Problem
You want your server to shutdown and restart when it receives a HUP
signal, just like inetd or httpd.
Solution
Catch the SIGHUP signal, and re-execute your
program:
$SELF = "/usr/local/libexec/myd"; # which program I am
@ARGS = qw(-l /var/log/myd -d); # program arguments
$SIG{HUP} = \&phoenix;
sub phoenix {
# close all your connections, kill your children, and
# generally prepare to be reincarnated with dignity.
exec($SELF, @ARGS) or die "Couldn't restart: $!\n";
}Discussion
It sure looks simple (“when I get a HUP signal, restart”)
but it’s tricky. You must know your own program name, and that
isn’t easy to find out. You could use $0 or
the FindBin module. For normal programs, this is fine, but critical
system utilities must be more cautious, as there’s no guarantee
that $0 is valid. You can hardcode the filename
and arguments into your program, as we do here. That’s not
necessarily the most convenient solution, however, so you might want
to read the program and arguments from an external file (using the
filesystem’s protections to ensure it hasn’t been
tampered with).
Be sure and install your signal handler after
you define $SELF and @ARGS,
otherwise there’s a race condition when a SIGHUP could run
restart but you don’t know the program to
run. This would cause your program to die.
Some servers don’t want restart on receiving a SIGHUP—they just want to reread their configuration file.
$CONFIG_FILE = "/usr/local/etc/myprog/server_conf.pl"; $SIG{HUP} ...