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Perl Cookbook by Nathan Torkington, Tom Christiansen

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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} ...

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