perlipc.pod: avoid race in "exec from signal handler" example

Instead of using SA_NODEFER, which makes the handler
function itself interruptible by another SIGHUP, use
a plain $SIG{HUP} handler and unblock SIGHUP using
sigprocmask from the main program.

Author: stepnem

pod/perlipc.pod

@@ -204,18 +204,15 @@ info to show that it works; it should be replaced with the real code.
   my $script = File::Basename::basename($0);
   my $SELF  = catfile($FindBin::Bin, $script);
 
-  # Use SA_NODEFER to prevent SIGHUP from being blocked in
-  # the new process (due to signal mask preservation across exec).
-  my $sigset = POSIX::SigSet->new();
-  my $action = POSIX::SigAction->new("sigHUP_handler",
-				     $sigset,
-				     &POSIX::SA_NODEFER);
-  POSIX::sigaction(&POSIX::SIGHUP, $action);
-
-  sub sigHUP_handler {
+  $SIG{HUP} = sub {
       print "got SIGHUP\n";
-      exec($SELF, @ARGV)        || die "$0: couldn't restart: $!";
-  }
+      exec($SELF, @ARGV) || die "$0: couldn't restart: $!";
+  };
+
+  # Make sure SIGHUP isn't blocked (as it normally would be, due to the
+  # handler function calling exec rather than returning)
+  POSIX::sigprocmask(&POSIX::SIG_UNBLOCK,
+    POSIX::SigSet->new(&POSIX::SIGHUP));
 
   code();