DNSSEC: prevent ipa-ods-exporter from looping after service auto-restart

It might happen that systemd will restart the service even if there is
no incomming connection to service socket. In that case we want to exit
because HSM synchronization is done before socket.accept() and we want
to synchronize HSM and DNS zones at the same time.

https://fedorahosted.org/freeipa/ticket/5273

Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Oleg Fayans <ofayans@redhat.com>

daemons/dnssec/ipa-ods-exporter

@@ -26,6 +26,7 @@ import logging
 import os
 import subprocess
 import socket
+import select
 import sys
 import systemd.daemon
 import systemd.journal
@@ -347,7 +348,12 @@ def receive_systemd_command(log):
         raise KeyError('Exactly one socket is expected.')
 
     sck = socket.fromfd(fds[0], socket.AF_UNIX, socket.SOCK_STREAM)
+    rlist, wlist, xlist = select.select([sck], [], [], 0)
+    if not rlist:
+        log.critical('socket activation did not return socket with a command')
+        sys.exit(0)
 
+    log.debug('accepting new connection')
     conn, addr = sck.accept()
     log.debug('accepted new connection %s', repr(conn))