[master] Disable load balancing when load balance max secs is 0

Merges in rt39669.

RELNOTES

@@ -177,6 +177,12 @@ dhcp-users@lists.isc.org.
   the MPL 2.0 license terms.
   [ISC-Bugs #45541]
 
+- Load balancing for failover peers can now be disabled by setting
+  "load balance max secs" to 0. Doing so for both peers means both
+  servers will respond to all DHCPDISCOVERs or DHCPREQUESTs as soon as
+  they are received.
+  [ISC-Bugs #39669]
+
 			Changes since 4.3.0 (bug fixes)
 
 - Tidy up several small tickets.

server/dhcpd.conf.5

@@ -749,6 +749,10 @@ failover peers gets into a state where it is responding to failover
 messages but not responding to some client requests, the other
 failover peer will take over its client load automatically as the
 clients retry.
+.PP
+It is possible to disable load balancing between peers by setting this
+value to 0 on both peers.  Bear in mind that this means both peers will
+respond to all DHCPDISCOVERs or DHCPREQUESTs.
 .RE
 .PP
 The

server/failover.c

@@ -76,11 +76,19 @@ void dhcp_failover_sanity_check() {
 				   state->name);
 			fail_count++;
 		}
+
+		if (state->load_balance_max_secs == 0) {
+			log_info ("WARNING: load balancing will be disabled "
+                                  "for failover peer, %s, "
+				  "because its load balance max secs is 0",
+				  state->name);
+		}
 	}
 
 	if (fail_count) {
 		log_fatal ("Failover configuration sanity check failed");
 	}
+
 }
 
 void dhcp_failover_startup ()
@@ -5968,7 +5976,8 @@ int load_balance_mine (struct packet *packet, dhcp_failover_state_t *state)
 	}
 #endif
 
-	if (state->load_balance_max_secs < ec) {
+	if ((state->load_balance_max_secs == 0) ||
+	    (state->load_balance_max_secs < ec)) {
 		return (1);
 	}