[master] Correct delayed-ack 0 prohibiting active lease updates to lease file

Merges in 47141.

RELNOTES

@@ -87,6 +87,16 @@ ISC DHCP is open source software maintained by Internet Systems
 Consortium.  This product includes cryptographic software written
 by Eric Young (eay@cryptsoft.com).
 
+                Changes since 4.4.0 (New Features)
+- none
+                Changes since 4.4.0 (Bugs)
+
+- A delayed-ack value of 0 (the default), now correctly disables the delayed
+  feature.  A change in 4.4.0 prohibited lease updates marking leases active
+  from be written to the lease file when delayed-ack is 0. This in turn,
+  caused servers to lose active lease assignments upon restart.
+  [ISC-Bugs #47141]
+
                 Changes since 4.4.0b1 (New Features)
 
 - Duplicate address detection when binding to a new IPv6 address was added

includes/dhcpd.h

@@ -821,7 +821,7 @@ struct lease_state {
 #endif
 
 #if !defined (DEFAULT_DELAYED_ACK)
-# define DEFAULT_DELAYED_ACK 0  /* default SO_SNDBUF size / 576 bytes */
+# define DEFAULT_DELAYED_ACK 0  /* default 0 disables delayed acking */
 #endif
 
 #if !defined (DEFAULT_ACK_DELAY_SECS)

server/dhcp.c

@@ -3,7 +3,7 @@
    DHCP Protocol engine. */
 
 /*
- * Copyright (c) 2004-2017 by Internet Systems Consortium, Inc. ("ISC")
+ * Copyright (c) 2004-2018 by Internet Systems Consortium, Inc. ("ISC")
  * Copyright (c) 1995-2003 by Internet Software Consortium
  *
  * This Source Code Form is subject to the terms of the Mozilla Public
@@ -3610,7 +3610,7 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 	} else {
   		lease->cltt = cur_time;
 #if defined(DELAYED_ACK)
-		if (enqueue && max_outstanding_acks != 0)
+		if (enqueue)
 			delayed_ack_enqueue(lease);
 		else 
 #endif