[3997] Corrected DHCP4 server crash on exit when DDNS is enabled

Rather that stopping it explicitly, the server was relying D2ClientMgr
to stop itself during its own destruction.  This was falling over during
process wind-down because the IfaceMgr singletone was being destroyed before
the D2ClientMgr instance.
The server destructor now explicitly stops the D2ClientMgr.

src/bin/dhcp4/dhcp4_messages.mes
    added DHCP4_SRV_D2STOP_ERROR log message

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::stopD2() - new method stops the NCR sender, causing it to
    unregister its WathSocket from IfaceMgr

    Dhcpv4Srv::~Dhcpv4Srv() - added call to Dhcpv4Srv::stopD2()

src/bin/dhcp4/tests/dhcp4_process_tests.sh.in
    Enabled DNS updates in the CONFIG string.  This ensures the D2
    client will be started during system tests

src/bin/dhcp4/dhcp4_messages.mes

@@ -610,6 +610,12 @@ core component within the DHCPv4 server (the Dhcpv4 server object)
 has failed.  As a result, the server will exit.  The reason for the
 failure is given within the message.
 
+% DHCP4_SRV_D2STOP_ERROR error stopping IO with DHCP_DDNS during shutdown: %1
+This error message indicates that during shutdown, an erro occurred while
+stopping IO between the DHCPv4 server and the DHCP_DDNS server.  This is
+probably due to a programmatic error is not likely to impact either server
+upon restart.  The reason for the failure is given within the message.
+
 % DHCP4_STARTED Kea DHCPv4 server version %1 started
 This informational message indicates that the DHCPv4 server has
 processed all configuration information and is ready to process

src/bin/dhcp4/dhcp4_srv.cc

@@ -263,6 +263,12 @@ Dhcpv4Srv::Dhcpv4Srv(uint16_t port, const bool use_bcast,
 }
 
 Dhcpv4Srv::~Dhcpv4Srv() {
+    try {
+        stopD2();
+    } catch(const std::exception& ex) {
+        // Highly unlikely, but lets Report it but go on
+        LOG_ERROR(dhcp4_logger, DHCP4_SRV_D2STOP_ERROR).arg(ex.what());
+    }
     IfaceMgr::instance().closeSockets();
 }
 
@@ -2259,6 +2265,15 @@ Dhcpv4Srv::startD2() {
 }
 
 void
+Dhcpv4Srv::stopD2() {
+    D2ClientMgr& d2_mgr = CfgMgr::instance().getD2ClientMgr();
+    if (d2_mgr.ddnsEnabled()) {
+        // Updates are enabled, so lets stop the sender
+        d2_mgr.stopSender();
+    }
+}
+
+void
 Dhcpv4Srv::d2ClientErrorHandler(const
                                 dhcp_ddns::NameChangeSender::Result result,
                                 dhcp_ddns::NameChangeRequestPtr& ncr) {

src/bin/dhcp4/dhcp4_srv.h

@@ -239,6 +239,13 @@ public:
     /// D2ClientErrors. This method does not catch exceptions.
     void startD2();
 
+    /// @brief Stops DHCP_DDNS client IO if DDNS updates are enabled.
+    ///
+    /// If updates are enabled, it instructs the D2ClientMgr singleton to
+    /// leave send mode.  If D2ClientMgr encounters errors it may throw
+    /// D2ClientErrors. This method does not catch exceptions.
+    void stopD2();
+
     /// @brief Implements the error handler for DHCP_DDNS IO errors
     ///
     /// Invoked when a NameChangeRequest send to kea-dhcp-ddns completes with

src/bin/dhcp4/tests/dhcp4_process_tests.sh.in

@@ -37,7 +37,11 @@ CONFIG="{
         {
             \"subnet\": \"10.0.0.0/8\",
             \"pools\": [ { \"pool\": \"10.0.0.10-10.0.0.100\" } ]
-        } ]
+        } ],
+       \"dhcp-ddns\": {
+            \"enable-updates\": true,
+            \"qualifying-suffix\": \"\"
+        }
     },
 
     \"Logging\":