[#1307] kea-dhcp4 now enforces parked-packet-limit

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::processDhcp4Query() - now drops packet and response
    if parking lot size reaches parked-packet-limit

src/bin/dhcp4/dhcp4_messages.*
    DHCP4_HOOK_LEASES4_PARKING_LOT_FULL - new message

src/bin/dhcp4/dhcp4_messages.cc

@@ -75,6 +75,7 @@ extern const isc::log::MessageID DHCP4_HOOK_DECLINE_SKIP = "DHCP4_HOOK_DECLINE_S
 extern const isc::log::MessageID DHCP4_HOOK_LEASE4_RELEASE_SKIP = "DHCP4_HOOK_LEASE4_RELEASE_SKIP";
 extern const isc::log::MessageID DHCP4_HOOK_LEASES4_COMMITTED_DROP = "DHCP4_HOOK_LEASES4_COMMITTED_DROP";
 extern const isc::log::MessageID DHCP4_HOOK_LEASES4_COMMITTED_PARK = "DHCP4_HOOK_LEASES4_COMMITTED_PARK";
+extern const isc::log::MessageID DHCP4_HOOK_LEASES4_PARKING_LOT_FULL = "DHCP4_HOOK_LEASES4_PARKING_LOT_FULL";
 extern const isc::log::MessageID DHCP4_HOOK_PACKET_RCVD_SKIP = "DHCP4_HOOK_PACKET_RCVD_SKIP";
 extern const isc::log::MessageID DHCP4_HOOK_PACKET_SEND_DROP = "DHCP4_HOOK_PACKET_SEND_DROP";
 extern const isc::log::MessageID DHCP4_HOOK_PACKET_SEND_SKIP = "DHCP4_HOOK_PACKET_SEND_SKIP";
@@ -230,6 +231,7 @@ const char* values[] = {
     "DHCP4_HOOK_LEASE4_RELEASE_SKIP", "%1: lease was not released because a callout set the next step to SKIP",
     "DHCP4_HOOK_LEASES4_COMMITTED_DROP", "%1: packet is dropped, because a callout set the next step to DROP",
     "DHCP4_HOOK_LEASES4_COMMITTED_PARK", "%1: packet is parked, because a callout set the next step to PARK",
+    "DHCP4_HOOK_LEASES4_PARKING_LOT_FULL", "The parked-packet-limit %1, has been reached, dropping query: %2",
     "DHCP4_HOOK_PACKET_RCVD_SKIP", "%1: packet is dropped, because a callout set the next step to SKIP",
     "DHCP4_HOOK_PACKET_SEND_DROP", "%1: prepared DHCPv4 response was not sent because a callout set the next ste to DROP",
     "DHCP4_HOOK_PACKET_SEND_SKIP", "%1: prepared response is not sent, because a callout set the next stp to SKIP",

src/bin/dhcp4/dhcp4_messages.h

@@ -76,6 +76,7 @@ extern const isc::log::MessageID DHCP4_HOOK_DECLINE_SKIP;
 extern const isc::log::MessageID DHCP4_HOOK_LEASE4_RELEASE_SKIP;
 extern const isc::log::MessageID DHCP4_HOOK_LEASES4_COMMITTED_DROP;
 extern const isc::log::MessageID DHCP4_HOOK_LEASES4_COMMITTED_PARK;
+extern const isc::log::MessageID DHCP4_HOOK_LEASES4_PARKING_LOT_FULL;
 extern const isc::log::MessageID DHCP4_HOOK_PACKET_RCVD_SKIP;
 extern const isc::log::MessageID DHCP4_HOOK_PACKET_SEND_DROP;
 extern const isc::log::MessageID DHCP4_HOOK_PACKET_SEND_SKIP;

src/bin/dhcp4/dhcp4_messages.mes

@@ -391,6 +391,14 @@ hook point sets the next step to DROP.
 This debug message is printed when a callout installed on the leases4_committed
 hook point sets the next step to PARK.
 
+% DHCP4_HOOK_LEASES4_PARKING_LOT_FULL The parked-packet-limit %1, has been reached, dropping query: %2
+This debug message occurs when the parking lot used to hold client queries
+while hook library work for them completes has reached or exceeded the
+limit set by the parked-packet-limit global parameter. This can occur when
+kea-dhcp4 is using hook libraries (e.g. HA) that implement the
+"leases4-committed" callout and client queries are arriving faster than
+those callouts can fulfill them.
+
 % DHCP4_HOOK_PACKET_RCVD_SKIP %1: packet is dropped, because a callout set the next step to SKIP
 This debug message is printed when a callout installed on the pkt4_receive
 hook point sets the next step to SKIP. For this particular hook point, the

src/bin/dhcp4/dhcp4_srv.cc

@@ -1356,6 +1356,31 @@ Dhcpv4Srv::processDhcp4Query(Pkt4Ptr& query, Pkt4Ptr& rsp,
         callout_handle->setArgument("deleted_leases4", deleted_leases);
 
         if (allow_packet_park) {
+            // Get the parking limit. Parsing should ensure the value is present.
+            uint32_t parked_packet_limit = 0;
+            data::ConstElementPtr ppl = CfgMgr::instance().
+                getCurrentCfg()->getConfiguredGlobal("parked-packet-limit");
+            if (ppl) {
+                parked_packet_limit = ppl->intValue();
+            }
+
+            if (parked_packet_limit) {
+                const auto& parking_lot = ServerHooks::getServerHooks().
+                    getParkingLotPtr("leases4_committed");
+
+                if (parking_lot && (parking_lot->size() >= parked_packet_limit)) {
+                    // We can't park it so we're going to throw it on the floor.
+                    LOG_DEBUG(packet4_logger, DBGLVL_PKT_HANDLING,
+                              DHCP4_HOOK_LEASES4_PARKING_LOT_FULL)
+                              .arg(parked_packet_limit)
+                              .arg(query->getLabel());
+                    isc::stats::StatsMgr::instance().addValue("pkt4-receive-drop",
+                                                              static_cast<int64_t>(1));
+                    rsp.reset();
+                    return;
+                }
+            }
+
             // We proactively park the packet. We'll unpark it without invoking
             // the callback (i.e. drop) unless the callout status is set to
             // NEXT_STEP_PARK.  Otherwise the callback we bind here will be