plugins/ipam: round robin ip allocation for host-local ipam

This changes the ip allocation logic to round robin. Before this, host-local IPAM searched for available IPs from start of subnet. Hence it tends to allocate IPs that had been used recently. This is not ideal since it may cause collisions.

plugins/ipam/host-local/allocator.go

@@ -16,6 +16,7 @@ package main
 
 import (
 	"fmt"
+	"log"
 	"net"
 
 	"github.com/containernetworking/cni/pkg/ip"
@@ -57,7 +58,6 @@ func NewIPAllocator(conf *IPAMConfig, store backend.Store) (*IPAllocator, error)
 		// RangeEnd is inclusive
 		end = ip.NextIP(conf.RangeEnd)
 	}
-
 	return &IPAllocator{start, end, conf, store}, nil
 }
 
@@ -112,7 +112,8 @@ func (a *IPAllocator) Get(id string) (*types.IPConfig, error) {
 		return nil, fmt.Errorf("requested IP address %q is not available in network: %s", requestedIP, a.conf.Name)
 	}
 
-	for cur := a.start; !cur.Equal(a.end); cur = ip.NextIP(cur) {
+	startIP, endIP := a.getSearchRange()
+	for cur := startIP; !cur.Equal(endIP); cur = a.nextIP(cur) {
 		// don't allocate gateway IP
 		if gw != nil && cur.Equal(gw) {
 			continue
@@ -163,3 +164,39 @@ func networkRange(ipnet *net.IPNet) (net.IP, net.IP, error) {
 	}
 	return ipnet.IP, end, nil
 }
+
+// nextIP returns the next ip of curIP within ipallocator's subnet
+func (a *IPAllocator) nextIP(curIP net.IP) net.IP {
+	if curIP.Equal(a.end) {
+		return a.start
+	}
+	return ip.NextIP(curIP)
+}
+
+// getSearchRange returns the start and end ip based on the last reserved ip
+func (a *IPAllocator) getSearchRange() (net.IP, net.IP) {
+	var startIP net.IP
+	var endIP net.IP
+	startFromLastReservedIP := false
+	lastReservedIP, err := a.store.LastReservedIP()
+	if err != nil {
+		log.Printf("Error retriving last reserved ip: %v", err)
+	} else if lastReservedIP != nil {
+		subnet := net.IPNet{
+			IP:   a.conf.Subnet.IP,
+			Mask: a.conf.Subnet.Mask,
+		}
+		err := validateRangeIP(lastReservedIP, &subnet)
+		if err == nil {
+			startFromLastReservedIP = true
+		}
+	}
+	if startFromLastReservedIP {
+		startIP = a.nextIP(lastReservedIP)
+		endIP = lastReservedIP
+	} else {
+		startIP = a.start
+		endIP = a.end
+	}
+	return startIP, endIP
+}