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/backend/disk/backend.go

@@ -15,12 +15,15 @@
 package disk
 
 import (
+	"fmt"
 	"io/ioutil"
 	"net"
 	"os"
 	"path/filepath"
 )
 
+const lastIPFile = "last_reserved_ip"
+
 var defaultDataDir = "/var/lib/cni/networks"
 
 type Store struct {
@@ -59,9 +62,25 @@ func (s *Store) Reserve(id string, ip net.IP) (bool, error) {
 		os.Remove(f.Name())
 		return false, err
 	}
+	// store the reserved ip in lastIPFile
+	ipfile := filepath.Join(s.dataDir, lastIPFile)
+	err = ioutil.WriteFile(ipfile, []byte(ip.String()), 0644)
+	if err != nil {
+		return false, err
+	}
 	return true, nil
 }
 
+// LastReservedIP returns the last reserved IP if exists
+func (s *Store) LastReservedIP() (net.IP, error) {
+	ipfile := filepath.Join(s.dataDir, lastIPFile)
+	data, err := ioutil.ReadFile(ipfile)
+	if err != nil {
+		return nil, fmt.Errorf("Failed to retrieve last reserved ip: %v", err)
+	}
+	return net.ParseIP(string(data)), nil
+}
+
 func (s *Store) Release(ip net.IP) error {
 	return os.Remove(filepath.Join(s.dataDir, ip.String()))
 }