Only allocate a gateway if the n/w driver wants one

A gateway address is always reserved before the network driver is
asked to create the network. But, the driver doesn't always need a
gateway address, so the address reservation can be unnecessary.

This means, for example, an "--internal" IPv4 "/31" network cannot
be used as a point-to-point link, because one of its two addresses
is always reserved for a gateway.

So, before allocating a gateway address, ask the network driver if
it will need one (based on options that only the network driver can
interpret). Implement that as an optional interface for network
drivers.

Signed-off-by: Rob Murray <rob.murray@docker.com>

libnetwork/controller.go

@@ -562,8 +562,14 @@ func (c *Controller) NewNetwork(networkType, name string, id string, options ...
 
 	// Make sure we have a driver available for this network type
 	// before we allocate anything.
-	if _, err := nw.driver(true); err != nil {
+	if d, err := nw.driver(true); err != nil {
 		return nil, err
+	} else if gac, ok := d.(driverapi.GwAllocChecker); ok {
+		// Give the driver a chance to say it doesn't need a gateway IP address.
+		nw.skipGwAllocIPv4, nw.skipGwAllocIPv6, err = gac.GetSkipGwAlloc(nw.generic)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	// From this point on, we need the network specific configuration,

libnetwork/driverapi/driverapi.go

@@ -3,6 +3,8 @@ package driverapi
 import (
 	"context"
 	"net"
+
+	"github.com/docker/docker/libnetwork/options"
 )
 
 // NetworkPluginEndpointType represents the Endpoint Type used by Plugin system
@@ -85,6 +87,13 @@ type Driver interface {
 	IsBuiltIn() bool
 }
 
+// GwAllocChecker is an optional interface for a network driver.
+type GwAllocChecker interface {
+	// GetSkipGwAlloc returns true if the opts describe a network
+	// that does not need a gateway IPv4/IPv6 address, else false.
+	GetSkipGwAlloc(opts options.Generic) (skipIPv4, skipIPv6 bool, err error)
+}
+
 // NetworkInfo provides a go interface for drivers to provide network
 // specific information to libnetwork.
 type NetworkInfo interface {

libnetwork/network.go

@@ -206,6 +206,8 @@ type Network struct {
 	configFrom       string
 	loadBalancerIP   net.IP
 	loadBalancerMode string
+	skipGwAllocIPv4  bool
+	skipGwAllocIPv6  bool
 	platformNetwork  //nolint:nolintlint,unused // only populated on windows
 	mu               sync.Mutex
 }
@@ -466,6 +468,8 @@ func (n *Network) CopyTo(o datastore.KVObject) error {
 	dstN.configFrom = n.configFrom
 	dstN.loadBalancerIP = n.loadBalancerIP
 	dstN.loadBalancerMode = n.loadBalancerMode
+	dstN.skipGwAllocIPv4 = n.skipGwAllocIPv4
+	dstN.skipGwAllocIPv6 = n.skipGwAllocIPv6
 
 	// copy labels
 	if dstN.labels == nil {
@@ -584,6 +588,8 @@ func (n *Network) MarshalJSON() ([]byte, error) {
 	netMap["configFrom"] = n.configFrom
 	netMap["loadBalancerIP"] = n.loadBalancerIP
 	netMap["loadBalancerMode"] = n.loadBalancerMode
+	netMap["skipGwAllocIPv4"] = n.skipGwAllocIPv4
+	netMap["skipGwAllocIPv6"] = n.skipGwAllocIPv6
 	return json.Marshal(netMap)
 }
 
@@ -707,6 +713,12 @@ func (n *Network) UnmarshalJSON(b []byte) (err error) {
 	if v, ok := netMap["loadBalancerMode"]; ok {
 		n.loadBalancerMode = v.(string)
 	}
+	if v, ok := netMap["skipGwAllocIPv4"]; ok {
+		n.skipGwAllocIPv4 = v.(bool)
+	}
+	if v, ok := netMap["skipGwAllocIPv6"]; ok {
+		n.skipGwAllocIPv6 = v.(bool)
+	}
 	return nil
 }
 
@@ -1515,18 +1527,21 @@ func (n *Network) ipamAllocate() (retErr error) {
 
 func (n *Network) ipamAllocateVersion(ipVer int, ipam ipamapi.Ipam) error {
 	var (
-		cfgList  *[]*IpamConf
-		infoList *[]*IpamInfo
-		err      error
+		cfgList     *[]*IpamConf
+		infoList    *[]*IpamInfo
+		skipGwAlloc bool
+		err         error
 	)
 
 	switch ipVer {
 	case 4:
 		cfgList = &n.ipamV4Config
 		infoList = &n.ipamV4Info
+		skipGwAlloc = n.skipGwAllocIPv4
 	case 6:
 		cfgList = &n.ipamV6Config
 		infoList = &n.ipamV6Info
+		skipGwAlloc = n.skipGwAllocIPv6
 	default:
 		return types.InternalErrorf("incorrect ip version passed to ipam allocate: %d", ipVer)
 	}
@@ -1587,8 +1602,9 @@ func (n *Network) ipamAllocateVersion(ipVer int, ipam ipamapi.Ipam) error {
 			}
 		}
 
-		// If there's still no gateway, reserve cfg.Gateway or let the IPAM driver select an address.
-		if d.Gateway == nil {
+		// If there's still no gateway, reserve cfg.Gateway if the user specified it. Else,
+		// if the driver wants a gateway, let the IPAM driver select an address.
+		if d.Gateway == nil && (cfg.Gateway != "" || !skipGwAlloc) {
 			gatewayOpts := map[string]string{
 				ipamapi.RequestAddressType: netlabel.Gateway,
 			}
@@ -1654,6 +1670,9 @@ func (n *Network) ipamReleaseVersion(ipVer int, ipam ipamapi.Ipam) {
 
 	for _, d := range *infoList {
 		if d.Gateway != nil {
+			// FIXME(robmry) - if an IPAM driver returned a gateway in Meta[netlabel.Gateway], and
+			// no user config overrode that address, it wasn't explicitly allocated so it shouldn't
+			// be released here?
 			if err := ipam.ReleaseAddress(d.PoolID, d.Gateway.IP); err != nil {
 				log.G(context.TODO()).Warnf("Failed to release gateway ip address %s on delete of network %s (%s): %v", d.Gateway.IP, n.Name(), n.ID(), err)
 			}