ipam/host-local: Accept ip ranges as a runtime argument

This allows for the runtime to dynamically request IP ranges.

Fixes: #95

plugins/ipam/host-local/README.md

@@ -120,6 +120,10 @@ The following [args conventions](https://github.com/containernetworking/cni/blob
 
 * `ips` (array of strings): A list of custom IPs to attempt to allocate
 
+The following [Capability Args](https://github.com/containernetworking/cni/blob/master/CONVENTIONS.md) are supported:
+
+* `ipRanges`: The exact same as the `ranges` array - a list of address pools
+
 ### Custom IP allocation
 For every requested custom IP, the `host-local` allocator will request that IP
 if it falls within one of the `range` objects. Thus it is possible to specify

plugins/ipam/host-local/backend/allocator/config.go

@@ -23,11 +23,15 @@ import (
 	types020 "github.com/containernetworking/cni/pkg/types/020"
 )
 
-// The top-level network config, just so we can get the IPAM block
+// The top-level network config - IPAM plugins are passed the full configuration
+// of the calling plugin, not just the IPAM section.
 type Net struct {
 	Name          string      `json:"name"`
 	CNIVersion    string      `json:"cniVersion"`
 	IPAM          *IPAMConfig `json:"ipam"`
+	RuntimeConfig struct {    // The capability arg
+		IPRanges []RangeSet `json:"ipRanges,omitempty"`
+	} `json:"runtimeConfig,omitempty"`
 	Args *struct {
 		A *IPAMArgs `json:"cni"`
 	} `json:"args"`
@@ -106,6 +110,11 @@ func LoadIPAMConfig(bytes []byte, envArgs string) (*IPAMConfig, string, error) {
 	}
 	n.IPAM.Range = nil
 
+	// If a range is supplied as a runtime config, prepend it to the Ranges
+	if len(n.RuntimeConfig.IPRanges) > 0 {
+		n.IPAM.Ranges = append(n.RuntimeConfig.IPRanges, n.IPAM.Ranges...)
+	}
+
 	if len(n.IPAM.Ranges) == 0 {
 		return nil, "", fmt.Errorf("no IP ranges specified")
 	}

plugins/ipam/host-local/backend/allocator/config_test.go

@@ -132,12 +132,18 @@ var _ = Describe("IPAM config", func() {
 		}))
 	})
 
-	It("Should parse a mixed config", func() {
+	It("Should parse a mixed config with runtime args", func() {
 		input := `{
 			"cniVersion": "0.3.1",
 			"name": "mynet",
 			"type": "ipvlan",
 			"master": "foo0",
+			"runtimeConfig": {
+				"irrelevant": "a",
+				"ipRanges": [
+					[{ "subnet": "12.1.3.0/24" }]
+				]
+			},
 			"ipam": {
 				"type": "host-local",
 				"subnet": "10.1.2.0/24",
@@ -162,6 +168,17 @@ var _ = Describe("IPAM config", func() {
 			Name: "mynet",
 			Type: "host-local",
 			Ranges: []RangeSet{
+				{ // The RuntimeConfig should always be first
+					{
+						RangeStart: net.IP{12, 1, 3, 1},
+						RangeEnd:   net.IP{12, 1, 3, 254},
+						Gateway:    net.IP{12, 1, 3, 1},
+						Subnet: types.IPNet{
+							IP:   net.IP{12, 1, 3, 0},
+							Mask: net.CIDRMask(24, 32),
+						},
+					},
+				},
 				{
 					{
 						RangeStart: net.IP{10, 1, 2, 9},