flannel: allow input ipam parameters as basis for delegate

This change allows providing an 'ipam' section as part of the
input network configuration for flannel. It is then used as
basis to construct the ipam parameters provided to the delegate.

All parameters from the input ipam are preserved except:
* 'subnet' which is set to the flannel host subnet
* 'routes' which is complemented by a route to the flannel
  network.

One use case of this feature is to allow adding back the routes
to the cluster services and/or to the hosts (HostPort) when
using isDefaultGateway=false. In that case, the bridge plugin
does not install a default route and, as a result, only pod-to-pod
connectivity would be available.

Example:
    {
      "name": "cbr0",
      "cniVersion": "0.3.1",
      "type": "flannel",
      "ipam": {
        "routes": [
          {
            "dst": "192.168.242.0/24"
          },
          {
            "dst": "10.96.0.0/12"
          }
        ],
        "unknown-param": "value"
      },
      "delegate": {
        "hairpinMode": true,
        "isDefaultGateway": false
      }
      ...
    }

This results in the following 'ipam' being provided to the delegate:
    {
      "routes" : [
        {
          "dst": "192.168.242.0/24"
        },
        {
          "dst": "10.96.0.0/12"
        },
        {
          "dst" : "10.1.0.0/16"
        }
      ],
      "subnet" : "10.1.17.0/24",
      "type" : "host-local"
      "unknown-param": "value"
    }

where "10.1.0.0/16" is the flannel network and "10.1.17.0/24" is
the host flannel subnet.

Note that this also allows setting a different ipam 'type' than
"host-local".

Signed-off-by: David Verbeiren <david.verbeiren@tessares.net>

plugins/meta/flannel/flannel_linux.go

@@ -22,12 +22,36 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"os"
+
 	"github.com/containernetworking/cni/pkg/invoke"
 	"github.com/containernetworking/cni/pkg/skel"
 	"github.com/containernetworking/cni/pkg/types"
-	"os"
 )
 
+// Return IPAM section for Delegate using input IPAM if present and replacing
+// or complementing as needed.
+func getDelegateIPAM(n *NetConf, fenv *subnetEnv) (map[string]interface{}, error) {
+	ipam := n.IPAM
+	if ipam == nil {
+		ipam = map[string]interface{}{}
+	}
+
+	if !hasKey(ipam, "type") {
+		ipam["type"] = "host-local"
+	}
+	ipam["subnet"] = fenv.sn.String()
+
+	rtes, err := getIPAMRoutes(n)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read IPAM routes: %w", err)
+	}
+	rtes = append(rtes, types.Route{Dst: *fenv.nw})
+	ipam["routes"] = rtes
+
+	return ipam, nil
+}
+
 func doCmdAdd(args *skel.CmdArgs, n *NetConf, fenv *subnetEnv) error {
 	n.Delegate["name"] = n.Name
 
@@ -55,15 +79,11 @@ func doCmdAdd(args *skel.CmdArgs, n *NetConf, fenv *subnetEnv) error {
 		n.Delegate["cniVersion"] = n.CNIVersion
 	}
 
-	n.Delegate["ipam"] = map[string]interface{}{
-		"type":   "host-local",
-		"subnet": fenv.sn.String(),
-		"routes": []types.Route{
-			{
-				Dst: *fenv.nw,
-			},
-		},
+	ipam, err := getDelegateIPAM(n, fenv)
+	if err != nil {
+		return fmt.Errorf("failed to assemble Delegate IPAM: %w", err)
 	}
+	n.Delegate["ipam"] = ipam
 
 	return delegateAdd(args.ContainerID, n.DataDir, n.Delegate)
 }