Initial merge of faas-containerd

This patch completes part of the work in #20 by porting the code
for faas-containerd in-tree. When tested, I was able to deploy
and then remove figlet from the store on `x86_64`.

In a follow-up PR, duplication will be removed where possible
and consolidated with updated documentation.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

pkg/provider/handlers/netns.go

@@ -0,0 +1,66 @@
+// +build go1.10
+
+// Copyright Weaveworks
+// github.com/weaveworks/weave/net
+
+package handlers
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+	"runtime"
+
+	"github.com/vishvananda/netlink"
+	"github.com/vishvananda/netns"
+)
+
+var ErrLinkNotFound = errors.New("Link not found")
+
+func WithNetNS(ns netns.NsHandle, work func() error) error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	oldNs, err := netns.Get()
+	if err == nil {
+		defer oldNs.Close()
+
+		err = netns.Set(ns)
+		if err == nil {
+			defer netns.Set(oldNs)
+
+			err = work()
+		}
+	}
+
+	return err
+}
+
+func WithNetNSLink(ns netns.NsHandle, ifName string, work func(link netlink.Link) error) error {
+	return WithNetNS(ns, func() error {
+		link, err := netlink.LinkByName(ifName)
+		if err != nil {
+			if err.Error() == errors.New("Link not found").Error() {
+				return ErrLinkNotFound
+			}
+			return err
+		}
+		return work(link)
+	})
+}
+
+func WithNetNSByPath(path string, work func() error) error {
+	ns, err := netns.GetFromPath(path)
+	if err != nil {
+		return err
+	}
+	return WithNetNS(ns, work)
+}
+
+func NSPathByPid(pid int) string {
+	return NSPathByPidWithRoot("/", pid)
+}
+
+func NSPathByPidWithRoot(root string, pid int) string {
+	return filepath.Join(root, fmt.Sprintf("/proc/%d/ns/net", pid))
+}