Add error message for ifname collision (#74)

* Add error message for ifname collision

This changes introduce to validateIfName, to check the validation
for interface name, i.e. same name among two or more interfaces.

vendor/github.com/vishvananda/netns/netns.go

@@ -0,0 +1,80 @@
+// Package netns allows ultra-simple network namespace handling. NsHandles
+// can be retrieved and set. Note that the current namespace is thread
+// local so actions that set and reset namespaces should use LockOSThread
+// to make sure the namespace doesn't change due to a goroutine switch.
+// It is best to close NsHandles when you are done with them. This can be
+// accomplished via a `defer ns.Close()` on the handle. Changing namespaces
+// requires elevated privileges, so in most cases this code needs to be run
+// as root.
+package netns
+
+import (
+	"fmt"
+	"syscall"
+)
+
+// NsHandle is a handle to a network namespace. It can be cast directly
+// to an int and used as a file descriptor.
+type NsHandle int
+
+// Equal determines if two network handles refer to the same network
+// namespace. This is done by comparing the device and inode that the
+// file descriptors point to.
+func (ns NsHandle) Equal(other NsHandle) bool {
+	if ns == other {
+		return true
+	}
+	var s1, s2 syscall.Stat_t
+	if err := syscall.Fstat(int(ns), &s1); err != nil {
+		return false
+	}
+	if err := syscall.Fstat(int(other), &s2); err != nil {
+		return false
+	}
+	return (s1.Dev == s2.Dev) && (s1.Ino == s2.Ino)
+}
+
+// String shows the file descriptor number and its dev and inode.
+func (ns NsHandle) String() string {
+	var s syscall.Stat_t
+	if ns == -1 {
+		return "NS(None)"
+	}
+	if err := syscall.Fstat(int(ns), &s); err != nil {
+		return fmt.Sprintf("NS(%d: unknown)", ns)
+	}
+	return fmt.Sprintf("NS(%d: %d, %d)", ns, s.Dev, s.Ino)
+}
+
+// UniqueId returns a string which uniquely identifies the namespace
+// associated with the network handle.
+func (ns NsHandle) UniqueId() string {
+	var s syscall.Stat_t
+	if ns == -1 {
+		return "NS(none)"
+	}
+	if err := syscall.Fstat(int(ns), &s); err != nil {
+		return "NS(unknown)"
+	}
+	return fmt.Sprintf("NS(%d:%d)", s.Dev, s.Ino)
+}
+
+// IsOpen returns true if Close() has not been called.
+func (ns NsHandle) IsOpen() bool {
+	return ns != -1
+}
+
+// Close closes the NsHandle and resets its file descriptor to -1.
+// It is not safe to use an NsHandle after Close() is called.
+func (ns *NsHandle) Close() error {
+	if err := syscall.Close(int(*ns)); err != nil {
+		return err
+	}
+	(*ns) = -1
+	return nil
+}
+
+// None gets an empty (closed) NsHandle.
+func None() NsHandle {
+	return NsHandle(-1)
+}