Vendor continuity

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

vendor/github.com/stevvooe/continuity/hardlinks_unix.go

@@ -0,0 +1,34 @@
+package continuity
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// hardlinkKey provides a tuple-key for managing hardlinks. This is system-
+// specific.
+type hardlinkKey struct {
+	dev   uint64
+	inode uint64
+}
+
+// newHardlinkKey returns a hardlink key for the provided file info. If the
+// resource does not represent a possible hardlink, errNotAHardLink will be
+// returned.
+func newHardlinkKey(fi os.FileInfo) (hardlinkKey, error) {
+	sys, ok := fi.Sys().(*syscall.Stat_t)
+	if !ok {
+		return hardlinkKey{}, fmt.Errorf("cannot resolve (*syscall.Stat_t) from os.FileInfo")
+	}
+
+	if sys.Nlink < 2 {
+		// NOTE(stevvooe): This is not always true for all filesystems. We
+		// should somehow detect this and provided a slow "polyfill" that
+		// leverages os.SameFile if we detect a filesystem where link counts
+		// is not really supported.
+		return hardlinkKey{}, errNotAHardLink
+	}
+
+	return hardlinkKey{dev: uint64(sys.Dev), inode: sys.Ino}, nil
+}