Implement btrfs usage

Implements btrfs usage using a double walking diff and
counting the result. Walking gives the most accurate
count and includes inode usage.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

fs/du_unix.go

@@ -3,17 +3,19 @@
 package fs
 
 import (
+	"context"
 	"os"
 	"path/filepath"
 	"syscall"
 )
 
+type inode struct {
+	// TODO(stevvooe): Can probably reduce memory usage by not tracking
+	// device, but we can leave this right for now.
+	dev, ino uint64
+}
+
 func diskUsage(roots ...string) (Usage, error) {
-	type inode struct {
-		// TODO(stevvooe): Can probably reduce memory usage by not tracking
-		// device, but we can leave this right for now.
-		dev, ino uint64
-	}
 
 	var (
 		size   int64
@@ -45,3 +47,37 @@ func diskUsage(roots ...string) (Usage, error) {
 		Size:   size,
 	}, nil
 }
+
+func diffUsage(ctx context.Context, a, b string) (Usage, error) {
+	var (
+		size   int64
+		inodes = map[inode]struct{}{} // expensive!
+	)
+
+	if err := Changes(ctx, a, b, func(kind ChangeKind, _ string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if kind == ChangeKindAdd || kind == ChangeKindModify {
+			stat := fi.Sys().(*syscall.Stat_t)
+
+			inoKey := inode{dev: uint64(stat.Dev), ino: uint64(stat.Ino)}
+			if _, ok := inodes[inoKey]; !ok {
+				inodes[inoKey] = struct{}{}
+				size += fi.Size()
+			}
+
+			return nil
+
+		}
+		return nil
+	}); err != nil {
+		return Usage{}, err
+	}
+
+	return Usage{
+		Inodes: int64(len(inodes)),
+		Size:   size,
+	}, nil
+}