fix FileSize to return correct disk usage recursively

extern/sector-storage/fsutil/filesize_unix.go

@@ -2,7 +2,7 @@ package fsutil
 
 import (
 	"os"
-	"syscall"
+	"path/filepath"
 
 	"golang.org/x/xerrors"
 )
@@ -11,19 +11,24 @@ type SizeInfo struct {
 	OnDisk int64
 }
 
-// FileSize returns bytes used by a file on disk
+// FileSize returns bytes used by a file or directory on disk
 func FileSize(path string) (SizeInfo, error) {
-	var stat syscall.Stat_t
-	if err := syscall.Stat(path, &stat); err != nil {
-		if err == syscall.ENOENT {
+	var size int64
+	err := filepath.Walk(path, func(_ string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if !info.IsDir() {
+			size += info.Size()
+		}
+		return err
+	})
+	if err != nil {
+		if os.IsNotExist(err) {
 			return SizeInfo{}, os.ErrNotExist
 		}
-		return SizeInfo{}, xerrors.Errorf("stat: %w", err)
+		return SizeInfo{}, xerrors.Errorf("filepath.Walk err: %w", err)
 	}
 
-	// NOTE: stat.Blocks is in 512B blocks, NOT in stat.Blksize
-	//  See https://www.gnu.org/software/libc/manual/html_node/Attribute-Meanings.html
-	return SizeInfo{
-		int64(stat.Blocks) * 512, // nolint NOTE: int64 cast is needed on osx
-	}, nil
+	return SizeInfo{size}, nil
 }