Add lib/diskusage to measure used/free on disks

lib/diskusage/diskusage_unix.go

@@ -0,0 +1,27 @@
+//go:build aix || android || darwin || dragonfly || freebsd || ios || linux
+
+package diskusage
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// New returns the disk status for dir.
+//
+// May return Unsupported error if it doesn't work on this platform.
+func New(dir string) (info Info, err error) {
+	var statfs unix.Statfs_t
+	err = unix.Statfs(dir, &statfs)
+	if err != nil {
+		return info, err
+	}
+	// Note that these can be different sizes on different OSes so
+	// we upcast them all to uint64
+	//nolint:unconvert
+	info.Free = uint64(statfs.Bfree) * uint64(statfs.Bsize)
+	//nolint:unconvert
+	info.Available = uint64(statfs.Bavail) * uint64(statfs.Bsize)
+	//nolint:unconvert
+	info.Total = uint64(statfs.Blocks) * uint64(statfs.Bsize)
+	return info, nil
+}