Name a filesystem by its UUID, its kernel id or its source, and keep nothing that has no name (hq ADR 0325 fifth review)

NFS and FUSE report a zero f_fsid, so another export or another FUSE mount at the
same point was named alike and kept without a warrant; xfs's f_fsid follows the
device number. A filesystem is now named by its UUID (its device matched in
/dev/disk/by-uuid), else a nonzero f_fsid, else mountinfo's source; one that cannot
be named is never kept or taken. Entries left on a filesystem not back within 30 days
are forgotten, said once, and the naming read is shared per mount.

internal/apply/walk_linux.go

@@ -548,3 +548,25 @@ var MountFsid = func(point string) string {
 	}
 	return fmt.Sprintf("%08x%08x", uint32(st.Fsid.Val[0]), uint32(st.Fsid.Val[1]))
 }
+
+// MountUUID is the UUID of the filesystem at point when it is on a block device: the name in /dev/disk/by-uuid of
+// the device the mount's own device number is. Empty for one on no block device (NFS, FUSE, ZFS, a btrfs
+// subvolume's own device). A variable so a test can stand in for disks it cannot make.
+var MountUUID = func(point string) string {
+	var st unix.Stat_t
+	if err := unix.Stat(point, &st); err != nil {
+		return ""
+	}
+	const dir = "/dev/disk/by-uuid"
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return ""
+	}
+	for _, e := range entries {
+		var dev unix.Stat_t
+		if unix.Stat(dir+"/"+e.Name(), &dev) == nil && dev.Mode&unix.S_IFMT == unix.S_IFBLK && dev.Rdev == st.Dev {
+			return e.Name()
+		}
+	}
+	return ""
+}