this is a forward of debian bug #814362, which lives over there: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814362.
on some files (think: holes) restore -C calls ftruncate(), but there are no suitable file handles in on-the-fly compare mode, so the ftruncate() fails. a patch to fix this is attached.
thanks, fixed in bd61936ee873fe4d857f858b680efb4f3ce75485