Comment 3 for bug 1986619

This occurred to me during an apt dist-upgrade.
   The file /etc/passwd.upwd-write does not exist.
   Attempting to "touch /etc/passwd.upwd-write" gives a permission denied error.
   I can write other files to the /etc/ directory.
   Nothing in lsof has and an fd open to this file.
   Forced a file system check made no difference.
   /etc/passwd is not immutable.
   Made a backup and new copy of /etc. No difference.