Trivial reproduction: $ true | tac - - tac: cannot create temporary file in `/tmp': Invalid argument
This is present in coreutils 8.14. The cause is the way "template" is reused in copy_to_temp(). The "XXXXXX" suffix is clobbered by the first call to mkstemp(), so the next call returns EINVAL. It looks like the intent is to call mkstemp() at most once and then reuse that file; for example, record_or_unlink_tempfile() will delete at most one file on exit.
