head: fails to leave remaining bytes of file available for subsequent commands

[jfinkels]

Environment: Ubuntu 20.04, uutils main branch (git commit dddbc17), GNU coreutils v9.5.218-7e5b6.

Steps to reproduce:

printf "a\nb\n" > f
(head -n1 >/dev/null; cat) < f

What happens now: uutils head outputs nothing.

What I expected to happen: GNU head outputs b\n, as expected.

Notes: this is causing a failure in the GNU test file tests/head/head-pos.sh.

We ran into a similar issue with dd here #3008 which was fixed by opening the file from its raw file descriptor here #4189. Maybe a similar solution can be applied to this issue?