Execute simple command at shipout, depending on value of a boolean

Question

I use only lualatex, but perhaps this is a general LaTeX question. Assume that TeXlive 2023 or later is used, since there is no need for backwards compatibility.

I have looked at atbegshi and some other docs, but they do not do what I need, or are much more complex than necessary. I also see that, not too long ago, someone asked a similar question, but without a useful answer: Use shipout hooks to manipulate global state

Situation: I have two commands, which I will call \foo and \unfoo. The \foo command includes \clearpage at its top, so I do not need to worry about where \foo appears when typeset. The \unfoo command finishes with \clearpage.

If a page has \foo, then the same page must have \unfoo. If the user fails to write \unfoo before the \foo page ships, it is an error. I know how to write a suitable error message, halting compile. I do not need to store, edit, discard, or otherwise manipulate the page contents.

My question: Is there a simple way to use the \shipout hook (or something similar) to do this? Although atbegshi looks like it should be capable, there is a lot going on with that package, which makes me nervous.

MWE:

\documentclass{article} % Compile with lualatex.
\usepackage{fontspec}
\newif\ifusedfoo
\def\foo{\clearpage Hello.\par\usedfootrue}
\def\unfoo{Goodbye.\par\usedfoofalse\clearpage}
% \WhenPageShips{ % This is what I need. I know how to create an error, instead of typeout.
%   \ifusedfoo
%     \typeout{EEEEK. The page with \string\foo\space did not have \string\unfoo.}
%   \fi
% }
\begin{document}
Welcome.\par
\foo
No problem here.\par
\unfoo
\foo % With too many lines on that page, causes problem:
x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\
x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\
x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\
x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\x\par
% Whether or not there is a later \unfoo, does not matter.
Moving right along\par
\clearpage
This is too late.\par
\unfoo
I cannot wait until this far in the document.\par
\end{document}

Also: I do not wish to use an aux file for this. The code does not appear within anything complicated (such as bibliography, table of contents, etc.). Just within running text.

EDIT: Made it clear that the problem should be detected quickly, not later in the document.

EDIT2: The accepted solution works in my specific situation but may not work in the general situation. See comments.

Answer 1

\documentclass{article} % Compile with lualatex.
\usepackage{fontspec}
\newif\ifusedfoo
\def\foo{\clearpage Hello.\par\usedfootrue}
\def\unfoo{Goodbye.\par\usedfoofalse\clearpage}

\AddToHook{shipout/before}[rallg]{%
% \WhenPageShips{ % This is what I need. I know how to create an error, instead of typeout.
  \ifusedfoo
    \typeout{EEEEK. The page with \string\foo did not have \string\unfoo.}
  \fi
}

\begin{document}
Welcome.\par
\foo
No problem here.\par
\unfoo
\foo % With too many lines on that page, causes problem:
x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\
x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\
x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\
x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\\x\par
% Whether or not there is a later \unfoo, does not matter.
\end{document}

This types out the message for the final two pages since both occur without \unfoo. If you want to avoid the second, you'll need to manage the conditional logic appropriately.