Size limit of an output file

Question

I am designing a set of macros to have exercises in a book and write the answers at the end while typesetting the answers just after the questions in the exercise file. (See MWE for an example). (The idea is to be able to renew the command \answer and print the answers right after the questions, but that another story).

I adapted the code of the package endnotes and use basically two commands for the answer \writesolutions and \appendsolutions that differ in the way they process fragile commands. The answers are written in a output file that is included using the command \printsolutions. Somehow, the output file is cut at the 16384th character. In the following MWE, the chapter of Problems expands when we add more lines of \input ex1 but after 27 such lines, the solution chapter does not expand anymore and we get some errors.

EDIT

The content of the file mwe.exo is not clear to me : after the run, it only contains, as noted by David Carlisle, \begin{questions} and \end{questions}, whereas it should contain the code of all answers. I don't understand this, but I suspect the issue is related to the truncation of the file. Here's how I determined the length of the answers's code that is truncated : I have defined a command

\def\abcdefghijklmnopqrstuvwxyz{\relax}

and if I insert it at the right position, I get errors as Undefined control sequence for \abcdefghijklmn. I know the length of one answer using also error messages. Note that with other content I get the same error with the token \addtoco or any other know command name cut in the middle, but then I can't count the number of characters.

This problem is far beyond my knowledge. Thanks to all who whould help !

MWE

\documentclass{book}
\usepackage{filecontents}
\usepackage{xkeyval}
\usepackage{etoolbox}
\usepackage{enumitem}

% File management adapted from endnotes.sty ©2002 John Lavagnino
\makeatletter
\newwrite\@exsol
\newif\if@exsolopen
\global\@exsolopenfalse

\def\@openexsol{\immediate\openout\@exsol=\jobname.exo\relax
      \global\@exsolopentrue}

% writesolutions does not expand fragile commands
% \strip@prefix trick given by egreg on TeX.SE (2016/11/09)
% http://tex.stackexchange.com/questions/338302/what-is-the-meaning-of-meaning
\long\def\writesolutions#1{%
     \if@exsolopen \else \@openexsol \fi
     \begingroup
        \def\solution{#1}%
        \newlinechar='40
        \immediate\write\@exsol{\expandafter\strip@prefix\meaning\solution}%
     \endgroup}

% \appendsolutions expands fragile commands first 
\long\def\appendsolutions#1{%
     \if@exsolopen \else \@openexsol \fi
     \begingroup
        \newlinechar='40
        \let\protect\string
        \immediate\write\@exsol{#1}%
     \endgroup}

% printsolutions
\long\def\printsolutions{
  \immediate\closeout\@exsol \global\@exsolopenfalse
  \chapter{Solutions}
  \input{\jobname.exo}}
% Settings for probleme environment in a book document
\newcounter{probleme}[chapter]
\def\theprobleme{\arabic{probleme}}
\def\problemename{Problem}

\newlist{questions}{enumerate}{10}
\setlist[questions]{label=\textit{\alph*}),ref=\textit{\alph*}}

\define@cmdkey[PRE]{problem}{title}{}
\presetkeys[PRE]{problem}{title=Problem}{}


\newenvironment{probleme}[1][]{
  \setkeys[PRE]{problem}{#1}
  \refstepcounter{probleme}
  \appendsolutions{%
     {\problemename~\theprobleme\, (page \thepage)}}
  \par\noindent\textbf{\theprobleme. \cmdPRE@problem@title}
}
{}

\newcommand\answer[1]{\writesolutions{\par #1}}
\newcommand\noanswer{\relax}

\AtBeginEnvironment{questions}{%
  \writesolutions{\begin{questions}}%
  \renewcommand\answer[1]{\writesolutions{\item{#1}}}%
  \renewcommand\noanswer{\writesolutions{\addtocounter{questionsi}{1}}}
}
\AtEndEnvironment{questions}{%
  \writesolutions{\end{questions}}%
  \renewcommand\answer[1]{\par #1}
  \renewcommand\noanswer{\relax}
}
\makeatother

\begin{document}
\begin{filecontents*}{ex1.tex}
\begin{probleme}[title=Problem]
\begin{questions}
\item 1+1 \answer{2}
\item 2+2 \noanswer
\item 3+3 \answer{6}
\item 4+4
\end{questions}
\end{probleme}
\end{filecontents*}

\mainmatter
\chapter{Problems}
\input ex1
\input ex1
\input ex1 
% Add as many similar lines as needed. 
% On my computer the error appears when ex1.tex 
% is included 27 times.

\backmatter
\printsolutions
\end{document}

Answer 1

I think that the problem is in \printsolutions. It inputs the file \jobname.exo which contains blocs like

{Problem\nobreakspace{}1\,(page 1)}
\begin{questions}
\item{2}
\addtocounter{questionsi}{1}
\item{6}
\end{questions}

Now, \begin{questions} writes to \jobname.exo which means that you write to the file you are reading. What seems to be a size limitation is probably just a matter of buffering of the read operation. I guess that the command \immediate\openout\@exsol=\jobname.exo destroys the content of \jobname.exo but a block has already been read and put in memory. When the read buffer is empty, a new block is read from the file, but at this time the file has changed.

By the way, the write operation is not completely correct either. The blocks written to the file actually look like

{Problem\nobreakspace
{}2\,
(page
1)}
\begin
{questions}
\item
{2}
\addtocounter
{questionsi}{1}
\item
{6}
\end
{questions}

It would at least require a % after \,

Answer 2

You're using the same environment for questions and solutions; unless your \printsolutions macro redefines questions, this environment will again write to the .exo file.

Just use a different name.

\documentclass{book}
\usepackage{filecontents}
\usepackage{xkeyval}
\usepackage{etoolbox}
\usepackage{enumitem}

% File management adapted from endnotes.sty ©2002 John Lavagnino
\makeatletter
\newwrite\@exsol
\newif\if@exsolopen
\global\@exsolopenfalse

\def\@openexsol{\immediate\openout\@exsol=\jobname.exo\relax
      \global\@exsolopentrue}

% writesolutions does not expand fragile commands
% \strip@prefix trick given by egreg on TeX.SE (2016/11/09)
% http://tex.stackexchange.com/questions/338302/what-is-the-meaning-of-meaning
\long\def\writesolutions#1{%
     \if@exsolopen \else \@openexsol \fi
     \begingroup
        \def\solution{#1}%
        \newlinechar='40
        \immediate\write\@exsol{\expandafter\strip@prefix\meaning\solution}%
     \endgroup}

% \appendsolutions expands fragile commands first 
\long\def\appendsolutions#1{%
     \if@exsolopen \else \@openexsol \fi
     \begingroup
        \newlinechar='40
        \let\protect\string
        \immediate\write\@exsol{#1}%
     \endgroup}

% printsolutions
\long\def\printsolutions{
  \immediate\closeout\@exsol \global\@exsolopenfalse
  \chapter{Solutions}
  \input{\jobname.exo}}
% Settings for probleme environment in a book document
\newcounter{probleme}[chapter]
\def\theprobleme{\arabic{probleme}}
\def\problemename{Problem}

\newlist{questions}{enumerate}{10}
\setlist[questions]{label=\textit{\alph*}),ref=\textit{\alph*}}

\define@cmdkey[PRE]{problem}{title}{}
\presetkeys[PRE]{problem}{title=Problem}{}


\newenvironment{probleme}[1][]{
  \setkeys[PRE]{problem}{#1}
  \refstepcounter{probleme}
  \appendsolutions{%
     {\problemename~\theprobleme\, (page \thepage)}}
  \par\noindent\textbf{\theprobleme. \cmdPRE@problem@title}
}
{}

\newcommand\answer[1]{\writesolutions{\par #1}}
\newcommand\noanswer{\relax}

\AtBeginEnvironment{questions}{%
  \writesolutions{\begin{questions}}%
  \renewcommand\answer[1]{\writesolutions{\item{#1}}}%
  \renewcommand\noanswer{\writesolutions{\addtocounter{questionsi}{1}}}
}
\AtEndEnvironment{questions}{%
  \writesolutions{\end{questions}}%
  \renewcommand\answer[1]{\par #1}
  \renewcommand\noanswer{\relax}
}
\makeatother

\begin{document}
\begin{filecontents*}{ex1.tex}
\begin{probleme}[title=Problem]
\begin{questions}
\item 1+1 \answer{2}
\item 2+2 \noanswer
\item 3+3 \answer{6}
\item 4+4
\end{questions}
\end{probleme}
\end{filecontents*}

\mainmatter
\chapter{Problems}
\input ex1
\input ex1
\input ex1 
% Add as many similar lines as needed. 
% On my computer the error appears when ex1.tex 
% is included 27 times.

\backmatter
\printsolutions
\end{document}

This is the contents of the .exo file:

{Problem\nobreakspace
{}1\,
(page
1)}
\begin
{solutions}
\item
{2}
\addtocounter
{questionsi}{1}
\item
{6}
\end
{solutions}
{Problem\nobreakspace
{}2\,
(page
1)}
\begin
{solutions}
\item
{2}
\addtocounter
{questionsi}{1}
\item
{6}
\end
{solutions}
{Problem\nobreakspace
{}3\,
(page
1)}
\begin
{solutions}
\item
{2}
\addtocounter
{questionsi}{1}
\item
{6}
\end
{solutions}