How to randomize the order of (sub)sections?

Update -- The number of sections per chapter is used now to provide the correct range for the random generator. No random number is used twice within a chapter.

The usage is \section{Section title}[section content], the list is cleared each time a new chapter is used and displayed with \displaysections, linking and labelling works, if the \label is placed within the final optional argument.

The generalization to subsections is similar.

Addition about the uniqueness of the random number:

\NewDocumentCommand{\displaysections}{}{%
  \f@rst=\@ne % Direct manipulation works ;-)
  \l@st=\seq_count:N \chapterwise_seq%
  \int_zero:N \l_tmpa_int
  \seq_clear:N \l_tmpa_seq
  \int_do_while:nn { \l_tmpa_int < \seq_count:N \chapterwise_seq} {
    \rand%
    \seq_if_in:NxF \l_tmpa_seq {\number\value{myrandcounter}} { %
      \int_incr:N \l_tmpa_int 
      \seq_put_right:Nx \l_tmpa_seq {\number\value{myrandcounter}}
    }
  }
  \seq_map_inline:Nn \l_tmpa_seq {%
    \seq_item:Nn \chapterwise_seq {##1}
  }
}

The \int_do_while:nn loops runs until all sections are displayed. The current random number is stored into a list if it is not already in the list -- this way, only unique numbers are added. This is done with

\seq_if_in:NxF \l_tmpa_seq {\number\value{myrandcounter}} { %
  \int_incr:N \l_tmpa_int 
  \seq_put_right:Nx \l_tmpa_seq {\number\value{myrandcounter}}
}

Now the code:

\documentclass{book}

\usepackage[seed=100,counter=myrandcounter,first=1,last=100]{lcg}
\usepackage{xpatch}
\usepackage{xparse}
\usepackage{blindtext} % Just for dummy text
\usepackage{hyperref}
\usepackage{cleveref}


\makeatletter
\ExplSyntaxOn
\seq_new:N \chapterwise_seq  
\let\latex@@section\section
\RenewDocumentCommand{\section}{som+O{}}{%
 \IfBooleanTF{#1}{%
   \latex@@section*{#3}%
 }{%
   \IfValueTF{#2}{%
     \seq_put_right:Nn \chapterwise_seq {\latex@@section[#2]{#3} #4}
   }{%
     \seq_put_right:Nn \chapterwise_seq {\latex@@section{#3} #4}
   }
 }
}

\xpretocmd{\chapter}{\seq_clear:N \chapterwise_seq}{}{}

\newcommand{\getnumofsections}{%
  \seq_count:N \chapterwise_seq%
}


\newcounter{tempcntr}

\NewDocumentCommand{\displaysections}{}{%
  \f@rst=\@ne % Direct manipulation works ;-)
  \l@st=\seq_count:N \chapterwise_seq%
  \int_zero:N \l_tmpa_int
  \seq_clear:N \l_tmpa_seq
  \int_do_while:nn { \l_tmpa_int < \seq_count:N \chapterwise_seq} {
    \rand%
    \seq_if_in:NxF \l_tmpa_seq {\number\value{myrandcounter}} { %
      \int_incr:N \l_tmpa_int 
      \seq_put_right:Nx \l_tmpa_seq {\number\value{myrandcounter}}
    }
  }
  \seq_map_inline:Nn \l_tmpa_seq {%
    \seq_item:Nn \chapterwise_seq {##1}
  }
}

\ExplSyntaxOff

\makeatother


\begin{document}
\chapter{First}

\section{A}[This is section A \label{Sec:A}] 

\section{B}[This is section B -- in \ref{Sec:A}]

\section{C}[This is section C]

\typeout{Foo}
\displaysections

\typeout{Foo}

\chapter{Second}

    \section{A}[This is another section A \label{Sec:AA}] 

\section{B}[This is another section B -- in \ref{Sec:AA}]

\section{C}[This is another section C  \label{Sec:CC} \subsection{Foo} \blindtext[50]]

\section{D}[This is another section D \label{Sec:DD}] 

\section{E}[This is another section E -- in \nameref{Sec:A} we see]

\section{F}[This is another section F  \label{Sec:FF} \subsection{Another Foo} \blindtext[10]]

\displaysections
\end{document}

This solution creates a list of filenames then reads them in random order.

\documentclass{report}
\usepackage{pgfmath}
\usepackage{lipsum}

\newcounter{index}[chapter]

\newcommand{\addfilename}[1]% #1=filename
{\stepcounter{index}%
 \expandafter\xdef\csname section\theindex\endcsname{#1}}

\newcommand{\randomize}%
{\bgroup% use local definitions (\total, \temp)
  \edef\total{\theindex}%
  \setcounter{index}{0}%
  \loop\stepcounter{index}% last loop does nothing
    \edef\temp{\csname section\theindex\endcsname}%
    \pgfmathparse{int((\total-\theindex)*rnd+\theindex)}%
    \expandafter\xdef\csname section\theindex\endcsname{\csname section\pgfmathresult\endcsname}%
    \expandafter\xdef\csname section\pgfmathresult\endcsname{\temp}%
  \ifnum\value{index}<\total\relax \repeat
%
  \setcounter{index}{0}%
  \loop\stepcounter{index}%
    \input{\csname section\theindex\endcsname}
  \ifnum\value{index}<\total\relax \repeat
\egroup}

\begin{document}

\chapter{First}
\addfilename{sectionA}
\addfilename{sectionB}
\addfilename{sectionC}
\addfilename{sectionD}
\randomize

\chapter{Second}
\addfilename{sectionA}
\addfilename{sectionB}
\addfilename{sectionC}
\addfilename{sectionD}
\randomize

\end{document}

where file sectionA is

\section{This is section A}
\lipsum[1]

file sectionB is

\section{This is section B}
\lipsum[2]

and so on.

Note that the order changes each time you run it.