Is it possible to abort loading a package if it's too old?

Question

I'm working on a package that has to work on both TeXlive 2009 (Ubuntu, so updating TeXlive is a nonstarter) and later versions, and I'd like to be able to use fontspec with LuaLaTeX if we're on TeXlive 2010 or later, but \RequirePackage{fontspec}[2008/08/10] (Ubuntu's TeXlive 2009 ships with a fontspec.sty from 2008/08/09) still tries to load fontspec.sty and the date argument only serves to make LaTeX issue a warning that it's too old - after barfing out errors because fontspec.sty has \RequireXeTeX in it.

Is there a way to abort loading a package if it's too old, or do I have to do something borderline insane involving (temporarily) redefining \ProvidesPackage. I have the following example cooked up, but I'd love to replace it with something saner:

\listfiles
\documentclass{article}
%\usepackage{trace}
\def\XID#1{\begingroup\ifdefined #1\aftergroup#1\fi\endgroup}
\XID\traceon
\let\OPP\ProvidesPackage
\def\ProvidesPackage#1[#2/#3]{%
  \let\ProvidesPackage\OPP
  \let\OPP\undefined
  \begingroup
    \ifnum #2 < 2009 % if fontspec is older than 2009, abort
      \aftergroup\endinput
      \XID\traceoff
    \fi
  \endgroup
  \XID\traceoff
  \ProvidesPackage#1[#2/#3]}

\usepackage{fontspec}
\ifdefined\setmainfont
  \message{^^J*** \string\setmainfont\space defined! ***^^J}
\else
  \message{^^J*** \string\setmainfont\space not defined! ***^^J}
\fi
\begin{document}
\end{document}

EDIT: After some fairly intense hacking, I came up with this shorter and saner replacement:

\documentclass{minimal}
\makeatletter
\def\IfNewer#1#2#3#4{% we chain into #5 if true and #6 if false using \@{first,second}oftwo.
  \begingroup %that we might forget the craziness that is to follow...
  \def\NeedsTeXFormat##1{\@ifnextchar[\relax\relax}
  % This is rather brittle and should probably use \@ifnextchar[
  \def\ProvidesPackage##1[##2/##3/##4 ##5]{%
    \newif\if@goahead % defaults to false, obviously.
    % Ugly test, but it works
    \ifnum ##2 < #2 % Year too old
    \else\ifnum ##2 = #2\ifnum ##3 < #3 % Year OK, month too old
    \else\ifnum ##2 = #2\ifnum ##3 = #3\ifnum ##4 < #4 % Year & month OK, day too old
    \else 
      \@goaheadtrue
    \fi\fi\fi\fi\fi\fi
    \endinput}
  \input #1.sty % use TeX's \input to avoid #1.sty being in \listfiles
  \if@goahead
    \endgroup % \if@goahead and our redifinitions disappear in a puff of logic.
    \expandafter\@firstoftwo
  \else
    \endgroup
    \expandafter\@secondoftwo
  \fi}
\makeatother
\IfNewer{fontspec}{2010}{01}{01}
  {\message{^^J*** New enough! ***^^J}}
  {\message{^^J*** Too old! ***^^J}}
\begin{document}
\end{document}

Answer 1

After looking through the related LaTeX core code I don't see any other method than redefining \ProvidesPackage. It is the earliest place where the version is known. However, I would reuse the existing macros as much as possible to keep it compatible.

The following code reads the two arguments of, feeds them to the original and then checks the package version as normal, aborting the loading if required. Note that the package version number still gets defined (globally!) and can be used afterwards as well. You should set it to \relax as shown to mark the package as not loaded.

\documentclass{article}

\makeatletter

\let\origProvidesPackage\ProvidesPackage
\def\ProvidesPackage#1{%
    \@testopt{\@ProvidesPackage{#1}}{}%
}
\def\@ProvidesPackage#1[#2]{%
    \let\ProvidesPackage\origProvidesPackage
    \ifx\\#2\\
        \ProvidesPackage{#1}%
    \else
        \ProvidesPackage{#1}[#2]%
    \fi
    \@ifpackagelater{fontspec}{2008/08/10}{%
    }{%
        \endinput
    }%
}


\usepackage{fontspec}
\@ifpackagelater{fontspec}{2008/08/10}{%
    \expandafter\let\csname ver@fontspec.sty\endcsname\relax%   Mark package as not loaded; this must be after `\usepackage` because of internal LaTeX core code.
}{}%

\makeatother


\begin{document}


\end{document}

Answer 2

Sorry, I did not understand the question - Werner explained it me.

Ok, after you loaded package, you can check its version, for example

\@ifpackagelater{amsmath}{1999/12/20}{\typeout{OK}}{\typeout{BAD}}

See this example:

\documentclass{article}
\RequirePackage{amsmath}
\makeatletter
\@ifpackagelater{amsmath}{1999/12/20}{\typeout{OK}}{\typeout{BAD}}
\@ifpackagelater{amsmath}{2013/12/20}{\typeout{OK}}{\typeout{BAD}}
\makeatother

\begin{document}
Text
\end{document}

The problem here is that you need first to load the package, and only then its version becomes known. Which is ok if you want to kill the compilation for older distributions, but might pose problems otherwise.

In the comments I suggested to write whether the package is ok to a special file, and on the next run to check it before loading the package. Probably first one needs to redefine \RequreXeTeX to make the first run end.