NAME

       Lexical::SealRequireHints - prevent leakage of lexical hints

SYNOPSIS

               use Lexical::SealRequireHints;

DESCRIPTION

       This module works around two historical bugs in Perl's handling of the "%^H" (lexical
       hints) variable.  One bug causes lexical state in one file to leak into another that is
       "require"d/"use"d from it.  This bug, [perl #68590], was present from Perl 5.6 up to Perl
       5.10, fixed in Perl 5.11.0.  The second bug causes lexical state (normally a blank "%^H"
       once the first bug is fixed) to leak outwards from "utf8.pm", if it is automatically
       loaded during Unicode regular expression matching, into whatever source is compiling at
       the time of the regexp match.  This bug, [perl #73174], was present from Perl 5.8.7 up to
       Perl 5.11.5, fixed in Perl 5.12.0.

       Both of these bugs seriously damage the usability of any module relying on "%^H" for
       lexical scoping, on the affected Perl versions.  It is in practice essential to work
       around these bugs when using such modules.  On versions of Perl that require such a
       workaround, this module globally changes the behaviour of "require", including "use" and
       the implicit "require" performed in Unicode regular expression matching, so that it no
       longer exhibits these bugs.

       The workaround supplied by this module takes effect the first time its "import" method is
       called.  Typically this will be done by means of a "use" statement.  This should be done
       as early as possible, because it only affects "require"/"use" statements that are compiled
       after the workaround goes into effect.  For "use" statements, and "require" statements
       that are executed immediately and only once, it suffices to invoke the workaround when
       loading the first module that will set up vulnerable lexical state.  Delayed-action
       "require" statements, however, are more troublesome, and can require the workaround to be
       loaded much earlier.  Ultimately, an affected Perl program may need to load the workaround
       as very nearly its first action.  Invoking this module multiple times, from multiple
       modules, is not a problem: the workaround is only applied once, and applies to everything
       subsequently compiled.

       This module is implemented in XS, with a pure Perl backup version for systems that can't
       handle XS modules.  The XS version has a better chance of playing nicely with other
       modules that modify "require" handling.  The pure Perl version can't work at all on some
       Perl versions; users of those versions must use the XS.  On all Perl versions suffering
       the underlying hint leakage bug, pure Perl hooking of "require" breaks the use of
       "require" without an explicit parameter (implicitly using $_).

PERL VERSION DIFFERENCES

       The history of the "%^H" bugs is complex.  Here is a chronological statement of the
       relevant changes.

       Perl 5.6.0
           "%^H" introduced.  It exists only as a hash at compile time.  It is not localised by
           "require", so lexical hints leak into every module loaded, which is bug [perl #68590].

           The "CORE::GLOBAL" mechanism doesn't work cleanly for "require", because overriding
           "require" loses the necessary special parsing of bareword arguments to it.  As a
           result, pure Perl code can't properly globally affect the behaviour of "require".
           Pure Perl code can localise "%^H" itself for any particular "require" invocation, but
           a global fix is only possible through XS.

       Perl 5.7.2
           The "CORE::GLOBAL" mechanism now works cleanly for "require", so pure Perl code can
           globally affect the behaviour of "require" to achieve a global fix for the bug.

       Perl 5.8.7
           When "utf8.pm" is automatically loaded during Unicode regular expression matching,
           "%^H" now leaks outward from it into whatever source is compiling at the time of the
           regexp match, which is bug [perl #73174].  It often goes unnoticed, because [perl
           #68590] makes "%^H" leak into "utf8.pm" which then doesn't modify it, so what leaks
           out tends to be identical to what leaked in.  If [perl #68590] is worked around,
           however, "%^H" tends to be (correctly) blank inside "utf8.pm", and this bug therefore
           blanks it for the outer module.

       Perl 5.9.4
           "%^H" now exists in two forms.  In addition to the relatively ordinary hash that is
           modified during compilation, the value that it had at each point in compilation is
           recorded in the compiled op tree, for later examination at runtime.  It is in a
           special representation-sharing format, and writes to "%^H" are meant to be performed
           on both forms.  "require" does not localise the runtime form of "%^H" (and still
           doesn't localise the compile-time form).

           A couple of special "%^H" entries are erroneously written only to the runtime form.

           Pure Perl code, although it can localise the compile-time "%^H" by normal means, can't
           adequately localise the runtime "%^H", except by using a string eval stack frame.
           This makes a satisfactory global fix for the leakage bug impossible in pure Perl.

       Perl 5.10.1
           "require" now properly localises the runtime form of "%^H", but still not the compile-
           time form.

           A global fix is once again possible in pure Perl, because the fix only needs to
           localise the compile-time form.

       Perl 5.11.0
           "require" now properly localises both forms of "%^H", fixing [perl #68590].  This
           makes [perl #73174] apparent without any workaround for [perl #68590].

           The special "%^H" entries are now correctly written to both forms of the hash.

       Perl 5.12.0
           The automatic loading of "utf8.pm" during Unicode regular expression matching now
           properly restores "%^H", fixing [perl #73174].

BUGS

       The operation of this module depends on influencing the compilation of "require".  As a
       result, it cannot prevent lexical state leakage through a "require" statement that was
       compiled before this module was invoked.  Where problems occur, this module must be
       invoked earlier.

       On all Perl versions that need a fix for the lexical hint leakage bug, the pure Perl
       implementation of this module unavoidably breaks the use of "require" without an explicit
       parameter (implicitly using $_).  This is due to another bug in the Perl core, fixed in
       Perl 5.15.5, and is inherent to the mechanism by which pure Perl code can hook "require".
       The use of implicit $_ with "require" is rare, so although this state of affairs is faulty
       it will actually work for most programs.  Perl versions 5.12.0 and greater, despite having
       the "require" hooking bug, don't actually exhibit a problem with the pure Perl version of
       this module, because with the lexical hint leakage bug fixed there is no need for this
       module to hook "require".

SEE ALSO

       perlpragma

AUTHOR

       Andrew Main (Zefram) <zefram@fysh.org>

COPYRIGHT

       Copyright (C) 2009, 2010, 2011, 2012, 2015, 2016, 2017 Andrew Main (Zefram)
       <zefram@fysh.org>

LICENSE

       This module is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.