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Lexical::SealRequireHiUser3Contributed Perl DocuLexical::SealRequireHints(3pm)

       Lexical::SealRequireHints - prevent leakage of lexical hints

               use Lexical::SealRequireHints;

       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 "", 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

       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.

       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 "" 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 "" 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 "", 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 "" during Unicode regular
           expression matching now properly restores "%^H", fixing [perl

       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.


       Andrew Main (Zefram) <>

       Copyright (C) 2009, 2010, 2011, 2012 Andrew Main (Zefram)

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

perl v5.20.0                      2014-08-15    Lexical::SealRequireHints(3pm)

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