Lexical::SealRequireHiUser3Contributed Perl DocuLexical::SealRequireHints(3pm)
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.
PERL VERION 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.
SEE ALSO
perlpragma
AUTHOR
Andrew Main (Zefram) <zefram@fysh.org>
COPYRIGHT
Copyright (C) 2009, 2010, 2011, 2012 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.
perl v5.20.0 2014-08-15 Lexical::SealRequireHints(3pm)
Czas wygenerowania: 0.00056 sek.
Created with the man page lookup class by Andrew Collington.
Based on a C man page viewer by Vadim Pavlov
Unicode soft-hyphen fix (as used by RedHat) by Dan Edwards
Some optimisations by Eli Argon
Caching idea and code contribution by James Richardson
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