Lexical::SealRequireHints - prevent leakage of lexical hints

	use Lexical::SealRequireHints;

There is a bug in Perl's handling of the %^H (lexical hints) variable that causes lexical state in one file to leak into another that is required/used from it. This bug will probably be fixed in Perl 5.10.2, and is definitely fixed in Perl 5.11.0, but in any earlier version it is necessary to work around it. On versions of Perl that require a fix, this module globally changes the behaviour of require and use so that they no longer exhibit the bug. This is the most convenient kind of workaround, and is meant to be invoked by modules that make use of lexical state.

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 before putting anything into %^H that would have a problem with leakage; usually it suffices to do this when loading the module that supplies the mechanism to set up the vulnerable lexical state. Invoking this module multiple times, from multiple lexical-related modules, is not a problem: the workaround is only applied once, and applies to everything.

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 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. This is not a problem when lexical state is managed in the usual ways: the leakage that is a problem is almost always through use statements, which are executed immediately after they are compiled. The situations that would escape the sealant of this module are rather convoluted. If such a problem does occur, a workaround is to invoke this module earlier.

This module applies its workaround on any version of Perl prior to 5.11.0. It is likely that a later version in the 5.10 series, probably 5.10.2, will incorporate a fix for the leakage bug, backported from 5.11. In that case, this module's workaround would be used redundantly on such later 5.10 versions. This shouldn't make lexical things behave any worse, but it would mean unnecessarily incurring the slight downsides of having the workaround in place. When such a Perl version is released, this module will be updated to detect it and avoid unnecessarily applying the workaround.

perlpragma

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

Copyright (C) 2009, 2010 Andrew Main (Zefram) <zefram@fysh.org>

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