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This queue is for tickets about the Package-Stash-XS CPAN distribution.

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The Basics
Id: 122525
Status: new
Priority: 0/
Queue: Package-Stash-XS

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Owner: ether [...] cpan.org
Requestors: zefram [...] fysh.org
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Subject: workaround for 5.15.<0-6> hash bug
Date: Tue, 18 Jul 2017 05:05:49 +0100
To: bug-Package-Stash-XS [...] rt.cpan.org
From: Zefram <zefram [...] fysh.org>
There's a hash bug in some old versions of the Perl core that Package::Stash::XS could helpfully work around. The attached patch does so. The bug is concerned with the timing of the freeing of a value deleted from a hash when hv_delete_ent() is passed the G_DISCARD flag. From Perl version 5.15.0 to 5.15.6 inclusive, the value would have its refcount decreased before actually being removed from the hash, so if the value has a destructor and that causes some operation on the hash then it sees the hash in an inconsistent state and mayhem can ensue. The bug is fiendishly difficult to invoke by anything other than a very artificial test case, which explains why it tends to go unnoticed. The situation in which I've actually run into it is that the use of G_DISCARD in Package::Stash::XS->remove_glob causes a failure in namespace::autoclean's test suite. (namespace::autoclean uses namespace::clean, which uses Package::Stash::XS. Neither namespace::clean nor Package::Stash::XS notice the problem in their test suites.) In addition to the patch, I'm attaching a test script which tickles the bug, using only Package::Stash::XS and B::Hooks::EndOfScope. It's derived from namespace::autoclean's failing test, by minimisation and the incorporation of the relevant parts of namespace::autoclean and namespace::clean. This test is almost suitable for adding to Package::Stash::XS's test suite, but unfortunately I haven't been able to eliminate the use of B::Hooks::EndOfScope from it, and that would be a new dependency for Package::Stash::XS. Of course, the affected Perl versions are only development versions, and are nominally obsolete, so the impact of this problem is limited. But it is useful to test modules against old dev versions, especially when a module has version-dependent problems, in which case seeing which dev versions are affected is very revealing. I therefore like to keep as much infrastructure as possible working on those old versions that aren't total lemons. Quite a few modules these days use namespace::autoclean, and testing those on old versions is how I ran into this problem. -zefram
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Download hash-discard.t
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