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Subject: reference count leak with take_imp_data
Date: Wed, 13 Apr 2011 17:49:17 +0100
From: Dave Mitchell <>
The following script, which is a much stripped down variant of DBD::Oracle's t/14threads.t: use threads; # or set PERL_DESTRUCT_LEVEL=2 use DBI; my $imp; for (1..10) { my $c = DBI->connect( "dbi:ExampleP:", '', '', { dbi_imp_data => $imp }) or die; $imp = $c->take_imp_data; } will generate an HV (with backref data), and with a reference count 10 higher than accounted for by anything that points to it. Between perl 5.13.4 and 5.13.11, this causes the following error: panic: magic_killbackrefs (flags=ff) during global destruction. That's now been fixed in bleedperl, but I thought I should report the underlying issue to you on the grounds that it feels like a bug in DBI. If you run the above code under gdb, and set a breakpoint on Perl_sv_clean_all, you'll see that it calls Perl_sv_clean_all 10 times. The action of this function, which is is perl's last-ditch claen-all function, is to, for every remaining SV, brute-force reduce its reference count by 1. The fact that Perl_sv_clean_all gets called 10 times is unusual, and indicates that its refcount is 10 higher than it should be. If after the second Perl_sv_clean_all, you set a breakpoint on Perl_sv_clear, then that will catch the bad HV that is finally being freed. I'm assuming this is the same HV corresponding to the orv arg in dbih_setup_handle, but after that I'm lost. -- Any [programming] language that doesn't occasionally surprise the novice will pay for it by continually surprising the expert. -- Larry Wall

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