[Xapian-discuss] Perl threads and Xapian - incompatibility?

[Olly Betts]

On Fri, Oct 19, 2007 at 02:15:52AM +0200, Ron Kass wrote:
> You can replace the line with a [my $Xapian_DB = 
> Search::Xapian::Database->new("test");] if you already have a test 
> database in place. Same error.

I used this variant.

> Notice that the bug happens on the SECOND call to a thread instance. It 
> can be a call to the detach method [->detach();].. same error.

If you move the creation of the database down a line or two, the SEGV
happens on the third thread.  If you move it down below the creation of
the threads, the code runs to completion.  This seems a reasonable
workaround as you can't share a Xapian object between threads anyway.

Now `perldoc threads' says that the Perl threads module starts a new
interpreter instance for each thread, and that variables aren't shared.
However, adding a printf to the XS wrapper for the Database destructor
shows it is called when each thread terminates!

If I add `print "$Xapian_DB\n"' to each thread routine, the output is:

    1
    Search::Xapian::Database=SCALAR(0x954a70)
    joins #1
    2
    Search::Xapian::Database=SCALAR(0xaffcd0)
    Database dtor called
    joins #2
    Database dtor called
    Segmentation fault (core dumped)

So it looks like the new interpreter for each thread makes a new Perl
Search::Xapian::Database object, but wrapped around the same C++
Xapian::Database object inside.  That's never going to work well!

I've no idea how to fix this (or if it is fixable even), but you have a
workaround.

Cheers,
    Olly