Xapian discuss - Oct 2007 - Perl threads and Xapian - incompatibility?

Try the following code:

    #!/usr/bin/perl -W

    use strict;
    use threads;
    use Search::Xapian;
    my $Xapian_DB    
Search::Xapian::WritableDatabase->new("test",Search::Xapian::DB_CREATE_OR_OPEN);

    my $Thr1 = threads->create(sub{print "1\n"});
    my $Thr2 = threads->create(sub{print "2\n"});
    my $Thr3 = threads->create(sub{print "3\n"});
    print "joins #1\n";
    $Thr1->join;
    print "joins #2\n";
    $Thr2->join;
    print "joins #3\n";
    $Thr3->join;  

here is what we get: # perl tst.pl

    joins #1
    1
    2
    3
    joins #2
    Segmentation fault  

Setting: Xapian 1.0.3 (flint backend), perl 5.8.8, threads 1.67

Notice that ALL it takes is an open Xapian DB. It doesn't have to be a 
writable one btw.
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.
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.
 
Anyone saw this bug? Any patches for that one?

Best regards,
Ron

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