MySQL driver segfaulting under mod_perl - where to look for issue

Question

I have a webapp that segfaults when the database in restarted and it tries to use the old connections. Running it under gdb --args apache -X leads to the following output:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1212868928 (LWP 16098)]
0xb7471c20 in mysql_send_query () from /usr/lib/libmysqlclient.so.15

I've checked that the drivers and database are all up to date (DBD::mysql 4.0008, MySQL 5.0.32-Debian_7etch6-log).

Annoyingly I can't reproduce this with a trivial script:

use DBI;
use Test::More tests => 2;

my $dbh = DBI->connect( "dbi:mysql:test", 'root' );

sub test_db {
    my ($number) = $dbh->selectrow_array("select 1 ");
    return $number;
}

is test_db, 1, "connected to db";

warn "restart db now";
getc;

is test_db, 1, "connected to db";

Which gives the following:

ok 1 - connected to db
restart db now at dbd-mysql-test.pl line 23.

DBD::mysql::db selectrow_array failed: MySQL server has gone away at dbd-mysql-test.pl line 17.
not ok 2 - connected to db
#   Failed test 'connected to db'
#   at dbd-mysql-test.pl line 26.
#          got: undef
#     expected: '1'

This behaves correctly, telling me why the request failed.

What stumps me is that it is segfaulting, which it shouldn't do. As it only appears to happen when the whole app is running (which uses DBIx::Class) it is hard to reduce it to a test case.

Where should I start to look to debug this? Has anyone else seen this?

UPDATE: further prodding showed that it being under mod_perl was a red herring. Having reduced it to a simple test script I've now posted to the DBI mailing list. Thanks for your answers.

mpeters mpeters · Accepted Answer · 2008-10-08T13:30:50

What this probably means is that there's a difference between your mod_perl environment and the one you were testing via your script. Some things to check:

Was your mod_perl compiled with the same version of Perl
Are the @INC's the same for both
Are you using threads in your mod_perl setup? I don't believe DBD::mysql is completely thread-safe.

MySQL driver segfaulting under mod_perl - where to look for issue

4 Answers