table locking with forking

I'm having issues with locking and forking code.  I've consolidated the 
problem into the below example code.

I expect the parent to start, lock the table, and spawn the child.  I 
then expect the child to die without inserting data, because the parent 
had the table locked longer than the child was allowed to wait.  The 
parent should then say "count: 0" and exit.

But instead, the child spawns, waits for the parent to unlock (ignoring 
the alarm?), and inserts the data.

What do I have to do to change the behavior so that the child times out 
and immediately stops trying to insert the data after a specified time 
period ?

my $dbh = DBI->connect($dsn, $dbun, $dbpw, {RaiseError => 1});

$dbh->do('DELETE FROM table1 WHERE field1 > 1');
$dbh->do('LOCK TABLE table1 READ');
my $pid = fork();
if($pid){
    print "PARENT: starting.\n";
    foreach(1..5){
       print "PARENT: sleeping [$_/5]\n";
       sleep 1;
    }
    $dbh->do('UNLOCK TABLES');
    print "PARENT: exiting.\n";

    my $sql = 'SELECT COUNT(*) FROM table1 WHERE field1 = 5';
    my $sth = $dbh->prepare($sql);
    $sth->execute;
    my $row = $sth->fetchrow_arrayref;
    print "PARENT: count $row->[0]\n";

}else{
    print "CHILD: starting.\n";
    $dbh->{InactiveDestroy} = 1; # dont kill the parent $dbh

    eval {
       local $SIG{ALRM} = sub { die "timeout."; };
       alarm(3);
       my $dbh = DBI->connect($dsn, $dbun, $dbpw, {RaiseError => 1});
       print "CHILD: db connection succeeded\n";

       my $sql = 'INSERT INTO table1 (field1) VALUES (5)';
       my $sth = $dbh->prepare($sql);
       $sth->execute;

       print "CHILD: inserted data.\n";
       alarm(0);
    };
    alarm(0);
    if($@){
       print "CHILD: $@\n";
    }
    print "CHILD: exiting.\n";
}

-- 

Jeremy Kister
http://jeremy.kister.net./

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