Docu limitations of using ptc with PXC. Update Cxn to get is_cluster_node(). Fix SELECT to make tests happy.

bin/pt-table-checksum

@@ -2987,6 +2987,7 @@ sub new {
       dbh_set      => 0,
       OptionParser => $o,
       DSNParser    => $dp,
+      is_cluster_node => undef,
    };
 
    return bless $self, $class;
@@ -3056,6 +3057,19 @@ sub name {
    return $self->{hostname} || $self->{dsn_name} || 'unknown host';
 }
 
+sub is_cluster_node {
+   my ($self) = @_;
+   return $self->{is_cluster_node} if defined $self->{is_cluster_node};
+   
+   my $sql = "SHOW VARIABLES LIKE 'wsrep_on'";
+   PTDEBUG && _d($sql);
+   my $row = $self->{dbh}->selectrow_arrayref($sql);
+   PTDEBUG && _d(defined $row ? @$row : 'undef');
+   $self->{is_cluster_node} = $row && $row->[0] ? 1 : 0;
+  
+   return $self->{is_cluster_node};
+}
+
 sub DESTROY {
    my ($self) = @_;
    if ( $self->{dbh}
@@ -8533,7 +8547,7 @@ sub main {
    my $checksum_dml = "REPLACE INTO $repl_table "
                     . "(db, tbl, chunk, chunk_index,"
                     . " lower_boundary, upper_boundary, this_cnt, this_crc) "
-                    . "SELECT "
+                    . "SELECT"
                     . ($master_cxn->is_cluster_node() ? '/*!99997*/' : '')
                     . " ?, ?, ?, ?, ?, ?,";
    my $past_cols    = " COUNT(*), '0'";
@@ -11028,6 +11042,14 @@ replicas do not honor this change.  Therefore, checksums will not replicate
 past any replicas using row-based replication that are masters for
 further replicas.  (L<Bug 899415|https://bugs.launchpad.net/percona-toolkit/+bug/899415>)
 
+=item Percona XtraDB Cluster
+
+pt-table-checksum works with Percona XtraDB Cluster 5.5.27-23.6 and newer.
+The C<dsn> method for L<"--recursion-method"> must be used to specify cluster
+nodes and regular replicas because nodes are not regular replicas so they
+cannot be detected automatically.  The lag check (see L<"REPLICA CHECKS">)
+is not performed for cluster nodes.
+
 =back
 
 =head1 BUGS