unclear documentation for get_slave_lag plugin of pt-online-schema-change

[Nickolay Ihalainen]

get_slave_lag should return a function with connection-to-slave argument.
For example:
pt-osc-plugin.pm
package pt_online_schema_change_plugin;

use strict;

sub new {
my ($class, %args) = @_;
my $self = { %args };
return bless $self, $class;
}

sub get_slave_lag {
  print "PLUGIN get_slave_lag\n";
  my ($self, %args) = @_;
  return sub {
    print "PLUGIN get_slave_lag call\n";
    my ($cxn) = @_;
    my $dbh = $cxn->dbh();
    my $row = $dbh->selectrow_arrayref(q(SELECT 2, @@hostname));
    print "The lag is $row->[0] on $row->[1].\n";
    return $row->[0];
  };
}

1;

Currently the plugin function is listed in
t/pt-online-schema-change/samples/plugins/all_hooks.pm
But example is too dump and not including connection to mysql slave.

[Nickolay Ihalainen (ihanick)]

How to reproduce:
I'm starting master-slave setup from:

https://github.com/ihanick/mysql-replica

with docker-compose up

docker exec -it mysqlreplica_master_1 bash

apt-get update;apt-get install -y percona-toolkit

using your plugin, adopted to my structure: my $row = $dbh->selectrow_arrayref(q(SELECT unix_timestamp(), @@hostname));

/usr/bin/pt-online-schema-change --no-check-alter --no-check-replication-filters --execute --chunk-size-limit=0 --analyze-before-swap --default-engine --plugin=./pt-osc-plugin.pm --recursion-method dsn=t=test.dsns --max-lag 3 --max-load Threads_running=50 --critical-load Threads_running=500 --alter "FORCE" D=test,t=t,u=root,p=master_passw0rd

Only single record in dsns table: insert into dsns values (1,1, 'h=172.19.0.2,u=root,p=slave_passw0rd');

table test.t structure is not important, create table t (c int, primary key(c)) engine=innodb;

root@a0702c18b089:~# /usr/bin/pt-online-schema-change --no-check-alter --no-check-replication-filters --execute --chunk-size-limit=0 --analyze-before-swap --default-engine --plugin=./pt-osc-plugin.pm --recursion-method dsn=t=test.dsns --max-lag 3 --max-load Threads_running=50 --critical-load

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PT-1464