PT-1757 Implemented fallback in NibbleIterator

Since now NibbleIterator can fallback to nibble (from one chunk) pt-osc
can nibble tables even if due to unacurate stats, it choses one nible as
the initial nibbling method.

t/pt-online-schema-change/pt_1757.t

@@ -0,0 +1,96 @@
+#!/usr/bin/env perl
+
+BEGIN {
+    die "The PERCONA_TOOLKIT_BRANCH environment variable is not set.\n"
+    unless $ENV{PERCONA_TOOLKIT_BRANCH} && -d $ENV{PERCONA_TOOLKIT_BRANCH};
+    unshift @INC, "$ENV{PERCONA_TOOLKIT_BRANCH}/lib";
+};
+
+use strict;
+use warnings FATAL => 'all';
+use threads;
+use threads::shared;
+use Thread::Semaphore;
+
+use English qw(-no_match_vars);
+use Test::More;
+
+use Data::Dumper;
+use PerconaTest;
+use Sandbox;
+use SqlModes;
+use File::Temp qw/ tempdir /;
+
+if ($sandbox_version lt '5.7') {
+    plan skip_all => 'This test needs MySQL 5.7+';
+} else {
+    plan tests => 3;
+}    
+
+require "$trunk/bin/pt-online-schema-change";
+
+my $dp = new DSNParser(opts=>$dsn_opts);
+my $sb = new Sandbox(basedir => '/tmp', DSNParser => $dp);
+
+my $dbh = $sb->get_dbh_for('master');
+my $dsn = $sb->dsn_for("master");
+
+my $slave_dbh = $sb->get_dbh_for('slave1');
+
+# The sandbox servers run with lock_wait_timeout=3 and it's not dynamic
+# so we need to specify --set-vars innodb_lock_wait_timeout=3 else the
+# tool will die.
+my @args = (qw(--set-vars innodb_lock_wait_timeout=3));
+my $output;
+my $exit_status;
+my $master_port = $sb->port_for('master');
+my $num_rows = 5000;
+
+$sb->load_file('master', "t/pt-online-schema-change/samples/pt-1757.sql");
+diag(`util/mysql_random_data_load --host=127.0.0.1 --port=$master_port --user=msandbox --password=msandbox test t1 $num_rows`);
+
+
+# Let's alter the stats to force this scenario:
+# 1) On master, we are going to put 100 as the number of rows in the table. This will make osc to try to run in one chunk
+# 2) On the slave, we are going to put the real number of rows. This will cause a fallback to nibble and pt-osc should call
+# NibbleIterator->switch_to_nibble()
+
+$dbh->do('SET @@SESSION.sql_log_bin=0');
+$dbh->do('update mysql.innodb_table_stats set n_rows=100 where table_name="t1"');
+$dbh->do('update mysql.innodb_index_stats set stat_value=100 where stat_description in("id") and table_name="t1"');
+$dbh->do('SET @@SESSION.sql_log_bin=1');
+
+$slave_dbh->do('SET @@SESSION.sql_log_bin=0');
+$slave_dbh->do("update mysql.innodb_table_stats set n_rows=$num_rows where table_name='t1'");
+$slave_dbh->do("update mysql.innodb_index_stats set stat_value=$num_rows where stat_description in('id') and table_name='t1'");
+$slave_dbh->do('SET @@SESSION.sql_log_bin=1');
+
+($output, $exit_status) = full_output(
+    sub { pt_online_schema_change::main(@args, "$dsn,D=test,t=t1",
+            '--execute', '--alter', "ADD COLUMN new_col INT NOT NULL DEFAULT 1",
+            '--chunk-size', '25',
+        ),
+    },
+    stderr => 1,
+);
+
+is(
+    $exit_status,
+    0,
+    "Altered OK status",
+);
+
+# The WHERE clause here is important as a double check that the table was altered and new_col exists
+my $rows = $dbh->selectrow_arrayref("SELECT COUNT(*) FROM test.t1 WHERE new_col = 1");
+is(
+   $rows->[0],
+   $num_rows,
+   "Correct rows count"
+) or diag(Dumper($rows));
+
+# #############################################################################
+# Done.
+# #############################################################################
+$sb->wipe_clean($dbh);
+ok($sb->ok(), "Sandbox servers") or BAIL_OUT(__FILE__ . " broke the sandbox");
+done_testing;

t/pt-online-schema-change/samples/pt-1757.sql

@@ -0,0 +1,15 @@
+DROP DATABASE IF EXISTS `test`;
+
+CREATE DATABASE `test`;
+
+CREATE TABLE `test`.`t1` (
+  `id` int(11) NOT NULL AUTO_INCREMENT,
+  `f2` char(32) DEFAULT NULL,
+  `f3` ENUM('red','green','blue'),
+  PRIMARY KEY (`id`),
+  UNIQUE KEY `c` (`f2`)
+) ENGINE=InnoDB 
+AUTO_INCREMENT=0 
+DEFAULT CHARSET=utf8
+STATS_AUTO_RECALC = 0
+STATS_PERSISTENT = 1;