PT-1423 - LP #1682929: pt-table-checksum: First chunk takes too long

- Modified the tool, so if both --chunk-size and --chunk-time are specified, it starts chunking with the indicated --chunk-size, and then adjusts chunk size to fit the time.

bin/pt-online-schema-change

@@ -8749,7 +8749,7 @@ sub main {
    }
 
    # Explicit --chunk-size disable auto chunk sizing.
-   $o->set('chunk-time', 0) if $o->got('chunk-size');
+   $o->set('chunk-time', 0) if $o->got('chunk-size') && !$o->got('chunk-time');
    if (!$o->get('swap-tables') && !$o->get('drop-triggers')) {
        PTDEBUG && _d('Enabling no-drop-new-table since no-swap-tables & no-drop-triggers were specified');
        $o->set('drop-new-table', 0);
@@ -13093,8 +13093,9 @@ This option can override the default behavior, which is to adjust chunk size
 dynamically to try to make chunks run in exactly L<"--chunk-time"> seconds.
 When this option isn't set explicitly, its default value is used as a starting
 point, but after that, the tool ignores this option's value.  If you set this
-option explicitly, however, then it disables the dynamic adjustment behavior and
-tries to make all chunks exactly the specified number of rows.
+option explicitly, and do not set L<"--chunk-time"> explicitly, then it disables
+the dynamic adjustment behavior and tries to make all chunks exactly the specified
+number of rows.
 
 =item --chunk-size-limit
 
@@ -13131,7 +13132,13 @@ changes in server load, the tool adapts quickly.
 If this option is set to zero, the chunk size doesn't auto-adjust, so query
 times will vary, but query chunk sizes will not. Another way to do the same
 thing is to specify a value for L<"--chunk-size"> explicitly, instead of leaving
-it at the default.
+it at the default, and omit the option L<"--chunk-time">.
+
+If both options L<"--chunk-size"> and L<"--chunk-time"> set explicitly, initial
+chunk size will be as specified by the option L<"--chunk-size">, but later it will
+be adjusted, so that the next query takes this amount of time (in seconds) 
+to execute. Use this option if the tool spends too much time copying the first chunk
+with default L<"--chunk-size">.
 
 =item --config
 

t/pt-online-schema-change/pt-1423.t

@@ -0,0 +1,106 @@
+#!/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 English qw(-no_match_vars);
+use Test::More;
+
+use Data::Dumper;
+use PerconaTest;
+use Sandbox;
+
+require "$trunk/bin/pt-online-schema-change";
+
+my $dp = new DSNParser(opts=>$dsn_opts);
+my $sb = new Sandbox(basedir => '/tmp', DSNParser => $dp);
+my $source_dbh = $sb->get_dbh_for('source');
+
+if ( !$source_dbh ) {
+   plan skip_all => 'Cannot connect to sandbox source';
+}
+
+# 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 $source_dsn = 'h=127.1,P=12345,u=msandbox,p=msandbox';
+my @args       = (qw(--set-vars innodb_lock_wait_timeout=3));
+my $sample     = "t/pt-online-schema-change/samples/";
+my $plugin     = "$trunk/$sample/plugins";
+my $output;
+my $exit_status;
+
+$sb->load_file('source', "$sample/basic_no_fks.sql");
+
+# Should be big enough, so we can watch how number of chunks changes with different options
+my $num_rows = 5000;
+diag("Loading $num_rows into the table. This might take some time.");
+diag(`util/mysql_random_data_load --host=127.0.0.1 --port=12345 --user=msandbox --password=msandbox pt_osc t $num_rows`);
+
+($output, $exit_status) = full_output(
+   sub { pt_online_schema_change::main(@args,
+      "$source_dsn,D=pt_osc,t=t",
+      '--progress', 'iterations,1',
+      "--alter", "FORCE",
+      '--chunk-size=100',
+      '--execute') },
+   stderr => 1,
+);
+
+my $chunks = () = $output =~ /Copying `pt_osc`.`t`:\s+\d+% \d\d:\d\d remain/gi;
+
+is(
+   $chunks,
+   51,
+   'Chunk size was not modified with default --chunk-time'
+) or diag($chunks, $output);
+
+($output, $exit_status) = full_output(
+   sub { pt_online_schema_change::main(@args,
+      "$source_dsn,D=pt_osc,t=t",
+      '--progress', 'iterations,1',
+      "--alter", "FORCE",
+      '--chunk-size=100',
+      '--chunk-time=0',
+      '--execute') },
+   stderr => 1,
+);
+
+$chunks = () = $output =~ /Copying `pt_osc`.`t`:\s+\d+% \d\d:\d\d remain/gi;
+
+is(
+   $chunks,
+   51,
+   'Chunk size was not modified with --chunk-time=0'
+) or diag($chunks, $output);
+
+($output, $exit_status) = full_output(
+   sub { pt_online_schema_change::main(@args,
+      "$source_dsn,D=pt_osc,t=t",
+      '--progress', 'iterations,1',
+      "--alter", "FORCE",
+      '--chunk-size=100',
+      '--chunk-time=0.5',
+      '--execute') },
+   stderr => 1,
+);
+
+$chunks = () = $output =~ /Copying `pt_osc`.`t`:\s+\d+% \d\d:\d\d remain/gi;
+
+is(
+   $chunks,
+   2,
+   'Chunk size was adjusted to fit specified --chunk-time'
+) or diag($chunks, $output);
+
+# #############################################################################
+# Done.
+# #############################################################################
+$sb->wipe_clean($source_dbh);
+ok($sb->ok(), "Sandbox servers") or BAIL_OUT(__FILE__ . " broke the sandbox");
+done_testing;