LP #1593184: pt-online-schema-change incorrectly reports to not drop tables

**Reported in Launchpad by Stefan Schlesinger last update 06-06-2017 04:17:58

In the following run pt-online-schema-change is incorrectly reporting
to not drop the old table.

Since it found that using drop_swap was required to update a foreign
key, it still drops the old table.

This is actually documented and a cosmetic issue.

pt-online-schema-change --alter "ADD COLUMN archived INT(11) DEFAULT NULL AFTER conflicted" --execute --alter-foreign-keys-method=auto D=production,t=shoppers
No slaves found. See --recursion-method if host db01 has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
analyze_table, 10, 1
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Child tables:
`production`.`shoppers` (approx. 129381829 rows)
`production`.`shoppers_log` (approx. 89381829 rows)
`production`.`shoppers_data` (approx. 89381829 rows)
Will automatically choose the method to update foreign keys.
Altering `production`.`shoppers`...
Creating new table...
Created new table production._shoppers_new OK.
Altering new table...
Altered `production`.`_shoppers_new` OK.
2016-06-16T10:23:33 Creating triggers...
2016-06-16T10:23:33 Created triggers OK.
2016-06-16T10:23:33 Copying approximately 120302383 rows...
Copying `production`.`shoppers`: 7% 62:04 remain
Copying `production`.`shoppers`: 15% 54:27 remain
Copying `production`.`shoppers`: 25% 52:19 remain
Copying `production`.`shoppers`: 34% 47:45 remain
Copying `production`.`shoppers`: 43% 40:15 remain
Copying `production`.`shoppers`: 52% 36:42 remain
Copying `production`.`shoppers`: 60% 30:15 remain
Copying `production`.`shoppers`: 67% 26:53 remain
Copying `production`.`shoppers`: 73% 20:39 remain
Copying `production`.`shoppers`: 78% 16:22 remain
Copying `production`.`shoppers`: 84% 10:02 remain
Copying `production`.`shoppers`: 88% 06:48 remain
2016-06-16T10:29:47 Copied rows OK.
2016-06-16T10:29:47 Max rows for the rebuild_constraints method: 72288
Determining the method to update foreign keys...
2016-06-16T10:29:47 `production`.`shoppers_log`: 6023 rows; can use rebuild_constraints
2016-06-16T10:29:47 `production`.`shoppers_data`: too many rows: 89381829; must use drop_swap
2016-06-16T10:29:47 Drop-swapping tables...
2016-06-16T10:29:47 Analyzing new table...
2016-06-16T10:29:48 Dropped and swapped tables OK.
Not dropping old table because --no-drop-old-table was specified.
2016-06-16T10:29:48 Dropping triggers...
2016-06-16T10:29:48 Dropped triggers OK.
Successfully altered `production`.`shoppers`.