LP #1727137: pt-online-schema change fails if foreign key name is too long

**Reported in Launchpad by Aleksandr Kuzminsky last update 25-10-2017 02:39:44

MySQL poses a limit on max length of foreign key name.

pt-osc renames foreign key by adding a prefix. If the initial FK name is as long as the limit renaming the FK will fail. pt-osc subsequently fails as well.

After the failure pt-osc leaves tables in broken state. The table is renamed but foreign keys refer to the _old table.