Advisory lock name easily exceeds length limit with MySQL 5.7

[hut8]

Hey. This doesn't affect MariaDB, which I use on my development machine, but unfortunately on MySQL 5.7 there's a length limit of 64 characters on locks. So I wound up with this error during a migration:

Mysql2::Error: Incorrect user-level lock name 'ClosureTree::CreateFrobulatorServiceTypeHierarchies::FrobulatorServiceType'.:
SELECT GET_LOCK('ClosureTree::CreateFrobulatorServiceTypeHierarchies::FrobulatorServiceType', 0) AS tdf2f8c4cdfaa43bc9026d2d607d0b5e7

I know that 64 seems like a pretty high limit, but the code in support_attributes.rb always prepends ClosureTree::, then (at least in my experience) Rails migrations tend to have very long names (CreateACertainTypeOfThing...), and it's best practice (or so I've heard) to redefine a stub model inside of that class (in case its implementation changes a lot after the migration runs). So in a migration, it's easy to exceed the length limit and it bit me today. And because MySQL doesn't have transactional DDL, it's especially painful to clean up after 😦

Potential Solution

Would you be open to a different implementation of https://github.com/mceachen/closure_tree/blob/master/lib/closure_tree/support_attributes.rb#L9 , perhaps one which uses the first 30 or so characters of the MD5 of the existing implementation?

[mceachen]

Using an md5 or sha1 of the lock name sounds good to me. Perhaps add it as an option?

I'm afraid I don't have time to add this feature, but I welcome pull requests!

[hut8]

Opened #234 for this

[hut8]

Thanks!