Don't clear ActiveRecord connections when run_synchronously is enabled

[tomgi]

Clearing active connection in run_synchronously mode can lead to unexpected behaviour, especially when executed within a database transaction.

Aa a result a connection with active transaction can be released back to the pool and picked up by another thread.

Have a look at the following simplified code snippet that illustrates the issue:

Que::Job.run_synchronously = true

ActiveRecord::Base.transaction do
  ActiveRecord::Base.connection_pool.active_connection? # true
  ActiveRecord::Base.connection.transaction_open? # true
  # ^ we have an active connection and are currently within a DB transaction

  SomeQueJob.enqueue(...)
  # ^ the job runs synchronously and calls ::ActiveRecord::Base.clear_active_connections!

  ActiveRecord::Base.connection_pool.active_connection? # nil
  # ^ our connection with open transaction got returned back to the pool
  
  # someone (possibly another thread) picks up a connection back from the pool
  ActiveRecord::Base.connection

  ActiveRecord::Base.connection_pool.active_connection? # true
  ActiveRecord::Base.connection.transaction_open? # true
  # ^ it's the same connection with the previously opened transaction
end

[ZimbiX]

Good catch! Thanks =)

[oeoeaio]

Yes, this seems sensible and I assume that anyone depending on this behaviour will be doing so by accident.