Connection closed during migrations under certain circumstances #41855

MVCE: https://github.com/joshleblanc/sharding-migrations-bug

Steps to reproduce

Create a new rails application

Create a sharded database with

# database.yml
development:
  default:
    database: default
  archive:
    database: archive

# application_record.rb
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to shards: {
    default: {writing: :default, reading: :default},
    archive: {writing: :archive, reading: :archive}
  }
end

Create any model, then add a second migration that queries the database using the model, then runs an execute statement.

class ThisShouldBreak < ActiveRecord::Migration[6.0]
  def up
    Author.count
    execute "SELECT * from authors"
  end

  def down
    # no-op
  end
end

Expected behavior

The migration should run

Actual behavior

The connection is closed during the migration, triggering a rollback.

System configuration

Rails version: Rails 6.1.3.1

Ruby version: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x64-mingw32]