require 'digest'


class SharedResourceIdAllocator
  SPACES = {
      'tags' => ['tags', 'external_tags'],
      'wiki' => ['tag_descriptions', 'tag_groups'] }.freeze
  LOCK_TIMEOUT = 10

  def self.assign! record
    space, tables = SPACES.find { |_name, names| names.include?(record.class.table_name) }
    raise ArgumentError, 'Unknown shared ID resource' unless tables

    connection = record.class.connection
    transaction = record.class.current_transaction
    raise ActiveRecord::ActiveRecordError, 'Shared ID requires a transaction' unless transaction.open?

    database = connection.pool.db_config.database
    name = "btrc:ids:#{ Digest::SHA256.hexdigest(database)[0, 16] }:#{ space }"
    quoted_name = connection.quote(name)
    acquired = record.class.uncached do
      connection.select_value("SELECT GET_LOCK(#{ quoted_name }, #{ LOCK_TIMEOUT })")
    end
    unless acquired.to_i == 1
      raise ActiveRecord::LockWaitTimeout, "Shared ID lock unavailable: #{ space }"
    end

    release = lambda do
      result = record.class.uncached do
        connection.select_value("SELECT RELEASE_LOCK(#{ quoted_name })")
      end
      unless result.to_i == 1
        raise ActiveRecord::ActiveRecordError, "Shared ID lock release failed: #{ space }"
      end
    end
    begin
      transaction.after_commit(&release)
      transaction.after_rollback(&release)
    rescue StandardError
      release.call
      raise
    end

    return if record.id.present?

    # Locking reads see committed maxima even after earlier repeatable reads.
    maxima = record.class.uncached do
      tables.map do |table|
        quoted_table = connection.quote_table_name(table)
        connection.select_value(
            "SELECT id FROM #{ quoted_table } ORDER BY id DESC LIMIT 1 FOR UPDATE").to_i
      end
    end
    record.id = maxima.max + 1
  end
end