Corrupt transaction counts with Rails 7.1 restarting savepoint transactions

[composerinteralia]

This is similar to #64, but I'm filing it separately since this one really is a bug.

What did you do?

I tried to run isolator at GitHub and ran into some Trying to finalize an untracked transaction warnings. Here's a failing test (against Rails main) to demonstrate the issue:

    context "when rolling back a restarting savepoint transaction" do
      specify do
        expect(Isolator).to_not be_within_transaction

        begin
          ActiveRecord::Base.logger = Logger.new(STDOUT)
          # RealTransaction (begin..rollback)
          ar_class.transaction do
            ar_class.first
            # Savepoint Transaction (savepoint..rollback)
            ar_class.transaction(requires_new: true) do
              # ResetParentTransaction (rollback to outer savepoint)
              ar_class.transaction(requires_new: true) do
                ar_class.first
                expect(Isolator).to be_within_transaction
                raise "Rollback"
              end
            end
          ensure
            expect(Isolator).to be_within_transaction # Oops, transaction count is already 0 so this test fails
          end
        rescue
        end

        expect(Isolator).to_not be_within_transaction
      end
    end

This happens because of rails/rails#44526, which can cause the same savepoint to get rolled back more than once. In older versions of Rails the transaction queries were:

BEGIN
SAVEPOINT active_record_1
SAVEPOINT active_record_2
ROLLBACK TO SAVEPOINT active_record_2
ROLLBACK TO SAVEPOINT active_record_1
ROLLBACK

But in Rails 7.1 it becomes:

BEGIN
SAVEPOINT active_record_1
ROLLBACK TO SAVEPOINT active_record_1
ROLLBACK TO SAVEPOINT active_record_1
ROLLBACK

That's 2 starts and 3 finishes, so we end up decrementing the count too far.

Environment

Ruby Version:

3.2

Framework Version (Rails, whatever):

Rails 7.1 (unreleased)

Isolator Version:

0.10.0

[composerinteralia]

rails/rails#49192 was merged, so that might be a more reliable option for the future