DEV: Update DB.after_commit to be compatible with 'real' transactions (#11294)

Previously it matched the behavior of standard ActiveRecord after_commit callbacks. They do not work well within `joinable: false` nested transactions. Now `DB.after_commit` callbacks will only be run when the outermost transaction has been committed.

Tests always run inside transactions, so this also introduces some logic to run callbacks once the test-wrapping transaction is reached.

lib/mini_sql_multisite_connection.rb

@@ -32,7 +32,12 @@ class MiniSqlMultisiteConnection < MiniSql::Postgres::Connection
     end
 
     def committed!(*)
-      @callback.call
+      if DB.transaction_open?
+        # Nested transaction. Pass the callback to the parent
+        ActiveRecord::Base.connection.add_transaction_record(self)
+      else
+        @callback.call
+      end
     end
 
     def before_committed!(*); end
@@ -42,16 +47,25 @@ class MiniSqlMultisiteConnection < MiniSql::Postgres::Connection
     end
   end
 
+  def transaction_open?
+    ActiveRecord::Base.connection.transaction_open?
+  end
+
+  if Rails.env.test?
+    def test_transaction=(transaction)
+      @test_transaction = transaction
+    end
+
+    def transaction_open?
+      ActiveRecord::Base.connection.current_transaction != @test_transaction
+    end
+  end
+
   # Allows running arbitrary code after the current transaction has been committed.
   # Works with nested ActiveRecord transaction blocks. Useful for scheduling sidekiq jobs.
   # If not currently in a transaction, will execute immediately
   def after_commit(&blk)
-    return blk.call if !ActiveRecord::Base.connection.transaction_open?
-
-    # In tests, everything is run inside a transaction.
-    # To run immediately, check for joinable? transaction
-    # This mimics core rails behavior: https://github.com/rails/rails/blob/348e142b/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb#L211
-    return blk.call if Rails.env.test? && !ActiveRecord::Base.connection.current_transaction.joinable?
+    return blk.call if !transaction_open?
 
     ActiveRecord::Base.connection.add_transaction_record(
       AfterCommitWrapper.new(&blk)