Reimplement Jeremy's PostgreSQL automatic transaction state introspection code.

- Fixed compatibility with the old 'postgres' driver which doesn't support
  transaction state introspection.
- Added unit tests for it.

activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb

@@ -54,6 +54,20 @@ module ActiveRecord
         delete_sql(sql, name)
       end
       
+      # Checks whether there is currently no transaction active. This is done
+      # by querying the database driver, and does not use the transaction
+      # house-keeping information recorded by #increment_open_transactions and
+      # friends.
+      #
+      # Returns true if there is no transaction active, false if there is a
+      # transaction active, and nil if this information is unknown.
+      #
+      # Not all adapters supports transaction state introspection. Currently,
+      # only the PostgreSQL adapter supports this.
+      def outside_transaction?
+        nil
+      end
+
       # Runs the given block in a database transaction, and returns the result
       # of the block.
       #
@@ -119,7 +133,7 @@ module ActiveRecord
             yield
           end
         rescue Exception => database_transaction_rollback
-          if transaction_open
+          if transaction_open && !outside_transaction?
             transaction_open = false
             decrement_open_transactions
             if open_transactions == 0
@@ -131,7 +145,9 @@ module ActiveRecord
           raise unless database_transaction_rollback.is_a? ActiveRecord::Rollback
         end
       ensure
-        if transaction_open
+        if outside_transaction?
+          @open_transactions = 0
+        elsif transaction_open
           decrement_open_transactions
           begin
             if open_transactions == 0

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -533,6 +533,16 @@ module ActiveRecord
         execute "ROLLBACK"
       end
       
+      if PGconn.public_method_defined?(:transaction_status)
+        # ruby-pg defines Ruby constants for transaction status,
+        # ruby-postgres does not.
+        PQTRANS_IDLE = defined?(PGconn::PQTRANS_IDLE) ? PGconn::PQTRANS_IDLE : 0
+        
+        def outside_transaction?
+          @connection.transaction_status == PQTRANS_IDLE
+        end
+      end
+
       def create_savepoint
         execute("SAVEPOINT #{current_savepoint_name}")
       end

activerecord/test/cases/transactions_test.rb

@@ -310,6 +310,7 @@ class TransactionTest < ActiveRecord::TestCase
     def test_rollback_when_commit_raises
       Topic.connection.expects(:begin_db_transaction)
       Topic.connection.expects(:commit_db_transaction).raises('OH NOES')
+      Topic.connection.expects(:outside_transaction?).returns(false)
       Topic.connection.expects(:rollback_db_transaction)
 
       assert_raise RuntimeError do
@@ -320,6 +321,39 @@ class TransactionTest < ActiveRecord::TestCase
     end
   end
   
+  if current_adapter?(:PostgreSQLAdapter) && PGconn.public_method_defined?(:transaction_status)
+    def test_outside_transaction_works
+      Topic.logger.info("-------------")
+      assert Topic.connection.outside_transaction?
+      Topic.connection.begin_db_transaction
+      assert !Topic.connection.outside_transaction?
+      Topic.connection.rollback_db_transaction
+      assert Topic.connection.outside_transaction?
+    end
+    
+    uses_mocha 'mocking connection.rollback_db_transaction' do
+      def test_rollback_wont_be_executed_if_no_transaction_active
+        assert_raise RuntimeError do
+          Topic.transaction do
+            Topic.connection.rollback_db_transaction
+            Topic.connection.expects(:rollback_db_transaction).never
+            raise "Rails doesn't scale!"
+          end
+        end
+      end
+    end
+    
+    def test_open_transactions_count_is_reset_to_zero_if_no_transaction_active
+      Topic.transaction do
+        Topic.transaction do
+          Topic.connection.rollback_db_transaction
+        end
+        assert_equal 0, Topic.connection.open_transactions
+      end
+      assert_equal 0, Topic.connection.open_transactions
+    end
+  end
+
   def test_sqlite_add_column_in_transaction_raises_statement_invalid
     return true unless current_adapter?(:SQLite3Adapter, :SQLiteAdapter)