class PG::Connection

+ROLLBACK+ if any exception occurs.
and a +COMMIT+ at the end of the block, or
Executes a +BEGIN+ at the start of the block,

conn.transaction { |conn| ... } -> result of the block
call-seq:

def transaction
	exec "BEGIN"
	res = yield(self)
rescue Exception
	cancel if transaction_status != PG::PQTRANS_IDLE
	block
	exec "ROLLBACK"
	raise
else
	exec "COMMIT"
	res
end