code: async def select(self, db):
          
          res = True
          async with self._cond:
              for i in range(self.freesize):
                  res = res and (await self._pool[i].select(db))
              else:
                  self._db = db
          return res
docs: Changes db index for all free connections.
  
          All previously acquired connections will be closed when released.