// zpop pops a value from the ZSET key using WATCH/MULTI/EXEC commands.
func zpop(c redis.Conn, key string) (result string, err error) {

	defer func() {
		// Return connection to normal state on error.
		if err != nil {
			c.Do("DISCARD")
		}
	}()

	// Loop until transaction is successful.
	for {
		if _, err := c.Do("WATCH", key); err != nil {
			return "", err
		}

		members, err := redis.Strings(c.Do("ZRANGE", key, 0, 0))
		if err != nil {
			return "", err
		}
		if len(members) != 1 {
			return "", redis.ErrNil
		}

		c.Send("MULTI")
		c.Send("ZREM", key, members[0])
		queued, err := c.Do("EXEC")
		if err != nil {
			return "", err
		}

		if queued != nil {
			result = members[0]
			break
		}
	}

	return result, nil
}
	actual   valueError
	expected valueError
}{
	{
		"ints([v1, v2])",
		ve(redis.Ints([]interface{}{[]byte("4"), []byte("5")}, nil)),
		ve([]int{4, 5}, nil),
	},
	{
		"ints(nil)",
		ve(redis.Ints(nil, nil)),
		ve([]int(nil), redis.ErrNil),
	},
	{
		"strings([v1, v2])",
		ve(redis.Strings([]interface{}{[]byte("v1"), []byte("v2")}, nil)),
		ve([]string{"v1", "v2"}, nil),
	},
	{
		"strings(nil)",
		ve(redis.Strings(nil, nil)),
		ve([]string(nil), redis.ErrNil),
	},
	{
		"values([v1, v2])",
		ve(redis.Values([]interface{}{[]byte("v1"), []byte("v2")}, nil)),
		ve([]interface{}{[]byte("v1"), []byte("v2")}, nil),
	},
	{
		"values(nil)",
		ve(redis.Values(nil, nil)),