multi: Properly reset client in case of empty transaction.

multi.go

@@ -53,12 +53,14 @@ func (c *MultiClient) Exec(do func()) ([]Req, error) {
 
 	c.mtx.Lock()
 	c.reqs = append(c.reqs, NewIfaceSliceReq("EXEC"))
-	if len(c.reqs) == 2 {
+
+	reqs := c.reqs
+	c.reqs = nil
+
+	if len(reqs) == 2 {
 		c.mtx.Unlock()
 		return []Req{}, nil
 	}
-	reqs := c.reqs
-	c.reqs = nil
 	c.mtx.Unlock()
 
 	conn, err := c.conn()

redis_test.go

@@ -2395,6 +2395,22 @@ func (t *RedisTest) TestMultiExecDiscard(c *C) {
 	c.Assert(get.Val(), Equals, "hello2")
 }
 
+func (t *RedisTest) TestMultiExecEmpty(c *C) {
+	multi, err := t.client.MultiClient()
+	c.Assert(err, IsNil)
+	defer func() {
+		c.Assert(multi.Close(), IsNil)
+	}()
+
+	reqs, err := multi.Exec(func() {})
+	c.Assert(err, IsNil)
+	c.Assert(reqs, HasLen, 0)
+
+	ping := multi.Ping()
+	c.Check(ping.Err(), IsNil)
+	c.Check(ping.Val(), Equals, "PONG")
+}
+
 func (t *RedisTest) TestMultiExecOnEmptyQueue(c *C) {
 	multi, err := t.client.MultiClient()
 	c.Assert(err, IsNil)