fix mapreduce problem when reducer doesn't write

example/mapreduce/deadlock/main.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"log"
+	"strconv"
+
+	"github.com/tal-tech/go-zero/core/mr"
+)
+
+type User struct {
+	Uid  int
+	Name string
+}
+
+func main() {
+	uids := []int{111, 222, 333}
+	res, err := mr.MapReduce(func(source chan<- interface{}) {
+		for _, uid := range uids {
+			source <- uid
+		}
+	}, func(item interface{}, writer mr.Writer, cancel func(error)) {
+		uid := item.(int)
+		user := &User{
+			Uid:  uid,
+			Name: strconv.Itoa(uid),
+		}
+		writer.Write(user)
+	}, func(pipe <-chan interface{}, writer mr.Writer, cancel func(error)) {
+		var users []*User
+		for p := range pipe {
+			users = append(users, p.(*User))
+		}
+		// missing writer.Write(...), should not panic
+	})
+	if err != nil {
+		log.Print(err)
+		return
+	}
+	log.Print(len(res.([]*User)))
+}