Golang Goroutine synchronization with channel - go

Question

I have the following program where the HTTP server is created using gorilla mux.
When any request comes, it starts goroutine 1. In processing, I am starting another goroutine 2.
I want to wait for goroutine 2's response in goroutine 1? How I can do that?
How to ensure that only goroutine 2 will give the response to goroutine 1?
There can be GR4 created by GR3 and GR 3 should wait for GR4 only.
GR = Goroutine
SERVER
package main
import (
"encoding/json"
"fmt"
"net/http"
"strconv"
"time"
"github.com/gorilla/mux"
)
type Post struct {
ID string `json:"id"`
Title string `json:"title"`
Body string `json:"body"`
}
var posts []Post
var i = 0
func getPosts(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "application/json")
i++
fmt.Println(i)
ch := make(chan int)
go getTitle(ch, i)
p := Post{
ID: "123",
}
// Wait for getTitle result and update variable P with title
s := <-ch
//
p.Title = strconv.Itoa(s) + strconv.Itoa(i)
json.NewEncoder(w).Encode(p)
}
func main() {
router := mux.NewRouter()
posts = append(posts, Post{ID: "1", Title: "My first post", Body: "This is the content of my first post"})
router.HandleFunc("/posts", getPosts).Methods("GET")
http.ListenAndServe(":9999", router)
}
func getTitle(resultCh chan int, m int) {
time.Sleep(2 * time.Second)
resultCh <- m
}
CLIENT
package main
import (
"fmt"
"net/http"
"io/ioutil"
"time"
)
func main(){
for i :=0;i <100 ;i++ {
go main2()
}
time.Sleep(200 * time.Second)
}
func main2() {
url := "http://localhost:9999/posts"
method := "GET"
client := &http.Client {
}
req, err := http.NewRequest(method, url, nil)
if err != nil {
fmt.Println(err)
}
res, err := client.Do(req)
defer res.Body.Close()
body, err := ioutil.ReadAll(res.Body)
fmt.Println(string(body))
}
RESULT ACTUAL
{"id":"123","title":"25115","body":""}
{"id":"123","title":"23115","body":""}
{"id":"123","title":"31115","body":""}
{"id":"123","title":"44115","body":""}
{"id":"123","title":"105115","body":""}
{"id":"123","title":"109115","body":""}
{"id":"123","title":"103115","body":""}
{"id":"123","title":"115115","body":""}
{"id":"123","title":"115115","body":""}
{"id":"123","title":"115115","body":""}
RESULT EXPECTED
{"id":"123","title":"112112","body":""}
{"id":"123","title":"113113","body":""}
{"id":"123","title":"115115","body":""}
{"id":"123","title":"116116","body":""}
{"id":"123","title":"117117","body":""}

Answer 1

there are few ways to do this , a simple way is to use channels
change the getTitle func to this
func getTitle(resultCh chan string) {
time.Sleep(2 * time.Second)
resultCh <- "Game Of Thrones"
}
and the getPosts will use it like this
func getPosts(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "application/json")
ch := make(chan string)
go getTitle(ch)
s := <-ch // this will wait until getTile inserts data to channel
p := Post{
ID: s,
}
json.NewEncoder(w).Encode(p)
}
i suspect you are new to go, this is a basic channel usage , check more details here Channels

Answer 2

So the problem you're having is that you haven't really leaned how to deal with concurrent code (not a dis, I was there once). Most of this centers not around channels. The channels are working correctly as #kojan's answer explains. Where things go awry is with the i variable. Firstly you have to understand that i is not being mutated atomically so if your client requests arrive in parallel you can mess up the number:
C1 : C2:
i == 6 i == 6
i++ i++
i == 7 i == 7
Two increments in software become one increment in actuality because i++ is really 3 operations: load, increment, store.
The second problem you have is that i is not a pointer, so when you pass i to your go routine you're making a copy. the i in the go routine is sent back on the channel and becomes the first number in your concatenated string which you can watch increment. However the i left behind which is used in the tail of the string has continued to be incremented by successive client invocations.