Golang在尝试使方法并发时没有任何回报

Question

I have a method that returns Json from the database and it works correctly, however when I try to make it concurrent it does not return anything and does not give any errors. For example this is the method working correctly

func Listing_Expiration(w http.ResponseWriter, r *http.Request)  {

        db,err := sql.Open("DB_Connect")

        if err != nil {
            log.Fatal(err)
            println(err)
        }



        var result string

        errr := db.QueryRow("select json_build_object('Expiration', array_to_json(array_agg(t))) from (select fullname,ad_end from profiles where id=32)t").Scan(&result)


        defer db.Close()
        switch {
        case errr != nil:
            log.Fatal(errr)


        }


        fmt.Fprintf(w,result)

}

The above method works correctly and the data is returned to the browser then I try to make this method async

 func Listing_Expiration(w http.ResponseWriter, r *http.Request)  {
        go func(){

            db,err := sql.Open("DB_Connect")

            if err != nil {
                log.Fatal(err)
                println(err)
            }



            var result string

            errr := db.QueryRow("select json_build_object('Expiration', array_to_json(array_agg(t))) from (select fullname,ad_end from profiles where id=32)t").Scan(&result)


            defer db.Close()
            switch {
            case errr != nil:
                log.Fatal(errr)


            }


            fmt.Fprintf(w,result)

        }()
    }

The async above returns nothing the only thing I have changed is that I added the Go func() inside the method so that it is async and everything else is the same . I checked and the database is returning the same content back just have no idea why the async is not printing the results back to the browser.

Answer 1

The net/http server completes the response when the handler returns. Anything written to the response after the handler returns is ignored.

The handler function Listing_Expiration is returning before the anonymous function executes fmt.Fprintf(w,result).

To fix this, use a sync.WaitGroup:

func Listing_Expiration(w http.ResponseWriter, r *http.Request)  {
  var wg sync.WaitGroup
  wg.Add(1)
  go func(){
    defer wg.Done()
    // same code here as in question
  }()
  wg.Wait()
}

The net/http server starts a goroutine for each connection and runs handlers in those goroutines. If the code in the question is the complete handler, then there's no reason to start yet another goroutine.