file_handler.go

package main

import (
    "encoding/json"
    "errors"
    "log"
    "net/http"
)

// mockInfoSaver mock implementation of infoSaver
type mockInfoSaver struct {
}

func (b mockInfoSaver) CreateInfo(i *infoModel) error {
    log.Println("saving to db", i)
    return nil
}

var (
    ErrNotFound = errors.New("Not Found")
    defaultInfoBatchRequester = &infoBatchRequester{
        pending:   map[string][]chan<- infoChan{},
        infoSaver: mockInfoSaver{},
    }
)

// getInfo acts like a failed db get operation
func getInfo(infoID string) (infoModel, error) {
    return infoModel{}, errors.New("info does not exists")
}

// handleSlowThirdParty http handler that makes the call to the third party api
func handleSlowThirdParty(w http.ResponseWriter, r *http.Request) {
    infoID := r.URL.Query().Get("info_id")

    // make a fake request to get the info
    info, err := getInfo(infoID)

    if err != nil {
        // make a request to get the info
        result := make(chan infoChan, 1)
        go defaultInfoBatchRequester.Request(infoID, result)

        select {
        //the request was canceled early
        case <-r.Context().Done():
            log.Println("request terminated")

        //result returned
        case rs := <-result:
            if rs.err != nil {
                http.Error(w, rs.err.Error(), http.StatusBadRequest)
            }

            info = rs.info
        }
    }

    b, err := json.Marshal(info)

    w.Write(b)

}