How to send external HTTP request in google app engine - google-app-engine

Question

I have a problem on issuing HTTP requests in appengine since it does not support http.Client. I'm creating a slackbot and want to create a delayed response. The logic is once I received slack's POST request, I'm going to respond successfully and spin a goroutine calling an external API and after I finish with it, create a new request to slack.
Seems easy enough, but the problem I encountered is when using appengine's urlfetch and NewContext, because NewContext takes in an *http.Request as an argument but since I'm responding immediately to the first slack request, the response Body closes before I can use it to issue the response to an external API. Is there an alternative?
code:
func Twit(w http.ResponseWriter, r *http.Request) {
defaultResponse := &SlashResponse{ResponseType: "ephemeral", Text: "success"}
// prepare to make a slack delayed response
responseURL := r.FormValue("response_url")
go sendDelayResponse(responseURL, r)
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(200)
json.NewEncoder(w).Encode(defaultResponse)
}
func sendDelayResponse(url string, r *http.Request) {
response := &SlashResponse{ResponseType: "in_channel", Text: twit.TweetTCL(r)}
b, _ := json.Marshal(response)
// send request to slack using given response_url
ctx := appengine.NewContext(r)
client := urlfetch.Client(ctx)
req, _ := http.NewRequest("POST", url, bytes.NewBuffer(b))
req.Header.Add("Content-Type", "application/json")
resp, err := client.Do(req)
defer resp.Body.Close()
if err != nil {
log.Println(err)
} else {
log.Println("success")
}
}

Answer 1

Use the delay package to execute functions outside the scope of a request.
You can also use the lower-level taskqueue package. The delay package is layer above the taskqueue package.
Declare package level variable with the delay function:
var laterFunc("dr", func(ctx context.Context, url string) {
response := &SlashResponse{ResponseType: "in_channel", Text: twit.TweetTCL(r)}
b, _ := json.Marshal(response)
// send request to slack using given response_url
client := urlfetch.Client(ctx)
req, _ := http.NewRequest("POST", url, bytes.NewBuffer(b))
req.Header.Add("Content-Type", "application/json")
resp, err := client.Do(req)
defer resp.Body.Close()
if err != nil {
log.Println(err)
} else {
log.Println("success")
}
})
Call it like this:
laterfunc.Call(appengine.NewContext(r), responseURL)