variable is empty but later has a value

Question

I'm trying to develop a Terraform provider but I have a problem of the first request body. Here is the code:

type Body struct {
    id string
}

func resourceServerCreate(d *schema.ResourceData, m interface{}) error {
    key := d.Get("key").(string)
    token := d.Get("token").(string)
    workspace_name := d.Get("workspace_name").(string)
    board_name := d.Get("board_name").(string)


    resp, err := http.Post("https://api.trello.com/1/organizations?key="+key+"&token="+token+"&displayName="+workspace_name,"application/json",nil)

    if err != nil {
            log.Fatalln(err)
    }

    defer resp.Body.Close()

    //lettura body.
    body := new(Body)
    json.NewDecoder(resp.Body).Decode(body)

    log.Println("[ORCA MADONNA] il log funzia "+body.id)
    d.Set("board_id",body.id)

    resp1, err1 := http.Post("https://api.trello.com/1/boards?key="+key+"&token="+token+"&idOrganization="+body.id+"&=&name="+board_name,"application/json",nil)
    
    if err1 != nil {
            log.Fatalln(resp1)
    }

    defer resp1.Body.Close()
    
    d.SetId(board_name)
    


    return resourceServerRead(d, m)

}

In the log is empty, but the second call have it and work fine. How is it possible?

yeah, but it have no sense because later in the second post body.id is used and work fine — Marco Bertelli, Jul 30 '21 at 14:17
you have deferred unmarshalling the resp.Body to `body` object, therefore body.id is expected to be empty — Wishwa Perera, Jul 30 '21 at 14:20
yeah sure, sorry Brand i don't understnad i have to leave to the Decode(i have try all for have this working) — Marco Bertelli, Jul 30 '21 at 14:26
handle the error of json decode. also in your error checks you are only logging the error and letting the code execute. do you see any fatal logs during execution — Wishwa Perera, Jul 30 '21 at 14:42
What is `body`? Unless you've implemented a `json.Unmarshaler` you're not going to have any value in the unexported `id` field. — JimB, Jul 30 '21 at 14:44
if someone have time to let me know how to parse correctly my body from post — Marco Bertelli, Jul 30 '21 at 14:50
You cannot write to an unexported field from another package. See https://stackoverflow.com/questions/26327391/json-marshalstruct-returns, and — JimB, Jul 30 '21 at 14:50
To put what `JimB` is saying another way, `id` is not exported from `main` module's `Body` to json because lowercase identifiers are always private in Go. Change the name to `Id`. — erik258, Jul 30 '21 at 16:08

Brent Bradburn · Answer 1 · 2021-07-30T18:57:32.720

Go doesn't force you to check error responses, therefore it's easy to make silly mistakes. Had you checked the return value from Decode(), you would have immediately discovered a problem.

err := json.NewDecoder(resp.Body).Decode(body)
if err != nil {
    log.Fatal("Decode error: ", err)
}

Decode error: json: Unmarshal(non-pointer main.Body)

So your most immediate fix is to use & to pass a pointer to Decode():

json.NewDecoder(resp.Body).Decode(&body)

Also of note, some programming editors will highlight this mistake for you:

Here's a working demonstration, including a corrected Body structure as described at json.Marshal(struct) returns “{}”:

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "log"
    "net/http"
    "time"
)

type JSON = map[string]interface{}
type JSONArray = []interface{}

func ErrFatal(err error, msg string) {
    if err != nil {
        log.Fatal(msg+": ", err)
    }
}

func handleTestRequest(w http.ResponseWriter, req *http.Request) {
    w.Write(([]byte)("{\"id\":\"yourid\"}"))
}

func launchTestServer() {
    http.HandleFunc("/", handleTestRequest)
    go http.ListenAndServe(":8080", nil)
    time.Sleep(1 * time.Second) // allow server to get started
}

// Medium: "Don’t use Go’s default HTTP client (in production)"
var restClient = &http.Client{
    Timeout: time.Second * 10,
}

func DoREST(method, url string, headers, payload JSON) *http.Response {

    requestPayload, err := json.Marshal(payload)
    ErrFatal(err, "json.Marshal(payload")

    request, err := http.NewRequest(method, url, bytes.NewBuffer(requestPayload))
    ErrFatal(err, "NewRequest "+method+" "+url)

    for k, v := range headers {
        request.Header.Add(k, v.(string))
    }

    response, err := restClient.Do(request)
    ErrFatal(err, "DoRest client.Do")
    return response
}

type Body struct {
    Id string `json:"id"`
}

func clientDemo() {
    response := DoREST("POST", "http://localhost:8080", JSON{}, JSON{})
    defer response.Body.Close()
    var body Body
    err := json.NewDecoder(response.Body).Decode(&body)
    ErrFatal(err, "Decode")
    fmt.Printf("Body: %#v\n", body)
}

func main() {
    launchTestServer()
    for i := 0; i < 5; i++ {
        clientDemo()
    }
}

A subtle implementation difference worthy of further investigation: `body := new(Body)` vs `var body Body` — Brent Bradburn, Aug 19 '21 at 17:28
Regarding `new`: [Pointers vs. values in parameters and return values](https://stackoverflow.com/a/57486190/86967) — Brent Bradburn, Sep 22 '21 at 15:58

variable is empty but later has a value

1 Answers1