🐛 BodyParser does not save empty string to field

[sandro]

Fiber version
Fiber v2.1.0

Issue description
I'm trying to use BodyParser to save an empty string to a field in a struct, but the empty string will not overwrite the existing data in the field.

The bug disappears when I'm able to set the decoder's zeroEmpty to true, i.e. decoder.ZeroEmpty(true).

I wonder if there's a way to allow users to change the decoder when necessary. Perhaps by allowing the user to pass a custom decoder into the BodyParser function?

Code Sample

package main

import (
	"fmt"
	"log"
	"net/http"

	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/utils"
)

type Post struct {
	Title string `form:"title"`
	Body  string `form:"body"`
}


func main() {
	app := fiber.New()

        // curl -d "title=&body=new_body" localhost:3000/posts
	app.Post("/posts", func(c *fiber.Ctx) error {
		p := Post{Title: "Existing title", Body: "Existing Body"}
		c.BodyParser(&p)
		fmt.Printf("form value of title %q\n", c.FormValue("title"))
		fmt.Printf("form value of title %q\n", c.FormValue("body"))
		fmt.Printf("%+v\n", p)
		return c.SendStatus(http.StatusOK)
	})

        // curl -d "title=&body=new_body" localhost:3000/posts_use_empty_string
	app.Post("/posts_use_empty_string", func(c *fiber.Ctx) error {
		p := Post{Title: "Existing title", Body: "Existing Body"}
		var decoder = NewDecoder()
		decoder.IgnoreUnknownKeys(true)
		decoder.ZeroEmpty(true)
		decoder.SetAliasTag("form")
		data := make(map[string][]string)
		c.Context().PostArgs().VisitAll(func(key []byte, val []byte) {
			data[utils.UnsafeString(key)] = append(data[utils.UnsafeString(key)], utils.UnsafeString(val))
		})
		decoder.Decode(&p, data)
		fmt.Printf("form value of title %q\n", c.FormValue("title"))
		fmt.Printf("form value of title %q\n", c.FormValue("body"))
		fmt.Printf("%+v\n", p)
		return c.SendStatus(http.StatusOK)
	})

	log.Fatal(app.Listen(":3000"))
}

Sending an empty string to BodyParser does not overwrite the existing value

$ curl -d "title=&body=new_body" localhost:3000/posts

form value of title ""
form value of title "new_body"
{Title:Existing title Body:new_body}

But when sending an empty string when the decoder has ZeroEmpty set to true, the decoder does overwrite the existing value.

curl -d "title=&body=new_body" localhost:3000/posts_use_empty_string

form value of title ""
form value of title "new_body"
{Title: Body:new_body}

[sandro]

@Fenny Thanks for your consideration. I've added a code sample.

[si3nloong]

Hi @sandro , is this issue still exists in the latest version?

[si3nloong]

If you use echo, you will realise they overwrite the default value if query string or form consists the key with empty value, i think we should enable ZeroEmpty(true) by default for decoder

Here is my pr, #1285

[sandro]

@si3nloong Thanks for the update. I haven't checked against the latest version, but I agree that setting ZeroEmpty to true by default makes sense. Unfortunately, that could break apps that depend on it being false. Is there any way to make ZeroEmpty configurable?

[si3nloong]

yes, it is, but I think the break will be minor. Since other famous frameworks also practicing the same way, we should also follow the same practice, to allow the user migrate their app from echo to fiber / fiber to echo seamlessly.

[Fenny]

#1285 merged 👍