Have I found a bug in BodyParser, or am I an idiot?

[lachdoug]

Question Description

I am new to Fiber so please forgive me is this is a noob issue, but I can't understand what is going wrong with my Fiber app code.

I am trying to achieve the following:

1. De-serialize a request body to a struct
2. Add the new struct to a collection
3. Repeat

package main

import (
	"fmt"
	"log"

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

type Record struct {
	Value string
}

func main() {
	records := []Record{}

	router := fiber.New(fiber.Config{})

	router.Post("/", func(ctx *fiber.Ctx) error {
		var record Record
		if err := ctx.BodyParser(&record); err != nil {
			return err
		}

		records = append(records, record)

		return ctx.SendString(fmt.Sprintf("%+v\n", records))
	})

	log.Fatal(router.Listen(":8080"))
}

When I add the first record things work as expected:

$ curl localhost:8080 --request POST --data value=1
[{Value:1}]

When I add a second record, the value in the first record is changed/corrupted.

$ curl localhost:8080 --request POST --data value=2
[{Value:2} {Value:2}]

I didn't expect that. I expected the first item to have a value of 1 and the second record to have a value of 2..

Here's what happens when more items are added.

$ curl localhost:8080 --request POST --data value=3
[{Value:3} {Value:3} {Value:3}]
$ curl localhost:8080 --request POST --data value=abc
[{Value:a} {Value:a} {Value:a} {Value:abc}]

At this point the collection should be [{Value:1} {Value:2} {Value:3} {Value:abc}]

What am I doing wrong?

Code Snippet (optional)

No response

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• I agree to follow Fiber's Code of Conduct.
• I have checked for existing issues that describe my questions prior to opening this one.
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[nickajacks1]

This is because by default, you cannot keep references to request data beyond the scope of the handler. What you're seeing is that the underlying buffer of the string is reused between requests.
See the fine print at the end of the BodyParser section: https://docs.gofiber.io/api/ctx#bodyparser
Please note that other methods have this same behavior.

You can get around this two ways:

• Use the Immutable config setting https://docs.gofiber.io/api/fiber#config . You most likely will never notice the performance hit from enabling this (depending on what you're doing, of course).
• Make a copy of the data, eg record.Value = strings.Clone(record.Value). Unfortunately, there isn't a super simple way to make a deep copy of a struct. You'd need to manually copy each string field in your struct.

[gaby]

@lachdoug https://docs.gofiber.io/#zero-allocation

[lachdoug]

Thank you @nickajacks1 and @gaby
That explains it. Much appreciated.