Cannot set cookies in goroutine

[kiura24metrics]

• go version: go1.10.1 linux/amd64
• gin version (or commit ref): 1.2.0
• operating system: linux

Description

In readme file it states that one must use a read-only copy of gin.Context in goroutines but in this case I cannot set cookie since there is no ResponseWriter in read-only copy of gin.Context. and it panics: panic: runtime error: invalid memory address or nil pointer dereference

If I use original context then it throws fatal error: concurrent map writes.

So the question is how to accomplish this properly?

[sairoutine]

You can not set cookies in the goroutine. This is because it is asynchronous and the user's request is completed before the cookie header is set.

You should wait for the goroutine and set cookies outside the goroutine.

Example

package main

import (
    "fmt"

    "github.com/gin-gonic/gin"
)

func main() {

    router := gin.Default()

    router.GET("/cookie", func(c *gin.Context) {
        cookie := ""
        ch := make(chan bool)
        go func() {
            cookie = "set_in_the_goroutine" // generate only the cookie variable
            ch <- true
        }()

        // wait for the goroutine
        <-ch

        // set cookie outside the goroutine
        c.SetCookie("gin_cookie", cookie, 3600, "/", "localhost", false, true)
        fmt.Printf("Cookie value: %s \n", cookie)
    })

    router.Run()
}

[kiura24metrics]

I see, I wanted to set cookie in the goroutine (even though I wait for it to finish in the main loop before the user's request is completed). The system we are building is really complex, so it would be much easier to be able to set it in the goroutine (since we are waiting for it to finish anyways).