ctx.Header is not thread safe

[saksham-arya-z]

Description

While writing to response header in goroutines we get fatal error: concurrent map writes.

How to reproduce

package main

import (
    "sync"

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

func setHeader(c *gin.Context) {
    c.Header("key", "value")
}

func main() {
    g := gin.Default()
    g.GET("/hello", func(c *gin.Context) {
        wg := sync.WaitGroup{}
        go func() {
            wg.Add(1)
            defer wg.Done()
            for i := 0; i < 100000; i++ {
                setHeader(c)
            }
        }()
        go func() {
            wg.Add(1)
            defer wg.Done()
            for i := 0; i < 100000; i++ {
                setHeader(c)
            }
        }()
        wg.Wait()
    })
    g.Run(":9000")
}

Expectations

$ curl http://localhost:9000/hello
Should not crash

Actual result

fatal error: concurrent map writes

goroutine 25 [running]:
net/textproto.MIMEHeader.Set(...)
        /opt/homebrew/Cellar/go/1.22.5/libexec/src/net/textproto/header.go:22
net/http.Header.Set(...)
        /opt/homebrew/Cellar/go/1.22.5/libexec/src/net/http/header.go:40
github.com/gin-gonic/gin.(*Context).Header(0x0?, {0x1002fe555, 0x3}, {0x1002fedc8, 0x5})
        /Users/admin/go/pkg/mod/github.com/gin-gonic/gin@v1.10.0/context.go:897 +0xb0
main.setHeader(...)
        /Users/admin/repos/scripts/main.go:12
main.main.func1.1()
        /Users/admin/repos/scripts/main.go:23 +0x94
created by main.main.func1 in goroutine 23
        /Users/admin/repos/scripts/main.go:19 +0x7c

Environment

• go version: 1.22.0
• gin version (or commit ref): 1.10.0
• operating system: Linux amd64

[flc1125]

For this reason, I suggest that the application side solve it.

https://github.com/gin-gonic/gin/pull/4101#issuecomment-2492897986