Why Does Using Goroutines with `WriteHeader` in Go's `net/http` Package Cause 'multiple response.WriteHeader calls'?

Handling Multiple WriteHeader Calls in Go's HTTP Server

In Go's net/http package, it's essential to avoid multiple calls to WriteHeader for a given request. Making a goroutine call within the main handler function can lead to this issue, as demonstrated in the following example:

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        fmt.Println(r.URL)
        go HandleIndex(w, r)
    })

    ...
}

Within the anonymous handler function, we're printing the URL and starting a goroutine that calls HandleIndex. However, this causes the error "multiple response.WriteHeader calls" in the console.

Why does this happen? By default, Go sets the response status to 200 (HTTP OK) if it's not explicitly set in the handler function. In this example, even though HandleIndex sets the header in a separate goroutine, the main handler function completes without writing anything to the response or setting the status. This triggers Go to automatically set the status, resulting in multiple header writes.

To resolve this issue, we can remove the go prefix from go HandleIndex so that it's executed in the same goroutine as the main handler function. Alternatively, in HandleIndex, we could set the response header before the main handler function returns.

Another option is to use a middleware to set the response header before the request reaches the main handler function. This way, the main handler function doesn't need to set the header, and the middleware ensures that the header is set only once:

func main() {
    http.HandleFunc("/", middleware(HandleIndex))

    ...
}

func middleware(next http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Content-Type", "text/plain")
        next(w, r)
    }
}

By using a middleware, we can ensure that the response header is set before handling the request, avoiding any potential issues with multiple WriteHeader calls.

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