Why am I getting the 'http: multiple response.WriteHeader calls' error in my Go HTTP server?

Multiple WriteHeader Calls in Simple HTTP Server

A basic HTTP server implemented in Go using the net/http package is exhibiting unusual behavior, as indicated by the error message:

http: multiple response.WriteHeader calls

This error suggests that the server is attempting to write the response header multiple times, which is not permitted by the HTTP specification. Let's analyze the code to understand why this issue arises:

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

    fmt.Println("Starting Server...")
    log.Fatal(http.ListenAndServe(":5678", nil))
}

func HandleIndex(w http.ResponseWriter, r *http.Request) {
    w.WriteHeader(200)
    w.Write([]byte("Hello, World!"))
}

Analysis

The main issue lies within the anonymous function registered as the handler for incoming requests:

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

This function prints the request URL and starts a new goroutine to call HandleIndex(). Then, it continues executing normally.

If the handler function doesn't set the response status or write anything to the response, Go will automatically set the status to 200 (HTTP OK) and return. This behavior applies to the anonymous function in our code, which doesn't explicitly set the response status or write to it before the go HandleIndex(w, r) line.

When HandleIndex() is invoked in a separate goroutine, the anonymous function continues its execution, ending with the request handling. Since the response status hasn't been set beforehand, Go will set it to 200 HTTP OK automatically. However, the spawned goroutine will also attempt to set the response status within HandleIndex(), leading to the "multiple response.WriteHeader calls" error.

Solution

To avoid this issue, either remove the "go" keyword from the line that starts the goroutine:

func(w http.ResponseWriter, r *http.Request) {
    fmt.Println(r.URL)
    HandleIndex(w, r)
}

or ensure that the anonymous function sets the response status and writes to it before it returns:

func(w http.ResponseWriter, r *http.Request) {
    fmt.Println(r.URL)
    w.Write([]byte("Hello, World!"))
    go HandleIndex(w, r)
}

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