Overview
This is a note on the side effects of Go's response.WriteHeader.
Superfluous response.WriteHeader
The following is a straightforward example, but calling WriteHeader multiple times like this will result in the error http: superfluous response.WriteHeader call from main.handler.
package main
import (
"fmt"
"net/http"
)
func handler(w http.ResponseWriter, r *http.Request) {
w.WriteHeader(http.StatusOK)
w.WriteHeader(http.StatusInternalServerError) // Error!
}
func main() {
http.HandleFunc("/", handler)
http.ListenAndServe(":8888", nil)
}
When called multiple times like this, the status code from the first call is adopted, and the status code from the last call is ignored. (Is this related to HTTP specifications?)
Looking into this gives a sense of the specifications.
Response
In the straightforward example above, it would suffice to adjust the implementation to call WriteHeader only once. However, suppose there are cases where WriteHeader is set multiple times due to implementation constraints.
import (
"bytes"
"html/template"
"net/http"
)
// Function called to render the error page
func ExecuteTpl(w http.ResponseWriter) error {
err := template.Must(template.ParseFiles("index.html")).Execute(w, nil)
w.WriteHeader(http.StatusInternalServerError)
if err != nil {
return err
}
return nil
}
Assuming that WriteHeader has already been called before this function, it would result in an error.
To avoid errors in such situations, you can write to a buffer first and then call WriteHeader at the end.
package main
import (
"bytes"
"html/template"
"net/http"
)
func ExecuteTpl(w http.ResponseWriter) error {
var buf bytes.Buffer
w.WriteHeader(http.StatusInternalServerError)
err := template.Must(template.ParseFiles("index.html")).Execute(w, nil)
if err != nil {
return err
}
buf.WriteTo(w)
return nil
}
Other Notes
It seems that when there is an error during the execution of the template, the execution stops, but there is a possibility that writing to the response has already begun. Therefore, it might be better to call WriteHeader before Execute. (Probably)
