chore: avoid superfluous WriteHeader call errors (#1275)

rest/internal/cors/handlers.go

@@ -1,6 +1,11 @@
 package cors
 
-import "net/http"
+import (
+	"bufio"
+	"errors"
+	"net"
+	"net/http"
+)
 
 const (
 	allowOrigin      = "Access-Control-Allow-Origin"
@@ -25,15 +30,16 @@ const (
 // At most one origin can be specified, other origins are ignored if given, default to be *.
 func NotAllowedHandler(fn func(w http.ResponseWriter), origins ...string) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		checkAndSetHeaders(w, r, origins)
+		gw := &guardedResponseWriter{w: w}
+		checkAndSetHeaders(gw, r, origins)
 		if fn != nil {
-			fn(w)
+			fn(gw)
 		}
 
-		if r.Method != http.MethodOptions {
-			w.WriteHeader(http.StatusNotFound)
+		if r.Method == http.MethodOptions {
+			gw.WriteHeader(http.StatusNoContent)
 		} else {
-			w.WriteHeader(http.StatusNoContent)
+			gw.WriteHeader(http.StatusNotFound)
 		}
 	})
 }
@@ -56,6 +62,44 @@ func Middleware(fn func(w http.ResponseWriter), origins ...string) func(http.Han
 	}
 }
 
+type guardedResponseWriter struct {
+	w           http.ResponseWriter
+	wroteHeader bool
+}
+
+func (w *guardedResponseWriter) Flush() {
+	if flusher, ok := w.w.(http.Flusher); ok {
+		flusher.Flush()
+	}
+}
+
+func (w *guardedResponseWriter) Header() http.Header {
+	return w.w.Header()
+}
+
+// Hijack implements the http.Hijacker interface.
+// This expands the Response to fulfill http.Hijacker if the underlying http.ResponseWriter supports it.
+func (w *guardedResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if hijacked, ok := w.w.(http.Hijacker); ok {
+		return hijacked.Hijack()
+	}
+
+	return nil, nil, errors.New("server doesn't support hijacking")
+}
+
+func (w *guardedResponseWriter) Write(bytes []byte) (int, error) {
+	return w.w.Write(bytes)
+}
+
+func (w *guardedResponseWriter) WriteHeader(code int) {
+	if w.wroteHeader {
+		return
+	}
+
+	w.w.WriteHeader(code)
+	w.wroteHeader = true
+}
+
 func checkAndSetHeaders(w http.ResponseWriter, r *http.Request, origins []string) {
 	setVaryHeaders(w, r)