added fallback middleware

pkg/middlewares/fallback.go

@@ -0,0 +1,61 @@
+package middlewares
+
+import (
+	"bytes"
+	"io/fs"
+	"log"
+	"net/http"
+)
+
+// FallbackFile serves a fallback file if the next handler returns a 404.
+// fs: The file system to serve the fallback file from.
+// name: The name of the fallback file.
+// next: The next handler to serve.
+func FallbackFile(fs fs.FS, name string, next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		bw := &buffedResponseWriter{
+			w,
+			0,
+			bytes.NewBuffer([]byte{}),
+		}
+		next.ServeHTTP(bw, r)
+
+		if bw.statusCode == http.StatusNotFound {
+			bw.Clear()
+
+			http.ServeFileFS(bw, r, fs, name)
+			bw.WriteHeader(http.StatusOK)
+			bw.Header().Add("Content-Type", "text/html; charset=utf-8")
+		}
+		bw.Apply()
+	})
+}
+
+type buffedResponseWriter struct {
+	http.ResponseWriter
+	statusCode int
+	buff       *bytes.Buffer
+}
+
+func (b *buffedResponseWriter) WriteHeader(code int) {
+	b.statusCode = code
+}
+
+func (b *buffedResponseWriter) Write(p []byte) (int, error) {
+	return b.buff.Write(p)
+}
+
+func (b *buffedResponseWriter) Clear() {
+	b.buff.Reset()
+}
+
+func (b *buffedResponseWriter) Apply() {
+	_, err := b.ResponseWriter.Write(b.buff.Bytes())
+	if err != nil {
+		log.Println("Error while applying buffedResponseWriter: ", err)
+		b.ResponseWriter.WriteHeader(http.StatusInternalServerError)
+	}
+	if b.statusCode != 200 {
+		b.ResponseWriter.WriteHeader(b.statusCode)
+	}
+}