Keep ServeContent on the sendfile path

main.go

@@ -632,6 +632,29 @@ func (rr *responseRecorder) Write(b []byte) (int, error) {
 	return n, err
 }
 
+// ReadFrom keeps downloads on the kernel's sendfile path. io.Copy — which is
+// where http.ServeContent ends up — only lets the destination take over the
+// copy when it implements io.ReaderFrom, so wrapping the ResponseWriter would
+// otherwise hide net/http's own implementation and push every byte through a
+// userspace buffer.
+func (rr *responseRecorder) ReadFrom(src io.Reader) (int64, error) {
+	if rr.status == 0 {
+		rr.status = http.StatusOK
+	}
+
+	var n int64
+	var err error
+	if rf, ok := rr.ResponseWriter.(io.ReaderFrom); ok {
+		n, err = rf.ReadFrom(src)
+	} else {
+		// Copy into the wrapped writer, not into rr, or this recurses.
+		n, err = io.Copy(rr.ResponseWriter, src)
+	}
+
+	rr.written += n
+	return n, err
+}
+
 // Unwrap lets http.ResponseController and baseWriter reach the original
 // ResponseWriter.
 func (rr *responseRecorder) Unwrap() http.ResponseWriter { return rr.ResponseWriter }