apply Range only to a 206 response in write_content_with_provider

httplib.h

@@ -11777,7 +11777,16 @@ Server::write_content_with_provider(Stream &strm, const Request &req,
   };
 
   if (res.content_length_ > 0) {
-    if (req.ranges.empty()) {
+    // Only a 206 response is served as a partial representation. That is the
+    // condition `apply_ranges()` used when it decided the Content-Length and
+    // the multipart boundary, and `detail::range_error()` only validates
+    // `req.ranges` against the content length for a 2xx status. Honouring the
+    // ranges here under any other status would write a body that disagrees
+    // with the header we already sent, from an offset nothing has checked.
+    auto partial =
+        !req.ranges.empty() && res.status == StatusCode::PartialContent_206;
+
+    if (!partial) {
       return detail::write_content(strm, res.content_provider_, 0,
                                    res.content_length_, is_shutting_down);
     } else if (req.ranges.size() == 1) {