Flutter send streamed multipart request not finishing and causing segfault - flutter

Question

I'm trying to send a multipart request via http.StreamedRequest. I am aware of http.MultipartRequest, however, it does not fit my needs (the API I'm trying to send the request to encodes arrays by sending the same attribute multiple times, which is not possible using the fields parameter of http.MultipartRequest since it is a Map<String, String>, which does not allow the same key to be inserted multiple times).
The file (with form-data name document) I'm trying to send is provided in form of a Uint8List (a .pdf file), in addition to a filename (in the following "some_filename.pdf", a pdf title (in the following "some_document_title") and an array of numbers (which I'm trying to add to the multipart request) with the attribute tags.
For instance, I want the array tags=[1, 2, 3], title="some_document_title" and file to be encoded as follows:
--dart-http-boundary-rVI-TO32QXZlS.VAGKWpuhbt99woLKVVvjToGoT-MVOU1YGSpnQ
Content-Disposition: form-data; name="title"
Content-type: text/plain
some_document_title
--dart-http-boundary-rVI-TO32QXZlS.VAGKWpuhbt99woLKVVvjToGoT-MVOU1YGSpnQ
Content-Disposition: form-data; name="tags"
Content-type: text/plain
1
--dart-http-boundary-rVI-TO32QXZlS.VAGKWpuhbt99woLKVVvjToGoT-MVOU1YGSpnQ
Content-Disposition: form-data; name="tags"
Content-type: text/plain
2
--dart-http-boundary-rVI-TO32QXZlS.VAGKWpuhbt99woLKVVvjToGoT-MVOU1YGSpnQ
Content-Disposition: form-data; name="tags"
Content-type: text/plain
3
--dart-http-boundary-rVI-TO32QXZlS.VAGKWpuhbt99woLKVVvjToGoT-MVOU1YGSpnQ
Content-Disposition: form-data; name="document"; filename="some_filename.pdf"
Content-type: application/octet-stream
After this part, I want to stream the file data.
The rough implementation looks like this:
Future<void> createDocument(Uint8List file, String filename, String title, List<int> tags) {
final StreamedRequest request = StreamedRequest('POST', Uri.parse('some_url'));
String boundary = "..."; // Generate random boundary string
String contentType = "application/octet-stream";
String body = "";
Map<String, String> fields = {
"title": title,
// Some more fields left out for readability
};
for (final fieldKey in fields.keys) {
body += _buildMultipartField(fieldKey, fields[fieldKey], boundary);
}
for (final tag in tags) {
body += _buildMultipartField('tags', tag.toString(), boundary);
}
// Prepare file part
body += "--$boundary" +
'\r\nContent-Disposition: form-data; name="document"; filename="$filename"' +
"\r\nContent-type: $contentType" +
"\r\n\r\n";
final closing = "\r\n--" + boundary + "--\r\n";
request.headers.addAll({
"Content-Type": "multipart/form-data; boundary=$boundary",
"Content-Length": "${body.length + closing.length + file.lengthInBytes}"
});
// Add current body
request.sink.add(utf8.encode(body));
Stream.fromIterable(file).listen((chunk) => request.sink.add([chunk]))
.onDone(() {
// Add closing to multipart request
request.sink.add(utf8.encode(closing));
request.sink.close();
});
final StreamdResponse response = await IOClient().send(request);
if (response.statusCode != 200) {
throw Error();
}
}
String _buildMultipartField(String fieldName, String value, String boundary) {
return '--$boundary' +
'\r\nContent-Disposition: form-data; name="$fieldName"' +
'\r\nContent-type: text/plain' +
'\r\n\r\n' +
value +
'\r\n';
}
Now, my current implementation produces the expected body, which is good, however, the streamed request does not seem to finish, and according to the DevTools Network page, the request is never sent.
Is the content-length relevant for the request to finish? I.e. is it possible that the request is not finishing because the server is still expecting data because the content-length is calculated incorrectly?
Also, I'm getting a segfault when hot restarting the application when the request is still pending (it never returns).
Any help is appreciated, thanks!