QNetworkReply error "Unable to write"

[Sheep]

Greetings to you all
Am trying to upload some file to a remote server using some api (Baidu Net Disk) but i keep getting QNetworkReply::UnknownNetworkError as the error and "Unable to write" as the error string.
This only happens when uploading a file greater than 4MBs ..but for small files (~ 1 mb) everything is fine.
Am using QHttpMultiPart and QHttpPart to upload the file.

First off is there some way i can get a more details error message than just Unable to write .

 QString formatedMd5;
// fileChunkSize holds the content of the file divided into segments ..each segment is 4mb or less 
  foreach (auto hash, mFileChuncks) {

    if (hash != mFileChuncks.last()) {
      formatedMd5.append(QString("\"%1\",").arg(
          QCryptographicHash::hash((hash), QCryptographicHash::Md5).toHex()));
    } else {
      formatedMd5.append(QString("\"%1\"").arg(
          QCryptographicHash::hash((hash), QCryptographicHash::Md5).toHex()));
    }
  }
  qDebug() << "formated md5  " << formatedMd5;


  QString targetData =
      QString("path=/%1&isdir=0&autoinit=1&rtype=3&block_list=[%2]&size=%3")
          .arg(remotePath)
          .arg(formatedMd5)
          .arg(file->size());
  qDebug() << "  target data is" << targetData;
  QNetworkRequest request(QString("http://pan.baidu.com/rest/2.0/xpan/"
                                  "file?method=precreate&access_token=%1")
                              .arg(access_token));
  request.setHeader(QNetworkRequest::ContentTypeHeader,
                    "application/x-www-form-urlencoded");
  request.setRawHeader(QByteArray("X-LC-Session"),
                       QString("4yj8b1l5kvo9wiww2st6jdjn3").toUtf8());

  QNetworkReply *mReply = networkManager->post(request, targetData.toUtf8());
  if (mReply) {
    
    connect(mReply, &QNetworkReply::finished, this,
            [=]() { this->finished(mReply); });
  }
}

This works well when uploading small files, but fails when uploading larger files....please help me!

[JonB]

@Sheep
First make sure that your server accepts >4MB, and across QHttpMultiPart.

You might read the long recent thread https://forum.qt.io/topic/143988/using-qhttpmultipart-to-send-big-files-via-http-freezes-my-applicatio and see how they claim it works.

First off is there some way i can get a more details error message than just Unable to write .

I don't think so, Qt has reported all it can. You know it is (supposedly) not one of the other errors in https://doc.qt.io/qt-6/qnetworkreply.html#NetworkError-enum. I guess it might be something has gone wrong with >4MB, or maybe that is the error you get if the server closes the connection during client writing, I don't know. You could take a chance at looking at errno, hoping it's left from whatever Qt did to cause the write error, maybe you'll get e.g. ECONNABORTED, ENOTCONN, EPIPE, or GetLastError() code under Windows.

[Sheep]

I tried using Wireshark to track the the network packets but I noticed something like " new fragment overlaps old data"....then qt reports the " unable to write" error!..

[JonB]

@Sheep

Am using QHttpMultiPart and QHttpPart to upload the file.

Call me stupid, but where in the code?

QNetworkReply *mReply = networkManager->post(request, targetData.toUtf8());

Don't you have to call
QNetworkReply *QNetworkAccessManager::post(const QNetworkRequest &request, QHttpMultiPart *multiPart)
rather than
QNetworkReply *QNetworkAccessManager::post(const QNetworkRequest &request, const QByteArray &data)
? Or are you saying the Qt code does this for you (I don't know)?

You should perhaps also connect() to QNetworkReply::errorOccurred() especially given your situation.

Ah, I think I might actually have a hunch! You send QString targetData local variable as the post(const QByteArray &data). I wonder if it's continually reading from this after the reply is established? The post(QIODevice *data) overload tells you the file must remain open for reading, and I guess the const QByteArray &data parameter may imply the same. Try making your targetData persist (new or class variable) till the QNetworkReply::finished() signal is received.

[Sheep]

@JonB ..am really sorry i posted the wrong code..
The API has three stages of uploading the files...and the code i initially was the preparation stage... that`s why i used the QNetworkReply *QNetworkAccessManager::post(const QNetworkRequest &request, const QByteArray &data) ..am really sorry about that..

But here the code for uploading the files

// chuckIterator is a QList<QByteArray>::iterator class member variable pointing to  member a QList<QByteArray> class member
// Since the api requires to divide any files greater than 4mb  into segments of 4mb (depending on the files size)

 if (!chuckIterator.hasNext()) {
    return;
  }
  QByteArray data_ = chuckIterator.next();
 
  int partSequence = mFileChuncks.indexOf(data_);

  qDebug() << "running part sequence " << partSequence;

  QString url = QString("https://d.pcs.baidu.com/rest/2.0/pcs/"
                        "superfile2?access_token=%1&method=upload&type=tmpfile&"
                        "path=/%2&uploadid=%3&partseq=%4")
                    .arg(access_token)
                    .arg(remotePath)
                    .arg(mUploadId)
                    .arg(partSequence);
  qDebug() << "Request Url " << url;
  auto path = localPath;
  QHttpMultiPart *multiPart = new QHttpMultiPart(QHttpMultiPart::FormDataType);
  QFileInfo info(path);

  QString fileName = info.fileName();
  QHttpPart filePart;

  filePart.setHeader(QNetworkRequest::ContentTypeHeader, QVariant("file/*"));
  filePart.setHeader(
      QNetworkRequest::ContentDispositionHeader,
      QVariant(QString("form-data; "
                       "name=\"file\";fieldname=\"chapter\";filename=\"%1\"")
                   .arg(fileName)));
  filePart.setHeader(QNetworkRequest::ContentLengthHeader,data_.size());

  filePart.setBody(data_);
  file->setParent(multiPart);
  multiPart->append(filePart);

  QNetworkRequest request(url);

  QNetworkReply *mReply = networkManager->post(request, multiPart);
  if (mReply) {
  
    connect(mReply, &QNetworkReply::finished, this,
            [=]() { this->finished(mReply); });
  }