QNetworkAccessManager re-sends destructive requests (POST, possibly others) without user intervention

When using QNAM to send a POST request, if there's an unexpected EOF or an error in the server's reply, QNAM sends the request again.

For instance:

    QNetworkAccessManager qnam;
    QNetworkRequest req(QUrl("http://localhost:12345/service"));
    auto reply = qnam.post(req, "hello\n");

Run a local faux server with something like nc -l 12345 -k. When the request arrives, type something into nc and press return – QNAM will re-send the POST.

A quick analysis of the problem:

QHttpProtocolHandler::_q_receiveReply will call m_channel->handleUnexpectedEOF(); in a number of cases. The common case seems to be if the server side times out and closes the connection after having sent a partial reply.¹

QHttpNetworkConnectionChannel::handleUnexpectedEOF will then try again, for a maximum of 4 times, before giving up. It will however not check if it is allowed to re-send the request (i.e. the request is idempotent). This will cause a POST to be sent again, causing data losses.

¹ But also in case of garbage, like the demo above shows. For instance if you just type nonsense, QHttpHeaderParser::parseStatus returns false. That causes QHttpNetworkReplyPrivate::readStatus to return -1:

            bool ok = parseStatus(fragment);
            state = ReadingHeaderState;
            fragment.clear();
            if (!ok) {
                return -1;
            }

but -1 is for EOF, not for parsing errors! This will also end up in QHttpNetworkConnectionChannel::handleUnexpectedEOF, and eventually cause it to emit the wrong error (RemoteHostClosedError, instead of a protocol-level error – "server sent garbage").