H2C - make sure we send the client preface and settings

It's required as a response to upgraded protocol and apparently some
servers would wait for it, not sending any frames. Becomes a problem
in case only one request was sent.

Fixes: QTBUG-83312
Change-Id: I90dc5c04095f0b78baa404466625d329dc4c6e21
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/access/qhttp2protocolhandler.cpp

@@ -220,6 +220,12 @@ void QHttp2ProtocolHandler::handleConnectionClosure()
     goingAway = true;
 }
 
+void QHttp2ProtocolHandler::ensureClientPrefaceSent()
+{
+    if (!prefaceSent)
+        sendClientPreface();
+}
+
 void QHttp2ProtocolHandler::_q_uploadDataReadyRead()
 {
     if (!sender()) // QueuedConnection, firing after sender (byte device) was deleted.

src/network/access/qhttp2protocolhandler_p.h

@@ -93,6 +93,7 @@ public:
     QHttp2ProtocolHandler &operator = (QHttp2ProtocolHandler &&rhs) = delete;
 
     Q_INVOKABLE void handleConnectionClosure();
+    Q_INVOKABLE void ensureClientPrefaceSent();
 
 private slots:
     void _q_uploadDataReadyRead();

src/network/access/qhttpnetworkconnectionchannel.cpp

@@ -488,6 +488,9 @@ void QHttpNetworkConnectionChannel::allDone()
             QHttp2ProtocolHandler *h2c = static_cast<QHttp2ProtocolHandler *>(protocolHandler.data());
             QMetaObject::invokeMethod(h2c, "_q_receiveReply", Qt::QueuedConnection);
             QMetaObject::invokeMethod(connection, "_q_startNextRequest", Qt::QueuedConnection);
+            // If we only had one request sent with H2 allowed, we may fail to send
+            // a client preface and SETTINGS, which is required by RFC 7540, 3.2.
+            QMetaObject::invokeMethod(h2c, "ensureClientPrefaceSent", Qt::QueuedConnection);
             return;
         } else {
             // Ok, whatever happened, we do not try HTTP/2 anymore ...