QNAM: Disable h2c by default

And since it's relatively unlikely to be used, just leave it
behind a environment variable for now.

[ChangeLog][QtNetwork][Potentially Source-Incompatible] Support for
clear-text http/2 was disabled due to incompatibility with certain
servers. If you were relying on this feature you must re-enable it by
setting the QT_NETWORK_ALLOW_H2C environment variable. For a later
version of Qt it will get a dedicated attribute.

Pick-to: 6.2
Task-number: QTBUG-98642
Change-Id: Id3e360726e285b3128e3e3f4bce9440404c9ad6e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Maximilian Goldstein <max.goldstein@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/access/qhttpnetworkrequest.cpp

@@ -365,6 +365,16 @@ void QHttpNetworkRequest::setHTTP2Direct(bool b)
     d->http2Direct = b;
 }
 
+bool QHttpNetworkRequest::isH2cAllowed() const
+{
+    return qEnvironmentVariableIsSet("QT_NETWORK_H2C_ALLOWED");
+}
+
+void QHttpNetworkRequest::setH2cAllowed(bool b)
+{
+    Q_UNUSED(b);
+}
+
 bool QHttpNetworkRequest::withCredentials() const
 {
     return d->withCredentials;

src/network/access/qhttpnetworkrequest_p.h

@@ -125,6 +125,9 @@ public:
     bool isHTTP2Direct() const;
     void setHTTP2Direct(bool b);
 
+    bool isH2cAllowed() const;
+    void setH2cAllowed(bool b);
+
     bool withCredentials() const;
     void setWithCredentials(bool b);
 

src/network/access/qhttpthreaddelegate.cpp

@@ -290,6 +290,12 @@ void QHttpThreadDelegate::startRequest()
         connectionType = QHttpNetworkConnection::ConnectionTypeHTTP2Direct;
     }
 
+    // Use HTTP/1.1 if h2c is not allowed and we would otherwise choose to use it
+    if (!ssl && connectionType == QHttpNetworkConnection::ConnectionTypeHTTP2
+        && !httpRequest.isH2cAllowed()) {
+        connectionType = QHttpNetworkConnection::ConnectionTypeHTTP;
+    }
+
 #if QT_CONFIG(ssl)
     // See qnetworkreplyhttpimpl, delegate's initialization code.
     Q_ASSERT(!ssl || incomingSslConfiguration.data());

tests/auto/network/access/http2/tst_http2.cpp

@@ -223,6 +223,7 @@ tst_Http2::~tst_Http2()
 void tst_Http2::init()
 {
     manager.reset(new QNetworkAccessManager);
+    qputenv("QT_NETWORK_H2C_ALLOWED", "1");
 }
 
 void tst_Http2::singleRequest_data()