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>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/network/access/qhttpthreaddelegate.cpp b/src/network/access/qhttpthreaddelegate.cpp
index 50a14b6258..243492d466 100644
--- a/src/network/access/qhttpthreaddelegate.cpp
+++ 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());