Fix excessive invalidation of QML disk caches

We use an MD5 checksum over the meta-object data to verify that the
types a QML file depends on haven't changed since the cache was
generated. However when the dependent types are QML types, then the
meta-object data contains dynamically generated type names such as
QMLTYPE_1234, which is non-deterministic. To address this, we resort to
the checksum over the meta-object data only for C++ types (if those
change it's likely an incompatible change) and for QML types use the
fact that all the information about the QML declared types comes from
the QML file only, which means we can in that case simply use a checksum
over the QV4::CompiledData memory chunk.

In addition we need to ensure that the generated CompiledData memory chunk
is deterministic by avoiding any uninitialized bytes (memset) and using
a map instead of a hash for the mapping of object index to object id.

Task-number: QTBUG-55926
Change-Id: I27c840b1960ad36b486198e504b70989c22a3972
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/qml/qml/qqmlpropertycache.cpp b/src/qml/qml/qqmlpropertycache.cpp
index f75ab9e25b..2610a807b5 100644
--- a/src/qml/qml/qqmlpropertycache.cpp
+++ b/src/qml/qml/qqmlpropertycache.cpp
@@ -1429,6 +1429,12 @@ QByteArray QQmlPropertyCache::checksum(bool *ok)
         return _checksum;
     }
 
+    // Generate a checksum on the meta-object data only on C++ types.
+    if (!_metaObject || _ownMetaObject) {
+        *ok = false;
+        return _checksum;
+    }
+
     QCryptographicHash hash(QCryptographicHash::Md5);
 
     if (_parent) {