Introduce protobuf-unsafe-registry feature

The feature guards the private code paths that enable or disable
the safe registering of the custom protobuf serializers.
The feature is useful when user want to trade the thread safety for
performance.

From the code perspective feature disables read/write locks when
accessing custom protobuf serializers registered privately.

Change-Id: I07cb622481a798e7aeba9c7afb29593e791e065e
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>

src/protobuf/configure.cmake

@@ -87,11 +87,20 @@ qt_feature("protobufquick" PUBLIC
     AUTODETECT TRUE
     CONDITION TARGET Qt6::Quick
 )
+
+qt_feature("protobuf-unsafe-registry" PRIVATE
+    SECTION "Qt Protobuf"
+    LABEL "Unsafe registry"
+    PURPOSE "Allows lock-free registry access for custom serializers"
+    AUTODETECT FALSE
+)
+
 qt_configure_add_summary_section(NAME "Qt Protobuf")
 qt_configure_add_summary_entry(ARGS "protobuf-qtcoretypes")
 qt_configure_add_summary_entry(ARGS "protobuf-qtguitypes")
 qt_configure_add_summary_entry(ARGS "protobuf-wellknowntypes")
 qt_configure_add_summary_entry(ARGS "protobufquick")
+qt_configure_add_summary_entry(ARGS "protobuf-unsafe-registry")
 qt_configure_end_summary_section()
 qt_configure_add_summary_section(NAME "Qt Protobuf tools")
 qt_configure_add_summary_entry(ARGS "qtprotobufgen")

src/protobuf/qprotobufjsonserializer.cpp

@@ -3,6 +3,7 @@
 
 #include <QtProtobuf/qprotobufjsonserializer.h>
 
+#include <QtProtobuf/private/qtprotobuf-config_p.h>
 #include <QtProtobuf/private/protobuffieldpresencechecker_p.h>
 #include <QtProtobuf/private/protobufscalarjsonserializers_p.h>
 #include <QtProtobuf/private/qprotobufdeserializerbase_p.h>
@@ -18,7 +19,9 @@
 #include <QtCore/qjsonarray.h>
 #include <QtCore/qjsondocument.h>
 #include <QtCore/qjsonobject.h>
-#include <QtCore/qreadwritelock.h>
+#if !QT_CONFIG(protobuf_unsafe_registry)
+#    include <QtCore/qreadwritelock.h>
+#endif
 #include <QtCore/qtimezone.h>
 #include <QtCore/qvariant.h>
 
@@ -52,13 +55,17 @@ struct JsonHandlerRegistry
     void registerHandler(QMetaType metaType, QtProtobufPrivate::CustomJsonSerializer serializer,
                          QtProtobufPrivate::CustomJsonDeserializer deserializer)
     {
+#if !QT_CONFIG(protobuf_unsafe_registry)
         QWriteLocker locker(&m_lock);
+#endif
         m_registry[metaType] = { serializer, deserializer };
     }
 
     QtProtobufPrivate::CustomJsonSerializer findSerializer(QMetaType metaType)
     {
+#if !QT_CONFIG(protobuf_unsafe_registry)
         QReadLocker locker(&m_lock);
+#endif
         const auto it = m_registry.constFind(metaType);
         if (it != m_registry.constEnd())
             return it.value().first;
@@ -67,7 +74,9 @@ struct JsonHandlerRegistry
 
     QtProtobufPrivate::CustomJsonDeserializer findDeserializer(QMetaType metaType)
     {
+#if !QT_CONFIG(protobuf_unsafe_registry)
         QReadLocker locker(&m_lock);
+#endif
         const auto it = m_registry.constFind(metaType);
         if (it != m_registry.constEnd())
             return it.value().second;
@@ -77,7 +86,9 @@ struct JsonHandlerRegistry
 private:
     using Handler = std::pair<QtProtobufPrivate::CustomJsonSerializer,
                               QtProtobufPrivate::CustomJsonDeserializer>;
+#if !QT_CONFIG(protobuf_unsafe_registry)
     QReadWriteLock m_lock;
+#endif
     QHash<QMetaType, Handler> m_registry;
 };
 Q_GLOBAL_STATIC(JsonHandlerRegistry, jsonHandlersRegistry)

src/protobuf/qprotobufregistration.cpp

@@ -3,12 +3,15 @@
 
 #include <QtProtobuf/qprotobufregistration.h>
 
+#include <QtProtobuf/private/qtprotobuf-config_p.h>
 #include <QtProtobuf/private/qprotobufregistration_p.h>
 #include <QtProtobuf/qtprotobuftypes.h>
 
 #include <QtCore/qmutex.h>
 #include <QtCore/qmetatype.h>
-#include <QtCore/qreadwritelock.h>
+#if !QT_CONFIG(protobuf_unsafe_registry)
+#   include <QtCore/qreadwritelock.h>
+#endif
 
 #include <mutex>
 
@@ -122,13 +125,17 @@ struct HandlersRegistry
     void registerHandler(QMetaType type, QtProtobufPrivate::Serializer serializer,
                          QtProtobufPrivate::Deserializer deserializer)
     {
+#if !QT_CONFIG(protobuf_unsafe_registry)
         QWriteLocker locker(&m_lock);
+#endif
         m_registry[type] = { serializer, deserializer };
     }
 
     QtProtobufPrivate::SerializationHandler findHandler(QMetaType type)
     {
+#if !QT_CONFIG(protobuf_unsafe_registry)
         QReadLocker locker(&m_lock);
+#endif
         auto it = m_registry.constFind(type);
         if (it != m_registry.constEnd())
             return it.value();
@@ -136,7 +143,9 @@ struct HandlersRegistry
     }
 
 private:
+# if !QT_CONFIG(protobuf_unsafe_registry)
     QReadWriteLock m_lock;
+# endif
     QHash<QMetaType, QtProtobufPrivate::SerializationHandler> m_registry;
 };
 Q_GLOBAL_STATIC(HandlersRegistry, handlersRegistry)