CMake: Introduce finalizer mode handling of static plugins

Allow linking all plugin initializer object libraries directly
into the final target (executable or shared library).

The finalizer mode is triggered when the project adds a call
to qt_import_plugins, as well when the project has an explicit
call to qt_finalize_executable or when it is defer called by
CMake 3.19+.

Otherwise the old non-finalizer mode is used, where each plugin
initializer object library is propagated via the usage
requirements of its associated module.

A user can explicitly opt in or out of the new mode by calling
qt_enable_import_plugins_finalizer_mode(target TRUE/FALSE)

The implementation, at configure time, recursively collects all
dependencies of the target to extract a list of used Qt modules.

From each module we extract its list of associated plugins and
their genex conditions. These genexes are used to conditionally
link the plugins and the initializers.

Renamed QT_PLUGINS property to _qt_plugins, so we can safely query the
property even on INTERFACE libraries with lower CMake versions.
QT_PLUGINS is kept for backwards compatibility with projects already
using it, but should be removed in Qt 7.

The upside of the finalizer mode is that it avoids creating link
cycles (e.g. Gui -> SvgPlugin -> Gui case) which causes CMake to
duplicate the library on the link line, slowing down link time as well
as possibly breaking link order dependencies.

The downside is that finalizer mode can't cope with generator
expressions at the moment. So if a Qt module target is wrapped in a
generator expression, it's plugins will not be detected and thus
linked.

Task-number: QTBUG-80863
Task-number: QTBUG-92933
Change-Id: Ic40c8ae5807a154ed18fcac18b25f00864c8f143
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 7 insertions, 1 deletions

diff --git a/cmake/QtPlugins.cmake.in b/cmake/QtPlugins.cmake.in
index 3a7b8ab30d..a96588cc5e 100644
--- a/cmake/QtPlugins.cmake.in
+++ b/cmake/QtPlugins.cmake.in
@@ -14,7 +14,10 @@ function(__qt_internal_add_static_plugins_once)
         set(_module_target ${_aliased_target})
     endif()
 
-    # Include all PluginConfig.cmake files and update the QT_PLUGINS property of the module.
+    # Include all PluginConfig.cmake files and update the _qt_plugins and QT_PLUGINS property of
+    # the module. The underscored version is the one we will use going forward to have compatibility
+    # with INTERFACE libraries. QT_PLUGINS is now deprecated and only kept so that we don't break
+    # existing projects using it (like CMake itself).
     file(GLOB _qt_plugin_config_files "${CMAKE_CURRENT_LIST_DIR}/@INSTALL_CMAKE_NAMESPACE@*PluginConfig.cmake")
     foreach(_config_file ${_qt_plugin_config_files})
         string(REGEX REPLACE "^.*/@INSTALL_CMAKE_NAMESPACE@(.*Plugin)Config.cmake$" "\\1" _qt_plugin "${_config_file}")
@@ -23,6 +26,9 @@ function(__qt_internal_add_static_plugins_once)
             list(APPEND _qt_plugins ${_qt_plugin})
         endif()
     endforeach()
+    set_property(TARGET ${_module_target} PROPERTY _qt_plugins ${_qt_plugins})
+
+    # TODO: Deprecated. Remove in Qt 7.
     set_property(TARGET ${_module_target} PROPERTY QT_PLUGINS ${_qt_plugins})
 
     get_target_property(_have_added_plugins_already ${_module_target} __qt_internal_plugins_added)