CMake: Fix QT_ADDITIONAL_PACKAGES_PREFIX_PATH for cross-builds

The QT_ADDITIONAL_PACKAGES_PREFIX_PATH variable was introduced to
allow specifying extra locations to find Qt packages.

The reason it was introduced instead of just using CMAKE_PREFIX_PATH
is because the Qt6 component find_package call uses NO_DEFAULT_PATH
which means CMAKE_PREFIX_PATH is ignored.
We use NO_DEFAULT_PATH to ensure we don't accidentally pick up
system / distro Qt packages.

The paths from QT_ADDITIONAL_PACKAGES_PREFIX_PATH are added to the
find_package PATHS option in the Qt6 package, each
ModuleDependencies.cmake file and some other places.

Unfortunately that's not enough to make it work for cross-builds.

Imagine the following scenario.

  host qtbase, qtdeclarative installed in /host_qt
  target qtbase installed in /target_qtbase
  target qtdeclarative installed in /target_qtdeclarative

We want to cross-build qtlottie.

We configure qtlottie as follows
 /target_qtbase/bin/qt-configure-module /qtlottie_src -- -DQT_ADDITIONAL_PACKAGES_PREFIX_PATH=/target_qtdeclarative

We expect the target QtQuick package to be found, but it won't be.

The reason is that QT_ADDITIONAL_PACKAGES_PREFIX_PATH is added to the
PATHs option, but we don't adjust CMAKE_FIND_ROOT_PATH.

Without adding the new paths in CMAKE_FIND_ROOT_PATH, CMake will
re-root the passed PATHs under the existing CMAKE_FIND_ROOT_PATH,
which is QT_TOOLCHAIN_RELOCATABLE_INSTALL_PREFIX, which evaluates to
/target_qtbase. There is no QtQuick package there.

To fix this, prepend the values of QT_ADDITIONAL_PACKAGES_PREFIX_PATH
to CMAKE_FIND_ROOT_PATH.

The location where we currently do CMAKE_FIND_ROOT_PATH manipulations
is in the qt.toolchain.cmake file, so to be consistent, we prepend the
new prefixes there as well.

We need to adjust both CMAKE_FIND_ROOT_PATH and CMAKE_PREFIX_PATH,
due the path re-rooting bug in CMake.
See https://gitlab.kitware.com/cmake/cmake/-/issues/21937 as well as
the existing comment in qt.toolchain.cmake marked with
REROOT_PATH_ISSUE_MARKER.

We also need to do a few more things to make the setup work

Because Qt6Config uses NO_DEFAULT_PATH, the CMAKE_PREFIX_PATH
adjustments we do in the toolchain file are not enough, so we still need
to add the same prefixes to the Qt6Config find_package PATHS option.

One would ask why do we need to adjust CMAKE_PREFIX_PATH at all then.
It's for find_package(Qt6Foo) calls to work which don't go through
the Qt6Config umbrella package.

To make the CMake re-rooting behavior happy, we need to ensure the
provided paths are absolute.
So we iterate over the values of QT_ADDITIONAL_PACKAGES_PREFIX_PATH,
to make them absolute. We do the same for the environment variable.

We need to append lib/cmake to the prefixes which are added to
CMAKE_PREFIX_PATH, otherwise the CMake re-rooting bug is hit.

We need to specify the Qt6 package location (${_qt_cmake_dir}) to the
PATHS option in the various Dependencies.cmake.in files, to ensure
that dependency resolution can jump around between the Qt6 dir and
the additional prefixes. Previously the dependency lookup code assumed
that all dependencies would be within the same prefix.

The same is needed for qt and qml plugin dependency lookup.

Amends 7bb91398f25cb2016c0558fd397b376f413e3e96
Amends 60c87c68016c6f02b0eddd4002f75a49ab51d4a8
Amends 5bbd700124d13a292ff8bae6045316112500e230

Pick-to: 6.2
Fixes: QTBUG-95854
Change-Id: I35ae82330fec427d0d38fc9a0542ffafff52556a
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 5 insertions, 2 deletions

diff --git a/cmake/QtPostProcessHelpers.cmake b/cmake/QtPostProcessHelpers.cmake
index 912a074422..4c898c5378 100644
--- a/cmake/QtPostProcessHelpers.cmake
+++ b/cmake/QtPostProcessHelpers.cmake
@@ -292,12 +292,15 @@ function(qt_internal_create_plugin_depends_file target)
 
     if(third_party_deps OR target_deps)
         # Setup build and install paths
-        set(find_dependency_paths "\${CMAKE_CURRENT_LIST_DIR}/..")
+
+        # Plugins should look for their dependencies in their associated module package folder as
+        # well as the Qt6 package folder which is stored by the Qt6 package in _qt_cmake_dir.
+        set(find_dependency_paths "\${CMAKE_CURRENT_LIST_DIR}/..;\${_qt_cmake_dir}")
         if(plugin_install_package_suffix)
             set(path_suffix "${INSTALL_CMAKE_NAMESPACE}${plugin_install_package_suffix}")
             if(plugin_install_package_suffix MATCHES "/QmlPlugins")
                 # Qml plugins are one folder deeper.
-                set(find_dependency_paths "\${CMAKE_CURRENT_LIST_DIR}/../..")
+                set(find_dependency_paths "\${CMAKE_CURRENT_LIST_DIR}/../..;\${_qt_cmake_dir}")
             endif()
 
         else()