CMake: Properly handle CONFIG += thread aka Threads::Threads

mkspecs/features/qt.prf adds a dependency on the system threading
library if the Qt Core thread feature is enabled. Because qt.prf is
loaded by any public or internal Qt project, it's essentially a public
dependency for any Qt consumer.

To mimic that in CMake, we check if the thread feature is enabled, and
and set the Threads::Threads library as a dependency of Qt6::Platform,
which is a public target used by all Qt modules and plugins and Qt
consumers.

We also need to create a Qt6Dependencies.cmake file so we
find_package(Threads) every time find_package(Qt6) is called.

For the .prl files to be usable, we have to filter out some
CMake implementation specific directory separator tokens
'CMAKE_DIRECTORY_ID_SEP' aka '::@', which are added because we call
target_link_libraries() with a target created in a different scope
(I think).

As a result of this change, we shouldn't have to hardcode
Threads::Threads in other projects, because it's now a global public
dependency.

Task-number: QTBUG-85801
Task-number: QTBUG-85877
Change-Id: Ib5d662c43b28e63f7da49d3bd77d0ad751220b31
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

cmake/QtBaseGlobalTargets.cmake

@@ -353,6 +353,7 @@ qt_copy_or_install(FILES
                    cmake/QtBuildInformation.cmake
                    cmake/QtCompilerFlags.cmake
                    cmake/QtCompilerOptimization.cmake
+                   cmake/QtConfigDependencies.cmake.in
                    cmake/QtFeature.cmake
                    cmake/QtFinishPrlFile.cmake
                    cmake/QtFindWrapHelper.cmake

cmake/QtBuild.cmake

@@ -3256,7 +3256,12 @@ function(qt_internal_walk_libs target out_var dict_name operation)
                 set(lib_target ${lib})
             endif()
 
-            if(TARGET ${lib_target})
+            # Skip CMAKE_DIRECTORY_ID_SEP. If a target_link_libraries is applied to a target
+            # that was defined in a different scope, CMake appends and prepends a special directory
+            # id separator. Filter those out.
+            if(lib_target MATCHES "^::@")
+                continue()
+            elseif(TARGET ${lib_target})
                 if ("${lib_target}" MATCHES "^Qt::(.*)")
                     # If both, Qt::Foo and Foo targets exist, prefer the target name without
                     # namespace. Which one is preferred doesn't really matter. This code exists to

cmake/QtConfig.cmake.in

@@ -49,6 +49,19 @@ if(APPLE AND (NOT CMAKE_SYSTEM_NAME OR CMAKE_SYSTEM_NAME STREQUAL "Darwin"))
     list(APPEND CMAKE_MODULE_PATH "${_qt_import_prefix}/macos")
 endif()
 
+# Find required dependencies, if any.
+include(CMakeFindDependencyMacro)
+set(@INSTALL_CMAKE_NAMESPACE@_DEPENDENCIES_FOUND TRUE)
+if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/@INSTALL_CMAKE_NAMESPACE@Dependencies.cmake")
+    include("${CMAKE_CURRENT_LIST_DIR}/@INSTALL_CMAKE_NAMESPACE@Dependencies.cmake")
+endif()
+if(NOT @INSTALL_CMAKE_NAMESPACE@_DEPENDENCIES_FOUND)
+    set(@INSTALL_CMAKE_NAMESPACE@_FOUND False)
+    message(FATAL_ERROR
+    "Failed to find Qt Platform dependency:
+    ${@INSTALL_CMAKE_NAMESPACE@_DEPENDENCY_NOT_FOUND_MESSAGE}")
+endif()
+
 foreach(module ${@INSTALL_CMAKE_NAMESPACE@_FIND_COMPONENTS})
     find_package(@INSTALL_CMAKE_NAMESPACE@${module}
         ${_@INSTALL_CMAKE_NAMESPACE@_FIND_PARTS_QUIET}

cmake/QtConfigDependencies.cmake.in

@@ -0,0 +1,35 @@
+# note: _third_party_deps example: "ICU\\;1.0\\;i18n uc data;ZLIB\\;\\;"
+set(_third_party_deps "@third_party_deps@")
+
+@third_party_extra@
+
+foreach(_target_dep ${_third_party_deps})
+    list(GET _target_dep 0 pkg)
+    list(GET _target_dep 1 version)
+    list(GET _target_dep 2 components)
+    set(find_package_args "${pkg}")
+    if(version)
+        list(APPEND find_package_args "${version}")
+    endif()
+
+    if(components)
+        string(REPLACE " " ";" components "${components}")
+        find_dependency(${find_package_args} COMPONENTS ${components})
+    else()
+        find_dependency(${find_package_args})
+    endif()
+
+    if (NOT ${pkg}_FOUND)
+        set(@INSTALL_CMAKE_NAMESPACE@_DEPENDENCIES_FOUND FALSE)
+        set(__@INSTALL_CMAKE_NAMESPACE@_message "\nPackage: ${pkg}")
+        if(version)
+            string(APPEND __@INSTALL_CMAKE_NAMESPACE@_message "\nVersion: ${version}")
+        endif()
+        if(components)
+            string(APPEND __@INSTALL_CMAKE_NAMESPACE@_message "\nComponents: ${components}")
+        endif()
+        set(@INSTALL_CMAKE_NAMESPACE@_DEPENDENCY_NOT_FOUND_MESSAGE
+            "${__@INSTALL_CMAKE_NAMESPACE@_message}")
+        return()
+    endif()
+endforeach()

cmake/QtPostProcess.cmake

@@ -262,10 +262,57 @@ function(qt_internal_create_plugin_depends_file target)
     endif()
 endfunction()
 
+function(qt_internal_create_qt6_dependencies_file)
+    # This is used for substitution in the configured file.
+    set(target "${INSTALL_CMAKE_NAMESPACE}")
+
+    # This is the actual target we're querying.
+    set(actual_target Platform)
+    get_target_property(public_depends "${actual_target}" INTERFACE_LINK_LIBRARIES)
+
+    # We need to collect third party deps that are set on the public Platform target,
+    # like Threads::Threads.
+    # This mimics find_package part of the CONFIG += thread assignment in mkspecs/features/qt.prf.
+    qt_collect_third_party_deps(${actual_target})
+
+    # For Threads we also need to write an extra variable assignment.
+    set(third_party_extra "")
+    if(third_party_deps MATCHES "Threads")
+        string(APPEND third_party_extra "if(NOT QT_NO_THREADS_PREFER_PTHREAD_FLAG)
+    set(THREADS_PREFER_PTHREAD_FLAG TRUE)
+endif()")
+    endif()
+
+    if(third_party_deps)
+        # Setup build and install paths.
+        set(path_suffix "${INSTALL_CMAKE_NAMESPACE}")
+
+        qt_path_join(config_build_dir ${QT_CONFIG_BUILD_DIR} ${path_suffix})
+        qt_path_join(config_install_dir ${QT_CONFIG_INSTALL_DIR} ${path_suffix})
+
+        # Configure and install QtDependencies file.
+        configure_file(
+            "${QT_CMAKE_DIR}/QtConfigDependencies.cmake.in"
+            "${config_build_dir}/${target}Dependencies.cmake"
+            @ONLY
+        )
+
+        qt_install(FILES
+            "${config_build_dir}/${target}Dependencies.cmake"
+            DESTINATION "${config_install_dir}"
+            COMPONENT Devel
+        )
+    endif()
+endfunction()
+
 # Create Depends.cmake & Depends.h files for all modules and plug-ins.
 function(qt_internal_create_depends_files)
     qt_internal_get_qt_repo_known_modules(repo_known_modules)
 
+    if(PROJECT_NAME STREQUAL "QtBase")
+        qt_internal_create_qt6_dependencies_file()
+    endif()
+
     foreach (target ${repo_known_modules})
         qt_internal_create_module_depends_file(${target})
     endforeach()

src/corelib/CMakeLists.txt

@@ -249,7 +249,6 @@ qt_add_module(Core
         ../3rdparty/tinycbor/src
     LIBRARIES
         Qt::GlobalConfigPrivate # special case
-        Threads::Threads # special case
     PRECOMPILED_HEADER
         "global/qt_pch.h"
     NO_PCH_SOURCES
@@ -276,6 +275,13 @@ endif()
 
 qt_generate_qconfig_cpp()
 
+# Handle qtConfig(thread): CONFIG += thread like in qt.prf.
+# Aka if the feature is enabled, publically link against the threading library.
+# This also ensures the link flag is in the .prl file.
+if(QT_FEATURE_thread)
+    target_link_libraries(Platform INTERFACE Threads::Threads)
+endif()
+
 # Handle QObject: Automoc does not work for this as it would
 # require to spill internals into users:
 qt_add_module(Core_qobject STATIC

src/plugins/sqldrivers/sqlite/CMakeLists.txt

@@ -33,7 +33,6 @@ qt_extend_target(QSQLiteDriverPlugin CONDITION QT_FEATURE_system_sqlite
 
 # special case begin
 if (NOT QT_FEATURE_system_sqlite)
-    qt_find_package(Threads REQUIRED PROVIDED_TARGETS Threads::Threads)
     # On newer compilers compiling sqlite.c produces warnings
     qt_disable_warnings(QSQLiteDriverPlugin)
 endif()
@@ -52,8 +51,6 @@ qt_extend_target(QSQLiteDriverPlugin CONDITION NOT QT_FEATURE_system_sqlite
         SQLITE_OMIT_COMPLETE
     INCLUDE_DIRECTORIES
         ../../../3rdparty/sqlite
-    PUBLIC_LIBRARIES # special case
-        Threads::Threads # special case
 )
 
 qt_extend_target(QSQLiteDriverPlugin CONDITION CMAKE_BUILD_TYPE STREQUAL Release AND NOT QT_FEATURE_system_sqlite

src/testlib/CMakeLists.txt

@@ -62,7 +62,6 @@ qt_add_module(Test
         QT_BUILD_TESTLIB_LIB # special case
     LIBRARIES
         Qt::CorePrivate
-        Threads::Threads # special case
     PUBLIC_LIBRARIES
         Qt::Core
     PRIVATE_MODULE_INTERFACE

tests/CMakeLists.txt

@@ -6,7 +6,6 @@ if(QT_BUILD_STANDALONE_TESTS)
     # special case begin
     qt_find_package(WrapDBus1 PROVIDED_TARGETS dbus-1)
     qt_find_package(ICU COMPONENTS i18n uc data PROVIDED_TARGETS ICU::i18n ICU::uc ICU::data)
-    qt_find_package(Threads PROVIDED_TARGETS Threads::Threads)
     qt_find_package(WrapOpenSSL PROVIDED_TARGETS WrapOpenSSL::WrapOpenSSL)
     qt_find_package(WrapOpenSSLHeaders PROVIDED_TARGETS WrapOpenSSLHeaders::WrapOpenSSLHeaders)
     # special case end

tests/auto/corelib/kernel/qmetatype/CMakeLists.txt

@@ -16,7 +16,6 @@ qt_add_test(tst_qmetatype
         ../../../other/qvariant_common
     PUBLIC_LIBRARIES
         Qt::CorePrivate
-        Threads::Threads # special case
     TESTDATA ${test_data}
 )
 

tests/auto/corelib/thread/qpromise/CMakeLists.txt

@@ -9,6 +9,4 @@ qt_add_test(tst_qpromise
         tst_qpromise.cpp
     PUBLIC_LIBRARIES
         Qt::CorePrivate
-    LIBRARIES # special case
-        Threads::Threads  # solves issue with libpthread linkage # special case
 )

tests/auto/corelib/thread/qthread/CMakeLists.txt

@@ -10,8 +10,6 @@ qt_add_test(tst_qthread
         tst_qthread.cpp
     INCLUDE_DIRECTORIES
         ../../../../shared
-    LIBRARIES # special case
-        Threads::Threads # special case
 )
 
 ## Scopes:

tests/auto/corelib/thread/qthreadstorage/CMakeLists.txt

@@ -8,9 +8,6 @@
 add_qt_test(tst_qthreadstorage
     SOURCES
         tst_qthreadstorage.cpp
-    LIBRARIES # special case
-        Threads::Threads # special case
-
 )
 
 ## Scopes:

tests/auto/gui/kernel/qguiapplication/CMakeLists.txt

@@ -28,7 +28,6 @@ qt_add_test(tst_qguiapplication
     PUBLIC_LIBRARIES
         Qt::CorePrivate
         Qt::GuiPrivate
-        Threads::Threads # special case
 )
 
 # Resources:

tests/auto/gui/kernel/qguitimer/CMakeLists.txt

@@ -10,5 +10,4 @@ qt_add_test(tst_qguitimer
     PUBLIC_LIBRARIES
         Qt::CorePrivate
         Qt::Gui
-        Threads::Threads # special case
 )

tests/auto/network/socket/qtcpsocket/test/CMakeLists.txt

@@ -21,12 +21,6 @@ qt_extend_target(tst_qtcpsocket CONDITION WIN32
         ws2_32
 )
 
-# special case begin
-set(THREADS_PREFER_PTHREAD_FLAG 1)
-find_package(Threads)
-target_link_libraries(tst_qtcpsocket PRIVATE Threads::Threads)
-# special case end
-
 #### Keys ignored in scope 4:.:.:test.pro:(CMAKE_BUILD_TYPE STREQUAL Debug):
 # DESTDIR = "../debug"
 

tests/benchmarks/corelib/kernel/qtimer_vs_qmetaobject/CMakeLists.txt

@@ -11,7 +11,6 @@ qt_add_benchmark(qtimer_vs_qmetaobject
         .
     PUBLIC_LIBRARIES
         Qt::Test
-        Threads::Threads # special case
 )
 
 #### Keys ignored in scope 1:.:.:qtimer_vs_qmetaobject.pro:<TRUE>: