CMake: Don't cache QT_SYNCQT across builds

There is no need for this variable to be stored in CMake's cache.  We
don't perform expensive operations to set up QT_SYNCQT, and we even
unset the cache variable to ensure it gets recomputed on
reconfiguration.

We still store QT_SYNCQT in a global property, because the function
qt_ensure_sync_qt is called in different directory scopes, and we want
to avoid re-calculations for every subdir.

It's now possible for the user to set QT_SYNCQT (see QTBUG-88088 for
motivation).  Also, in a non-prefix build, changes to syncqt.pl in the
source dir are reflected upon re-configuration in the build
tree (because qt_copy_or_install is called on every configure).

Pick-to: 6.2
Fixes: QTBUG-88088
Task-number: QTBUG-75290
Change-Id: I6137b060d200d3dafd4a64d5a6c1bd2549723d78
Reviewed-by: Alexandru Croitor <[email protected]>

cmake/QtBuild.cmake

@@ -401,10 +401,6 @@ qt_internal_set_qt_known_plugins("")
 
 set(QT_KNOWN_MODULES_WITH_TOOLS "" CACHE INTERNAL "Known Qt modules with tools" FORCE)
 
-# Reset syncqt cache variable, to make sure it gets recomputed on reconfiguration, otherwise
-# it might not get installed.
-unset(QT_SYNCQT CACHE)
-
 # For adjusting variables when running tests, we need to know what
 # the correct variable is for separating entries in PATH-alike
 # variables.

cmake/QtSyncQtHelpers.cmake

@@ -14,11 +14,17 @@ function(qt_ensure_sync_qt)
         return()
     endif()
 
+    get_property(QT_SYNCQT GLOBAL PROPERTY _qt_syncqt)
+    if(NOT "${QT_SYNCQT}" STREQUAL "")
+        set(QT_SYNCQT "${QT_SYNCQT}" PARENT_SCOPE)
+        return()
+    endif()
+
     # When building qtbase, use the source syncqt, otherwise use the installed one.
     set(SYNCQT_FROM_SOURCE "${QtBase_SOURCE_DIR}/libexec/syncqt.pl")
     if(NOT ("${QtBase_SOURCE_DIR}" STREQUAL "") AND EXISTS "${SYNCQT_FROM_SOURCE}")
-        set(QT_SYNCQT "${SYNCQT_FROM_SOURCE}" CACHE FILEPATH "syncqt script")
-        message(STATUS "Using source syncqt found at: ${QT_SYNCQT}")
+        set(syncqt_absolute_path "${SYNCQT_FROM_SOURCE}")
+        message(STATUS "Using source syncqt found at: ${syncqt_absolute_path}")
 
         qt_path_join(syncqt_install_dir ${QT_INSTALL_DIR} ${INSTALL_LIBEXECDIR})
         qt_copy_or_install(PROGRAMS "${SYNCQT_FROM_SOURCE}"
@@ -27,15 +33,16 @@ function(qt_ensure_sync_qt)
         get_filename_component(syncqt_absolute_path
                                "${QT_HOST_PATH}/${QT${PROJECT_VERSION_MAJOR}_HOST_INFO_LIBEXECDIR}/syncqt.pl"
                                ABSOLUTE)
-        set(QT_SYNCQT "${syncqt_absolute_path}" CACHE FILEPATH "syncqt script")
-        message(STATUS "Using host syncqt found at: ${QT_SYNCQT}")
+        message(STATUS "Using host syncqt found at: ${syncqt_absolute_path}")
     else()
         get_filename_component(syncqt_absolute_path
                                "${QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX}/${INSTALL_LIBEXECDIR}/syncqt.pl"
                                ABSOLUTE)
-        set(QT_SYNCQT "${syncqt_absolute_path}" CACHE FILEPATH "syncqt script")
-        message(STATUS "Using installed syncqt found at: ${QT_SYNCQT}")
+        message(STATUS "Using installed syncqt found at: ${syncqt_absolute_path}")
     endif()
+
+    set(QT_SYNCQT "${syncqt_absolute_path}" PARENT_SCOPE)
+    set_property(GLOBAL PROPERTY _qt_syncqt "${syncqt_absolute_path}")
 endfunction()
 
 function(qt_install_injections target build_dir install_dir)