Enchance qt_find_package with opt-in debug capability

Sometimes when writing a qt_find_package and configuring a project,
you might get the following error from CMake:

CMake Error at QtBuild.cmake (set_property)
Attempt to promote imported target "WrapFreetype::WrapFreetype" to
global scope (by setting IMPORTED_GLOBAL) which is not built in this
directory.

This means that another find_package call, found WrapFreetype
in another directory scope other than the current one, and thus the
found target cat not be made global. Sometimes that implies that the
qt_find_package might not be needed if WrapFreeType will always be
found via a transitive depdendency.

By setting QT_DEBUG_QT_FIND_PACKAGE=1 on the command line, you can
make qt_find_package skip all of its behavior if the package was
already found and the provided targets were also found.

Unfortunately this behavior can not be made the default, because there
is no way to find out in what scope the package was found, and if it's
legal to make the targets global. At least I haven't found a way to do
that yet.

Thus the opt-in QT_DEBUG_QT_FIND_PACKAGE is a means to help with
debugging such cases.

Change-Id: I04242ed0f2fd0a75bc199386d28a1a0bd92da41a
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 27 insertions, 3 deletions

diff --git a/cmake/QtBuild.cmake b/cmake/QtBuild.cmake
index d3fe4bc96e..3a08ca5745 100644
--- a/cmake/QtBuild.cmake
+++ b/cmake/QtBuild.cmake
@@ -2829,6 +2829,30 @@ macro(qt_find_package)
     set(multiValueArgs PROVIDED_TARGETS COMPONENTS)
     cmake_parse_arguments(arg "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
 
+    # If some Qt internal project calls qt_find_package(WrapFreeType), but WrapFreeType was already
+    # found as part of a find_dependency() call from a ModuleDependencies.cmake file (or similar),
+    # and the provided target is also found, that means this might have been an unnecessary
+    # qt_find_package() call, because the dependency was already found via some other transitive
+    # dependency. Return early, so that CMake doesn't fail wiht an error with trying to promote the
+    # targets to be global. This behavior is not enabled by default, because there are cases
+    # when a regular find_package() (non qt_) can find a package (Freetype -> PNG), and a subsequent
+    # qt_find_package(PNG PROVIDED_TARGET PNG::PNG) still needs to succeed and register the provided
+    # targets. To enable the debugging behavior, set QT_DEBUG_QT_FIND_PACKAGE to 1.
+    set(_qt_find_package_skip_find_package FALSE)
+    if(QT_DEBUG_QT_FIND_PACKAGE AND ${ARGV0}_FOUND AND arg_PROVIDED_TARGETS)
+        set(_qt_find_package_skip_find_package TRUE)
+        foreach(qt_find_package_target_name ${arg_PROVIDED_TARGETS})
+            if(NOT TARGET ${qt_find_package_target_name})
+                set(_qt_find_package_skip_find_package FALSE)
+            endif()
+        endforeach()
+
+        if(_qt_find_package_skip_find_package)
+            message(AUTHOR_WARNING "qt_find_package(${ARGV0}) called even though the package "
+                   "was already found. Consider removing the call.")
+        endif()
+    endif()
+
     # Get the version if specified.
     set(package_version "")
     if(${ARGC} GREATER_EQUAL 2)
@@ -2842,7 +2866,7 @@ macro(qt_find_package)
         list(APPEND arg_UNPARSED_ARGUMENTS "COMPONENTS;${arg_COMPONENTS}")
     endif()
 
-    if(NOT (arg_CONFIG OR arg_NO_MODULE OR arg_MODULE))
+    if(NOT (arg_CONFIG OR arg_NO_MODULE OR arg_MODULE) AND NOT _qt_find_package_skip_find_package)
         # Try to find a config package first in quiet mode
         set(config_package_arg ${arg_UNPARSED_ARGUMENTS})
         list(APPEND config_package_arg "CONFIG;QUIET")
@@ -2875,7 +2899,7 @@ macro(qt_find_package)
         endif()
     endforeach()
 
-    if (NOT ${ARGV0}_FOUND)
+    if (NOT ${ARGV0}_FOUND AND NOT _qt_find_package_skip_find_package)
         # Unset the NOTFOUND ${package}_DIR var that might have been set by the previous
         # find_package call, to get rid of "not found" messagees in the feature summary
         # if the package is found by the next find_package call.
@@ -2887,7 +2911,7 @@ macro(qt_find_package)
         find_package(${arg_UNPARSED_ARGUMENTS})
     endif()
 
-    if(${ARGV0}_FOUND AND arg_PROVIDED_TARGETS)
+    if(${ARGV0}_FOUND AND arg_PROVIDED_TARGETS AND NOT _qt_find_package_skip_find_package)
         # If package was found, associate each target with its package name. This will be used
         # later when creating Config files for Qt libraries, to generate correct find_dependency()
         # calls. Also make the provided targets global, so that the properties can be read in