FindPkgConfig: Restore legacy behavior when CMP0126 is set to NEW

Module behavior must be independent from `CMP0126` policy.

Fixes: #22526

Modules/FindPkgConfig.cmake

@@ -79,10 +79,18 @@ set(PKG_CONFIG_FOUND "${PKGCONFIG_FOUND}")
 
 # Unsets the given variables
 macro(_pkgconfig_unset var)
+  # Clear normal variable (possibly set by project code).
+  unset(${var})
+  # Store as cache variable.
+  # FIXME: Add a policy to switch to a normal variable.
   set(${var} "" CACHE INTERNAL "")
 endmacro()
 
 macro(_pkgconfig_set var value)
+  # Clear normal variable (possibly set by project code).
+  unset(${var})
+  # Store as cache variable.
+  # FIXME: Add a policy to switch to a normal variable.
   set(${var} ${value} CACHE INTERNAL "")
 endmacro()
 

Tests/RunCMake/FindPkgConfig/FindPkgConfig_CMP0126_NEW.cmake

@@ -0,0 +1,64 @@
+
+cmake_policy(SET CMP0126 NEW)
+
+# Needed for CMAKE_SYSTEM_NAME, CMAKE_LIBRARY_ARCHITECTURE, FIND_LIBRARY_USE_LIB32_PATHS and FIND_LIBRARY_USE_LIB64_PATHS
+enable_language(C)
+
+# Prepare environment and variables
+set(PKG_CONFIG_USE_CMAKE_PREFIX_PATH TRUE)
+set(CMAKE_PREFIX_PATH "${CMAKE_CURRENT_SOURCE_DIR}/pc-foo")
+if(WIN32)
+    set(PKG_CONFIG_EXECUTABLE "${CMAKE_CURRENT_SOURCE_DIR}\\dummy-pkg-config.bat")
+    set(ENV{CMAKE_PREFIX_PATH} "${CMAKE_CURRENT_SOURCE_DIR}\\pc-bar;X:\\this\\directory\\should\\not\\exist\\in\\the\\filesystem")
+    set(ENV{PKG_CONFIG_PATH} "C:\\baz")
+else()
+    set(PKG_CONFIG_EXECUTABLE "${CMAKE_CURRENT_SOURCE_DIR}/dummy-pkg-config.sh")
+    set(ENV{CMAKE_PREFIX_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/pc-bar:/this/directory/should/not/exist/in/the/filesystem")
+    set(ENV{PKG_CONFIG_PATH} "/baz")
+endif()
+
+
+find_package(PkgConfig)
+
+
+if(NOT DEFINED CMAKE_SYSTEM_NAME
+    OR (CMAKE_SYSTEM_NAME MATCHES "^(Linux|kFreeBSD|GNU)$"
+    AND NOT CMAKE_CROSSCOMPILING))
+  if(EXISTS "/etc/debian_version") # is this a debian system ?
+    if(CMAKE_LIBRARY_ARCHITECTURE MATCHES "^(i386-linux-gnu|x86_64-linux-gnu)$")
+      # Cannot create directories for all the existing architectures...
+      set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/${CMAKE_LIBRARY_ARCHITECTURE}/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/${CMAKE_LIBRARY_ARCHITECTURE}/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+    else()
+      set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+    endif()
+  else()
+    # not debian, check the FIND_LIBRARY_USE_LIB32_PATHS and FIND_LIBRARY_USE_LIB64_PATHS properties
+    get_property(uselibx32 GLOBAL PROPERTY FIND_LIBRARY_USE_LIBX32_PATHS)
+    get_property(uselib32 GLOBAL PROPERTY FIND_LIBRARY_USE_LIB32_PATHS)
+    get_property(uselib64 GLOBAL PROPERTY FIND_LIBRARY_USE_LIB64_PATHS)
+    if(uselibx32 AND CMAKE_INTERNAL_PLATFORM_ABI STREQUAL "ELF X32")
+      set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/libx32/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/libx32/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+    elseif(uselib32 AND CMAKE_SIZEOF_VOID_P EQUAL 4)
+      set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib32/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib32/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+    elseif(uselib64 AND CMAKE_SIZEOF_VOID_P EQUAL 8)
+      set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib64/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib64/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+    else()
+      set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+    endif()
+  endif()
+else()
+  if(WIN32)
+    set(expected_path "C:\\baz;${CMAKE_CURRENT_SOURCE_DIR}\\pc-foo\\lib\\pkgconfig;${CMAKE_CURRENT_SOURCE_DIR}\\pc-bar\\lib\\pkgconfig")
+  else()
+    set(expected_path "/baz:${CMAKE_CURRENT_SOURCE_DIR}/pc-foo/lib/pkgconfig:${CMAKE_CURRENT_SOURCE_DIR}/pc-bar/lib/pkgconfig")
+  endif()
+endif()
+
+
+set(FOO_FOUND FALSE)
+
+pkg_check_modules(FOO "${expected_path}")
+
+if(NOT FOO_FOUND)
+  message(FATAL_ERROR "Expected FOO_FOUND == 1.")
+endif()

Tests/RunCMake/FindPkgConfig/RunCMakeTest.cmake

@@ -7,6 +7,7 @@ set(ENV{CMAKE_FRAMEWORK_PATH} "")
 
 run_cmake(PkgConfigDoesNotExist)
 
+run_cmake(FindPkgConfig_CMP0126_NEW)
 run_cmake(FindPkgConfig_NO_PKGCONFIG_PATH)
 run_cmake(FindPkgConfig_PKGCONFIG_PATH)
 run_cmake(FindPkgConfig_PKGCONFIG_PATH_NO_CMAKE_PATH)