ExternalProject: expose _ep_cache_args_script to the caller

This is needed so that the caller can add a dependency on the cache
file.

Fixes: #20668

Modules/ExternalProject.cmake

@@ -2974,6 +2974,10 @@ function(_ep_extract_configure_command var name)
       endif()
       _ep_write_initial_cache(${name} "${_ep_cache_args_script}" "${script_initial_cache_force}${script_initial_cache_default}")
       list(APPEND cmd "-C${_ep_cache_args_script}")
+      _ep_replace_location_tags(${name} _ep_cache_args_script)
+      set(_ep_cache_args_script
+        "${_ep_cache_args_script}"
+        PARENT_SCOPE)
     endif()
 
     list(APPEND cmd "<SOURCE_DIR><SOURCE_SUBDIR>")

Tests/RunCMake/BuildDepends/ExternalProject/CMakeLists.txt

@@ -0,0 +1,14 @@
+cmake_minimum_required(VERSION 3.12)
+project(External NONE)
+
+if (DEFINED cache_arg)
+  message("configured with: ${cache_arg}")
+else ()
+  message("cache_arg is undefined")
+endif ()
+
+if (DEFINED second_cache_arg)
+  message("configured again with: ${second_cache_arg}")
+else ()
+  message("second_cache_arg is undefined")
+endif ()

Tests/RunCMake/BuildDepends/ExternalProjectCacheArgs-build1-stdout.txt

@@ -0,0 +1,2 @@
+.*configured with: first
+.*second_cache_arg is undefined

Tests/RunCMake/BuildDepends/ExternalProjectCacheArgs-build2-stdout.txt

@@ -0,0 +1,2 @@
+.*configured with: first
+.*configured again with: second

Tests/RunCMake/BuildDepends/ExternalProjectCacheArgs.cmake

@@ -0,0 +1,19 @@
+include("${CMAKE_CURRENT_BINARY_DIR}/data.cmake")
+
+include(ExternalProject)
+ExternalProject_add(external
+  SOURCE_DIR "${CMAKE_CURRENT_LIST_DIR}/ExternalProject"
+  CMAKE_CACHE_ARGS
+    ${cache_args}
+  BUILD_COMMAND ""
+  INSTALL_COMMAND "")
+
+set(cache_args_path "<TMP_DIR>/external-cache-$<CONFIG>.cmake")
+set(cmake_cache_path "<BINARY_DIR>/CMakeCache.txt")
+_ep_replace_location_tags(external cache_args_path cmake_cache_path)
+
+file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/check-$<LOWER_CASE:$<CONFIG>>.cmake" CONTENT "
+set(check_pairs
+  \"${cmake_cache_path}|${cache_args_path}\"
+)
+")

Tests/RunCMake/BuildDepends/ExternalProjectCacheArgs.step1.cmake

@@ -0,0 +1,3 @@
+file(WRITE "${RunCMake_TEST_BINARY_DIR}/data.cmake"
+  "set(cache_args -Dcache_arg:STRING=first)
+")

Tests/RunCMake/BuildDepends/ExternalProjectCacheArgs.step2.cmake

@@ -0,0 +1,3 @@
+file(WRITE "${RunCMake_TEST_BINARY_DIR}/data.cmake"
+  "set(cache_args -Dsecond_cache_arg:STRING=second)
+")

Tests/RunCMake/BuildDepends/RunCMakeTest.cmake

@@ -46,6 +46,11 @@ endif()
 run_BuildDepends(Custom-Symbolic-and-Byproduct)
 run_BuildDepends(Custom-Always)
 
+set(RunCMake_TEST_OUTPUT_MERGE_save "${RunCMake_TEST_OUTPUT_MERGE}")
+set(RunCMake_TEST_OUTPUT_MERGE 1)
+run_BuildDepends(ExternalProjectCacheArgs)
+set(RunCMake_TEST_OUTPUT_MERGE "${RunCMake_TEST_OUTPUT_MERGE_save}")
+
 # Test header dependencies with a build tree underneath a source tree.
 set(RunCMake_TEST_SOURCE_DIR "${RunCMake_BINARY_DIR}/BuildUnderSource")
 set(RunCMake_TEST_BINARY_DIR "${RunCMake_BINARY_DIR}/BuildUnderSource/build")