Tests/RunCMake/execute_process: Check STARTUPINFOW reserved members

Verify that `execute_process` launches processes on Windows such that
`GetStartupInfoW` in the child does not populate `STARTUPINFOW` members
reserved for the MSVC C run-time.

Issue: #25996

Tests/RunCMake/CMakeLists.txt

@@ -513,6 +513,10 @@ set(execute_process_ARGS
 if(NOT CMake_TEST_EXTERNAL_CMAKE)
   list(APPEND execute_process_ARGS -DTEST_ENCODING_EXE=$<TARGET_FILE:testEncoding>)
 endif()
+if(WIN32)
+  add_executable(testStartupInfo testStartupInfo.c)
+  list(APPEND execute_process_ARGS -DTEST_STARTUPINFO_EXE=$<TARGET_FILE:testStartupInfo>)
+endif()
 add_RunCMake_test(execute_process)
 add_RunCMake_test(export)
 if(CMake_TEST_MSYSTEM_PREFIX)

Tests/RunCMake/execute_process/RunCMakeTest.cmake

@@ -53,3 +53,7 @@ if(WIN32 OR CYGWIN)
   set(RunCMake_TEST_NO_CLEAN 1)
   run_cmake_command(WindowsNoExtension-build ${CMAKE_COMMAND} --build . --config Debug --target RunScript)
 endif()
+
+if(TEST_STARTUPINFO_EXE)
+  run_cmake_script(StartupInfo -DTEST_STARTUPINFO_EXE=${TEST_STARTUPINFO_EXE})
+endif()

Tests/RunCMake/execute_process/StartupInfo.cmake

@@ -0,0 +1 @@
+execute_process(COMMAND "${TEST_STARTUPINFO_EXE}" COMMAND_ERROR_IS_FATAL ANY)

Tests/RunCMake/testStartupInfo.c

@@ -0,0 +1,25 @@
+#ifndef _CRT_SECURE_NO_WARNINGS
+#  define _CRT_SECURE_NO_WARNINGS
+#endif
+
+#if defined(_MSC_VER) && _MSC_VER >= 1928
+#  pragma warning(disable : 5105) /* macro expansion warning in windows.h */
+#endif
+
+#include <windows.h>
+
+#include <stdio.h>
+#include <string.h>
+
+int main(void)
+{
+  STARTUPINFOW si;
+  memset(&si, 0, sizeof(si));
+  GetStartupInfoW(&si);
+  if (si.cbReserved2 != 0 || si.lpReserved2 != NULL) {
+    fprintf(stderr, "si.cbReserved2: %u\n", si.cbReserved2);
+    fprintf(stderr, "si.lpReserved2: %p\n", si.lpReserved2);
+    return 1;
+  }
+  return 0;
+}