Reapply [libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (#86020)

mstorsjo · tstellar · commit 0cd4babe0342 · 2024-03-28T14:18:41.000-07:00
This reapplies 272d1b4 (from #85756), which was reverted in 4079370. In the previous attempt, empty CMAKE_INSTALL_PREFIX was handled by quoting them, in d209d13. That made the calls to cmake_path(ABSOLUTE_PATH) succeed, but the output paths of that weren't actually absolute, which was required by file(RELATIVE_PATH). Avoid this issue by constructing a non-empty base directory variable to use for calculating the relative path. (cherry picked from commit 50801f1)
diff --git a/libcxx/modules/CMakeLists.txt b/libcxx/modules/CMakeLists.txt
@@ -206,9 +206,20 @@ add_custom_target(generate-cxx-modules
 # Configure the modules manifest.
 # Use the relative path between the installation and the module in the json
 # file. This allows moving the entire installation to a different location.
+if("${CMAKE_INSTALL_PREFIX}" STREQUAL "")
+  set(BASE_DIRECTORY "/")
+else()
+  set(BASE_DIRECTORY ${CMAKE_INSTALL_PREFIX})
+endif()
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_LIBRARY_DIR
+  BASE_DIRECTORY ${BASE_DIRECTORY}
+  OUTPUT_VARIABLE ABS_LIBRARY_DIR)
+cmake_path(ABSOLUTE_PATH LIBCXX_INSTALL_MODULES_DIR
+  BASE_DIRECTORY ${BASE_DIRECTORY}
+  OUTPUT_VARIABLE ABS_MODULES_DIR)
 file(RELATIVE_PATH LIBCXX_MODULE_RELATIVE_PATH
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_LIBRARY_DIR}
-  ${CMAKE_INSTALL_PREFIX}/${LIBCXX_INSTALL_MODULES_DIR})
+  ${ABS_LIBRARY_DIR}
+  ${ABS_MODULES_DIR})
 configure_file(
   "modules.json.in"
   "${LIBCXX_LIBRARY_DIR}/libc++.modules.json"
