cmake: fix .git-dir detection with using git-rev-parse (regression since 4.0)

[pboettch]

Summary

When using clang as a link in llvm/tools in conjunction with using git repositories running ninja fails with

ninja: error: '/home/.../llvm/.git/modules/external/clang/logs/HEAD', needed by 'tools/clang/lib/Basic/SVNVersion.inc', missing and no known rule to make it

If a .git-file or path is found, we need to determine the real .git-path, which could be anywhere when this repo is a submodule, or ln'd
into tools/.

This patch fixes it by using git rev-parse as llvm/include/llvm/Support/CMakeLists.txt is doing it.

Introduced with https://reviews.llvm.org/D34955

[modocache]

Thanks for the improvement!

lib/Basic/CMakeLists.txt

24

Reading llvm/include/llvm/Support/CMakeLists.txt, I didn't see very much that was borrowed here. I did see similar code in llvm/cmake/modules/VersionFromVCS.cmake, though. Is this a typo? Or maybe I'm misreading?

lib/Basic/CMakeLists.txt

 macro(find_first_existing_vc_file out_var path)
   set(git_path "${path}/.git")
 
-  # Normally '.git' is a directory that contains a 'logs/HEAD' file that
-  # is updated as modifications are made to the repository. In case the
-  # repository is a Git submodule, '.git' is a file that contains text that
-  # indicates where the repository's Git directory exists.
-  if (EXISTS "${git_path}" AND NOT IS_DIRECTORY "${git_path}")
-    FILE(READ "${git_path}" file_contents)
-    if("${file_contents}" MATCHES "^gitdir: ([^\n]+)")
-      # '.git' is indeed a link to the submodule's Git directory.
-      # Use the path to that Git directory.
-      set(git_path "${path}/${CMAKE_MATCH_1}")
+  # if a .git-file or path is found, determine the real .git-path,
+  # which could be anywhere when this repo is a submodule, or ln'd
+  # into tools/, so we use git rev-parse.
+  #
+  # Code is borrowed from llvm/include/llvm/Support/CMakeLists.txt
+  if (EXISTS "${git_path}")
+    find_program(git_executable NAMES git git.exe git.cmd)
+    # Run from a subdirectory to force git to print an absolute path.
+    execute_process(COMMAND ${git_executable} rev-parse --git-dir
+      WORKING_DIRECTORY ${path}/cmake
+      RESULT_VARIABLE git_result
+      OUTPUT_VARIABLE git_dir
+      ERROR_QUIET)
+    if(git_result EQUAL 0)
+      string(STRIP "${git_dir}" git_dir)
+      set(${out_var} "${git_dir}/logs/HEAD")
+      # some branchless cases (e.g. 'repo') may not yet have .git/logs/HEAD
+      if (NOT EXISTS "${git_dir}/logs/HEAD")
+        file(WRITE "${git_dir}/logs/HEAD" "")
+      endif()
+    else()
+      find_first_existing_file(${out_var}
+        "${path}/.svn/wc.db"   # SVN 1.7
+        "${path}/.svn/entries" # SVN 1.6
+      )
     endif()
   endif()
-
-  find_first_existing_file(${out_var}
-    "${git_path}/logs/HEAD"  # Git or Git submodule
-    "${path}/.svn/wc.db"     # SVN 1.7
-    "${path}/.svn/entries"   # SVN 1.6
-    )
 endmacro()
 
 find_first_existing_vc_file(llvm_vc "${LLVM_MAIN_SRC_DIR}")