[CMake] Avoid __FakeVCSRevision.h with no git repository

[Hahnfeld]

Summary

Set the return variable to "" in find_first_existing_vc_file to
say that there is a repository, but no file to depend on. This works
transparently for all other callers that handle undefinedness and
equality to an empty string the same way.

Use the knowledge to avoid depending on __FakeVCSRevision.h if there
is no git repository at all (for example when building a release) as
there is no point in regenerating an empty VCSRevision.h.

[Hahnfeld]

Ping

[scott.linder]

The change makes sense to me, but at this point it might be good to add a comment for find_first_existing_vc_file to say what it does and what each possible return value means. Maybe something like:

# If the sources being configured are under version control, set `out_var`
# to the the path of a file which will be modified when the VCS revision
# changes, attempting to create that file if it does not exist; if no such
# file exists and one cannot be created, instead set `out_var` to the
# empty string.
#
# If the sources being configured are not under version control, do not
# define `out_var`.

[Hahnfeld]

Add comment for find_first_existing_vc_file, thanks @scott.linder.

[scott.linder]

LGTM, thank you!

llvm/cmake/modules/AddLLVM.cmake

function(setup_dependency_debugging name)
 
   set(deny_attributes_inc "(deny file* (literal \"${LLVM_BINARY_DIR}/include/llvm/IR/Attributes.inc\"))")
   set(deny_intrinsics_inc "(deny file* (literal \"${LLVM_BINARY_DIR}/include/llvm/IR/Intrinsics.inc\"))")
 
   set(sandbox_command "sandbox-exec -p '(version 1) (allow default) ${deny_attributes_inc} ${deny_intrinsics_inc}'")
   set_target_properties(${name} PROPERTIES RULE_LAUNCH_COMPILE ${sandbox_command})
 endfunction()
 
+# If the sources at the given `path` are under version control, set `out_var`
+# to the the path of a file which will be modified when the VCS revision
+# changes, attempting to create that file if it does not exist; if no such
+# file exists and one cannot be created, instead set `out_var` to the
+# empty string.
+#
+# If the sources are not under version control, do not define `out_var`.
 function(find_first_existing_vc_file path out_var)
   if(NOT EXISTS "${path}")
     return()
   endif()
   find_package(Git)
   if(GIT_FOUND)
     execute_process(COMMAND ${GIT_EXECUTABLE} rev-parse --git-dir
       WORKING_DIRECTORY ${path}
       RESULT_VARIABLE git_result
       OUTPUT_VARIABLE git_output
       ERROR_QUIET)
     if(git_result EQUAL 0)
       string(STRIP "${git_output}" git_output)
       get_filename_component(git_dir ${git_output} ABSOLUTE BASE_DIR ${path})
       # Some branchless cases (e.g. 'repo') may not yet have .git/logs/HEAD
       if (NOT EXISTS "${git_dir}/logs/HEAD")
         execute_process(COMMAND ${CMAKE_COMMAND} -E touch HEAD
           WORKING_DIRECTORY "${git_dir}/logs"
           RESULT_VARIABLE touch_head_result
           ERROR_QUIET)
         if (NOT touch_head_result EQUAL 0)
+          set(${out_var} "" PARENT_SCOPE)
           return()
         endif()
       endif()
       set(${out_var} "${git_dir}/logs/HEAD" PARENT_SCOPE)
     endif()
   endif()
 endfunction()

llvm/include/llvm/Support/CMakeLists.txt

 find_first_existing_vc_file("${LLVM_MAIN_SRC_DIR}" llvm_vc)
 
 # The VC revision include that we want to generate.
 set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/VCSRevision.h")
 
 set(generate_vcs_version_script "${LLVM_CMAKE_PATH}/GenerateVersionFromVCS.cmake")
 
 if(LLVM_APPEND_VC_REV)
   set(llvm_source_dir ${LLVM_MAIN_SRC_DIR})
 
   # A fake version file and is not expected to exist. It is being used to
   # force regeneration of VCSRevision.h for source directory with no write
   # permission available.
-  if (NOT llvm_vc)
+  if (llvm_vc STREQUAL "")
     set(fake_version_inc "${CMAKE_CURRENT_BINARY_DIR}/__FakeVCSRevision.h")
   endif()
 endif()
 
 set(generated_files "${version_inc}")
 if (fake_version_inc)
   list(APPEND generated_files "${fake_version_inc}")
 endif()