[Basic] Detect Git submodule version in CMake

[modocache]

Summary

When searching for Git version control information, libBasic's CMake
checks for the path '.git/logs/HEAD'. However, when LLVM is included as
a Git submodule, this path does not exist. Instead, it contains a '.git'
file with the following:

gitdir: ../../.git/modules/external/llvm

Where '../..' is the relative path to the root repository that contains
the LLVM Git submodule.

Because of this discrepancy, clang --version does not output source
control information if built from a Git submodule.

To fix, check whether '.git' is a directory or a file. If it's a
directory, simply use the '.git/logs/HEAD' path. If it's a file, read it
and check for the tell-tale sign of a Git submodule: the text "gitdir:".
If it exists, follow that path and use the 'logs/HEAD' at that location
instead. This allows not only the correct revision information to be
retrieved, but also uses a file that will change with each source
control revision.

Test Plan:

1. Before applying this change, build Clang as a Git submodule in a repository that places it in external/clang, and confirm no revision information is output when clang --version is invoked (just "clang 5.0.0" is output, no Git hashes).
2. Apply these changes and build Clang as a Git repository nested under llvm/tools/clang, and confirm that clang --version displays correct version information.
3. Apply these changes and build Clang as a Git submodule using the structure described in (1), and confirm version control information is output as in (2).

[jordan_rose]

If I'm remembering correctly from when I set this up, this isn't just about detecting which version control system you're using; it's about finding a file that will change on every commit. Otherwise, the generated Version.cpp won't be rebuilt after you update.

If you don't want to go looking for a better choice for this that would handle submodules, a stopgap answer would be to add a second entry that just looks for .git in addition to the one looking for HEAD.

[modocache]

Oh, nice catch @jordan_rose, you're absolutely right. I just tried updating my Git submodule reference for Clang and rebuilding my project, but the commit hash shown for Clang didn't change accordingly.

I'll need to find a file that changes on every commit. Following the Git submodule path, gitdir: ../../.git/modules/external/llvm and checking for ../../.git/modules/external/llvm/HEAD appears to work. I might rewrite the patch to do that, or try migrating this to use LLVM's source control checking (I think it exists in llvm/cmake/modules/VersionFromVCS.cmake).

[pcc]

FYI, I don't think HEAD on its own will work because it is not necessarily updated on every commit, see discussion on D31985.

[jordan_rose]

I'm not sure why we would care if a commit is made on a branch. The important information here is what commit is checked out, and that's what HEAD refers to.

[modocache]

Use submodule's .git directory.

[modocache]

Update commit message.

[jordan_rose]

lib/Basic/CMakeLists.txt

27

How about

if("${file_contents}" MATCHES "^gitdir: ([^\n]+)")
  set(git_path "${git_path}${CMAKE_MATCH_0}")
endif()

and then no stripping or replacing later at all? This admittedly doesn't handle some future submodule format that doesn't put gitdir on the first line, but neither does the code that's there.

(This also doesn't handle absolute path submodule references, but looking around a bit makes it sound like those aren't supposed to occur in practice and are considered bugs when git does generate them. I could be wrong about that though.)

[modocache]

Use CMAKE_MATCH_0 instead of using REGEX REPLACE. Thanks for the suggestion!

[modocache]

Friendly ping! I'm pretty excited about this change, it helps a pet project of mine, which builds Clang and LLVM checked out as submodules, display correct revision information. Let me know if I can modify this change at all, to make it easier to accept.

[jordan_rose]

Whoops, yes, this new version looks good!

[modocache]

Awesome, thanks @jordan_rose!

lib/Basic/CMakeLists.txt

foreach(file ${ARGN})
     if(EXISTS "${file}")
       set(${out_var} "${file}" PARENT_SCOPE)
       return()
     endif()
   endforeach()
 endfunction()
 
 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}/${CMAKE_MATCH_1}")
+    endif()
+  endif()
+
   find_first_existing_file(${out_var}
-    "${path}/.git/logs/HEAD" # Git
+    "${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 "${LLVM_MAIN_SRC_DIR}")
 find_first_existing_vc_file(clang_vc "${CLANG_SOURCE_DIR}")