Apply clang format

[kxl-adsk]

Time to start leveraging clang-format in maya-usd. Below are updated instructions from #843

Instructions for applying clang-format to an outstanding branch / PR

These instructions assume that:

• $MAYA_USD_REPO points to the root directory of a checkout of the maya-usd source repository
• $COMMIT_BEFORE_FORMAT points at the commit in the dev branch immediately before clang-format is applied (this is 0fe90bc)
• $FORMAT_COMMIT points at the commit in the dev branch where clang-format was applied (this is 7c64b5d)
• $FORMAT_BRANCH points at the head of the branch, containing any commits after $FORMAT_COMMIT (dev branch)
• $YOUR_BRANCH is the name of the branch which does not have clang-format applied, but want to merge dev (with clang-format) into

Download and install clang-tools 10.0 (generally included as part of llvm):

• MacOS: use brew or https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/clang+llvm-10.0.0-x86_64-apple-darwin.tar.xz
• Windows: VS2019 comes with v10.0 or https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/LLVM-10.0.0-win64.exe
• Ubuntu 18.04: https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/clang+llvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz
• CentOS: build yourself - rough instructions follow:
  Make sure a modern version of cmake, and python-3, are both on your path, then:

  sudo yum install libedit-devel libxml2-devel ncurses-devel swig ninja-build devtoolset-9
  source /opt/rh/devtoolset-9/enable
  git clone -b release/10.x --single-branch --depth 1 https://github.com/llvm/llvm-project.git
  cd llvm-project
  mkdir build
  cd build
  cmake -D CMAKE_INSTALL_PREFIX=/usr/local/clang-10.0 -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_VERBOSE_MAKEFILE=1 -D LLVM_ENABLE_PROJECTS="clang;clang-tools-extra;compiler-rt;debuginfo-tests;libc;libclc;libcxx;libcxxabi;libunwind;lld;lldb;mlir;openmp;parallel-libs;polly;pstl" -D LLVM_ENABLE_BINDINGS=0 -D LLVM_TARGETS_TO_BUILD=all -GNinja ../llvm
  ninja-build install
  

Merge + apply clang-format to your branch (these instructions assume Linux, you may need to adapt to other OSes):

cd $MAYA_USD_REPO
git checkout $YOUR_BRANCH
git merge $COMMIT_BEFORE_FORMAT
# now resolve any merge errors that may have resulted from previous command

# this next command will technically create a merge in git, but won't actually change the
# state of any files
git merge -s ours $FORMAT_COMMIT

# now, apply the formatting to your branch...for Linux:
find . -regextype posix-egrep -regex "$(echo '.*('$(echo $(cat .clang-format-include) | tr ' ' '\|')')')" -exec clang-format -i {} \+
# for MacOS
# find -E . -regex "$(echo '.*('$(echo $(cat .clang-format-include) | tr ' ' '\|')')')" -exec clang-format -i {} \+

# ...then ammend the commit, to make this the new "merged" result
git commit -a --amend --no-edit

# now merge in any of the latest stuff, that came AFTER the clang-format changes
git merge $FORMAT_BRANCH

# OPTIONAL: do a sanity check, to make sure that the diff of your branch to
#           the format branch now only contains "substantive" changes (ie, not a
#           bunch of formatting changes!)
git diff $FORMAT_BRANCH
# ...or, if you have a gui difftool set up in your .gitconfig:
git difftool $FORMAT_BRANCH

[aloysbaillet]

Great to see this happening!
It would be nice to add the reformat commit sha to a file named .git-blame-ignore-revs (which can be used by the git blame command, see https://www.moxio.com/blog/43/ignoring-bulk-change-commits-with-git-blame for example).

[kxl-adsk]

I like this suggestion @aloysbaillet, see #895