Drake: Incremental submodule update not working after lcm repository switched

[jwnimmer-tri]

Reproduction steps:

git clone 'https://[email protected]/jwnimmer-tri/drake.git' drake-distro
cd drake-distro
git remote add upstream 'https://[email protected]/RobotLocomotion/drake.git'
git remote set-url --push upstream no_push
git reset --hard 8535fc31ec624e632b05c6212a7134947a59a28f   # Just before #2809.
mkdir -p build/debug
cd build/debug
env PATH=/usr/lib/ccache:$PATH CC=gcc-4.9 CXX=g++-4.9 \
  cmake ../../ -G Ninja -DCMAKE_BUILD_TYPE:STRING=Debug
env PATH=/usr/lib/ccache:$PATH CC=gcc-4.9 CXX=g++-4.9 ninja -j1 -v
git fetch upstream && git merge --ff-only upstream/master  # Takes on #2809.
env PATH=/usr/lib/ccache:$PATH CC=gcc-4.9 CXX=g++-4.9 ninja -j1 -v

The last build line (after merging 2809, then re-rerunning ninja), yields:

[27/88] cd /home/jwnimmer/jwnimmer-tri/drake-distro/externals/lcm && /home/jwnimmer/jwnimmer-tri/cmake-3.5.2-Linux-x86_64/bin/cmake -E chdir /home/jwnimmer/jwnimmer-tri/drake-distro /usr/bin/git submodule update --init --recursive -- externals/lcm && /home/jwnimmer/jwnimmer-tri/cmake-3.5.2-Linux-x86_64/bin/cmake -E touch /home/jwnimmer/jwnimmer-tri/drake-distro/build/debug/lcm-prefix/src/lcm-stamp/lcm-update
FAILED: cd /home/jwnimmer/jwnimmer-tri/drake-distro/externals/lcm && /home/jwnimmer/jwnimmer-tri/cmake-3.5.2-Linux-x86_64/bin/cmake -E chdir /home/jwnimmer/jwnimmer-tri/drake-distro /usr/bin/git submodule update --init --recursive -- externals/lcm && /home/jwnimmer/jwnimmer-tri/cmake-3.5.2-Linux-x86_64/bin/cmake -E touch /home/jwnimmer/jwnimmer-tri/drake-distro/build/debug/lcm-prefix/src/lcm-stamp/lcm-update
fatal: reference is not a tree: 9c9e22cf23663ef9cd0e5ec83932ff7f70cd7b18
Unable to checkout '9c9e22cf23663ef9cd0e5ec83932ff7f70cd7b18' in submodule path 'externals/lcm'
ninja: build stopped: subcommand failed.

[jwnimmer-tri]

Tagging #2757 as the broader discussion for related issues.

[jwnimmer-tri]

@bradking What am I doing wrong here?

[jwnimmer-tri]

/CC @david-german-tri

[jwnimmer-tri]

I guess the work-around of nuking drake-distro and cloning again is plausible, so I'll drop this down to medium.

[bradking]

The problem is that the git submodule sync step only runs when CMake re-runs, and in this particular case nothing caused CMake to re-run. I'll look at making a build step that does it too.

[bradking]

@jwnimmer-tri all you need to do is run cmake . at the top of your build tree to move forward for now.