Find core files and print stack if build failed

.travis.yml

@@ -11,6 +11,13 @@ env:
 - PURPOSE=unittest
 - PURPOSE=compile-with-bazel
 
+before_script:
+- ulimit -c unlimited -S       # enable core dumps
+
+after_failure:
+- COREFILE=$(find . -maxdepth 2 -name "core*" | head -n 1) # find core file
+- if [[ -f "$COREFILE" ]]; then gdb -c "$COREFILE" example -ex "thread apply all bt" -ex "set pagination 0" -batch; fi
+
 before_install:
 - wget --no-clobber https://github.com/bazelbuild/bazel/releases/download/0.8.1/bazel_0.8.1-linux-x86_64.deb
 - sudo dpkg -i bazel_0.8.1-linux-x86_64.deb
@@ -18,6 +25,7 @@ before_install:
 install:
 - sudo apt-get install -qq realpath libgflags-dev libprotobuf-dev libprotoc-dev protobuf-compiler libleveldb-dev libgoogle-perftools-dev
 - sudo apt-get install libgtest-dev && cd /usr/src/gtest && sudo env "PATH=$PATH" cmake . && sudo make && sudo mv libgtest* /usr/lib/ && cd -
+- sudo apt-get install -y gdb  # install gdb
 
 script:
 - sh build_in_travis_ci.sh