docker_functions.sh

# `prepare_dir_tarball some_dir`
#
# Given a directory name in the pwd (with NO trailing slash), create or update
# `some_dir.tar.gz` only if the contents of the tarball would actually change.
#
# Although Dockerfiles allow you to ADD a directory, this isn't cacheable.
# Instead, maintain a tarball which allows you to ADD a host directory as a file
# which *is* cacheable. I.E.:
#
# Prior to running your container:
# `pushd .; cd path/to; prepare_dir_tarball some_dir; popd`
#
# Then, inside your Dockerfile
# `ADD path/to/some_dir.tar.gz`
#
# The above is a cacheable, functional equivalent of:
# `ADD path/to/some_dir/`
prepare_dir_tarball() {
    SRC_DIR=$1
    if [ -z "$1" ]; then
        echo "ERROR: prepare_dir_tarball(): expected directory name"
        exit 1
    fi
    SRC_TAR="${SRC_DIR}.tar.gz"
    if [ -f "$SRC_TAR" ]; then
        echo "$SRC_TAR exists"
        TMP_TAR="${SRC_DIR}.tmp.tar.gz"
        tar -czf "$TMP_TAR" "$SRC_DIR"
        SRC_SHA=`tar -xf "$SRC_TAR" --to-stdout | shasum`
        TMP_SHA=`tar -xf "$TMP_TAR" --to-stdout | shasum`
        if [ "$SRC_SHA" == "$TMP_SHA" ]; then
            echo "$SRC_TAR has not changed, shasum $SRC_SHA"
        else
            echo "$SRC_TAR has changed, shasums:"
            echo "    old: $SRC_SHA"
            echo "    new: $TMP_SHA"
            mv "$TMP_TAR" "$SRC_TAR"
            echo "$SRC_TAR replaced"
        fi
    else
        echo "$SRC_TAR does not exist, creating"
        tar -czf "$SRC_TAR" "$SRC_DIR"
    fi
}