rootfs/sbin/mount-copybind

#!/bin/sh
#
# Perform a bind mount, copying existing files as we do so to ensure the
# overlaid path has the necessary content.

if [ $# -lt 2 ]; then
    echo >&2 "Usage: $0 spec mountpoint [OPTIONS]"
    exit 1
fi

# e.g. /var/volatile/lib
spec=$1

# e.g. /var/lib
mountpoint=$2

if [ $# -gt 2 ]; then
    options=$3
else
    options=
fi

[ -n "$options" ] && options=",$options"

mkdir -p "${spec%/*}"

if [ -d "$mountpoint" ]; then

    if [ -d "$spec" ]; then
        specdir_existed=yes
    else
        specdir_existed=no
        mkdir "$spec"
    fi

    # Fast version of calculating `dirname ${spec}`/.`basename ${spec}`-work
    overlay_workdir="${spec%/*}/.${spec##*/}-work"
    mkdir "${overlay_workdir}"

    # Try to mount using overlay, which is must faster than copying files.
    # If that fails, fall back to slower copy.
    if ! mount -t overlay overlay -olowerdir="$mountpoint",upperdir="$spec",workdir="$overlay_workdir" "$mountpoint" > /dev/null 2>&1; then

        if [ "$specdir_existed" != "yes" ]; then
            cp -aPR "$mountpoint"/. "$spec/"
        fi

        mount -o "bind$options" "$spec" "$mountpoint"
    fi
elif [ -f "$mountpoint" ]; then
    if [ ! -f "$spec" ]; then
        cp -aP "$mountpoint" "$spec"
    fi

    mount -o "bind$options" "$spec" "$mountpoint"
fi