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#!/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 [ "$specdir_existed" != "yes" ]; then
cp -pPR "$mountpoint"/. "$spec/"
fi
mount -o "bind$options" "$spec" "$mountpoint"
fi
elif [ -f "$mountpoint" ]; then
if [ ! -f "$spec" ]; then
cp -pP "$mountpoint" "$spec"
fi
mount -o "bind$options" "$spec" "$mountpoint"
fi
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