#!/bin/sh
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#
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# Perform a bind mount, copying existing files as we do so to ensure the
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# overlaid path has the necessary content.
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if [ $# -lt 2 ]; then
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echo >&2 "Usage: $0 spec mountpoint [OPTIONS]"
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exit 1
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fi
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# e.g. /var/volatile/lib
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spec=$1
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# e.g. /var/lib
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mountpoint=$2
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if [ $# -gt 2 ]; then
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options=$3
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else
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options=
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fi
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[ -n "$options" ] && options=",$options"
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mkdir -p "${spec%/*}"
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if [ -d "$mountpoint" ]; then
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if [ -d "$spec" ]; then
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specdir_existed=yes
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else
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specdir_existed=no
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mkdir "$spec"
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fi
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# Fast version of calculating `dirname ${spec}`/.`basename ${spec}`-work
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overlay_workdir="${spec%/*}/.${spec##*/}-work"
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mkdir "${overlay_workdir}"
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# Try to mount using overlay, which is must faster than copying files.
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# If that fails, fall back to slower copy.
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if ! mount -t overlay overlay -olowerdir="$mountpoint",upperdir="$spec",workdir="$overlay_workdir" "$mountpoint" > /dev/null 2>&1; then
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if [ "$specdir_existed" != "yes" ]; then
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cp -aPR "$mountpoint"/. "$spec/"
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fi
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mount -o "bind$options" "$spec" "$mountpoint"
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fi
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elif [ -f "$mountpoint" ]; then
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if [ ! -f "$spec" ]; then
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cp -aP "$mountpoint" "$spec"
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fi
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mount -o "bind$options" "$spec" "$mountpoint"
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fi
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