/lib/partman/commit.d/01unmount_busy is in ubiquity 2.10.16.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112  | #! /bin/sh
# This should largely be unnecessary now that partman-base/init.d/parted
# checks for mounted partitions at startup, but it may still serve as
# insurance against partitions being mounted while the partitioner is
# running.
. /lib/partman/lib/base.sh
db_get ubiquity/partman-skip-unmount
if [ "$RET" = true ]; then
    exit 0
fi
if [ -f /etc/mtab ]; then
    MTAB=/etc/mtab
else
    MTAB=/proc/mounts
fi
busy_partitions=
for dev in $DEVICES/*; do
    [ -d "$dev" ] || continue
    cd $dev
    partitions=
    found_mounted=false
    open_dialog PARTITIONS
    while { read_line num id size type fs path name; [ "$id" ]; }; do
	[ "$fs" != free ] || continue
	partitions="$partitions $id,$path"
	if [ -f "$id/mountpoint" ]; then
	    mp="$(cat "$id/mountpoint")"
	    case $mp in
		/media/*)
		    ;;
		*)
		    found_mounted=:
		    ;;
	    esac
	fi
    done
    close_dialog
    if ! $found_mounted; then
	open_dialog IS_CHANGED
	read_line changed
	close_dialog
	[ "$changed" = yes ] || continue
    fi
    for part in $partitions; do
	id="${part%,*}"
	path="${part#*,}"
	open_dialog IS_BUSY "$id"
	read_line busy
	close_dialog
	[ "$busy" = yes ] || continue
	busy_partitions="$busy_partitions $path"
    done
done
[ "$busy_partitions" ] || exit 0
noninteractive=:
while :; do
    failed_mountpoints=
    for part in $busy_partitions; do
	for mp in $(grep "^$part " "$MTAB" | cut -d' ' -f2); do
	    # This is impressively horrible, but it's the best way I can
	    # come up with to unmangle mtab entries in shell.
	    new_mp=
	    while [ "$mp" ]; do
		case $mp in
		    \\[0-9][0-9][0-9]*)
			octal="$(echo "$mp" | sed 's/^\\\([0-9][0-9][0-9]\).*/\1/')"
			new_mp="$new_mp$(printf "\\$octal")"
			mp="${mp#\\[0-9][0-9][0-9]}"
			;;
		    \\*)
			new_mp="$new_mp\\"
			mp="${mp#\\}"
			;;
		    *\\*)
			new_mp="$new_mp${mp%%\\*}"
			mp="\\${mp#*\\}"
			;;
		    *)
			new_mp="$new_mp$mp"
			mp=
			;;
		esac
	    done
	    umount "$new_mp" || failed_mountpoints="${failed_mountpoints:+$failed_mountpoints }$new_mp"
	done
    done
    if [ -z "$failed_mountpoints" ]; then
	break
    fi
    db_reset ubiquity/partman-failed-unmount
    db_subst ubiquity/partman-failed-unmount MOUNTED "$failed_mountpoints"
    db_input critical ubiquity/partman-failed-unmount || $noninteractive
    db_go || exit 1
    db_get ubiquity/partman-failed-unmount
    [ "$RET" = true ] || exit 1
    noninteractive='exit 1'
done
exit 0
 |