/usr/lib/linux-boot-probes/50mounted-tests is in os-prober 1.65+deb8u1.
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 | #!/bin/sh
# Sub-tests that require a mounted partition.
. /usr/share/os-prober/common.sh
set -e
partition="$1"
types="$(fs_type "$partition")" || types=NOT-DETECTED
if [ "$types" = NOT-DETECTED ]; then
	debug "$1 type not recognised; skipping"
	exit 0
elif [ "$types" = swap ]; then
	debug "$1 is a swap partition; skipping"
	exit 0
elif [ "$types" = crypto_LUKS ]; then
	debug "$1 is a LUKS partition; skipping"
	exit 0
elif [ "$types" = ntfs ]; then
	if type ntfs-3g >/dev/null 2>&1; then
		types='ntfs-3g ntfs'
	fi
elif [ -z "$types" ]; then
	if type cryptsetup >/dev/null 2>&1 && \
	   cryptsetup luksDump "$partition" >/dev/null 2>&1; then
		debug "$1 is a LUKS partition; skipping"
		exit 0
	fi
	types="$(grep -v nodev /proc/filesystems)"
fi
tmpmnt=/var/lib/os-prober/mount
if [ ! -d "$tmpmnt" ]; then
	mkdir "$tmpmnt"
fi
mounted=
if type grub-mount >/dev/null 2>&1 && \
   type grub-probe >/dev/null 2>&1 && \
   grub-mount "$partition" "$tmpmnt" 2>/dev/null; then
	mounted=1
	type="$(grub-probe -d "$partition" -t fs)"
	[ "$type" ] || type=fuseblk
else
	ro_partition "$partition"
	for type in $types; do
		if mount -o ro -t "$type" "$partition" "$tmpmnt" 2>/dev/null; then
			mounted=1
			break
		fi
	done
fi
if [ "$mounted" ]; then
	linux_mount_boot "$partition" "$tmpmnt"
	bootpart="${mountboot%% *}"
	mounted="${mountboot#* }"
	for test in /usr/lib/linux-boot-probes/mounted/*; do
		if [ -f "$test" ] && [ -x "$test" ]; then
			debug "running $test $partition $bootpart $tmpmnt $type"
			if $test "$partition" "$bootpart" "$tmpmnt" "$type"; then
				debug "$test succeeded"
				umount "$tmpmnt/boot" 2>/dev/null || true 	
				if ! umount "$tmpmnt"; then
					warn "failed to umount $tmpmnt"
				fi
				rmdir "$tmpmnt" || true
				exit 0
			fi
		fi
	done
	umount "$tmpmnt/boot" 2>/dev/null || true 	
	if ! umount "$tmpmnt"; then
		warn "failed to umount $tmpmnt"
	fi
fi
rmdir "$tmpmnt" || true
# No tests found anything.
exit 1
 |