qcow2: Add iotest for an empty refcount table

This patch adds a simple iotest in which we try to write to an image
with an empty refcount table (i.e. with all entries set to 0).

This scenario was already handled by the existing consistency checks,
but we add an explicit test case for completeness.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 7e48b0e2ae1a0a18e0ee303b3045f130feec0474.1509718618.git.berto@igalia.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/060

@@ -242,6 +242,13 @@ poke_file "$TEST_IMG" "$(($l2_offset+8))" "\x80\x00\x00\x00\x00\x06\x2a\x00"
 # Should emit two error messages
 $QEMU_IO -c "discard 0 64k" -c "read 64k 64k" "$TEST_IMG" | _filter_qemu_io
 
+echo
+echo "=== Testing empty refcount table ==="
+echo
+_make_test_img 64M
+poke_file "$TEST_IMG" "$rt_offset"        "\x00\x00\x00\x00\x00\x00\x00\x00"
+$QEMU_IO -c "write 0 64k" "$TEST_IMG" | _filter_qemu_io
+
 echo
 echo "=== Testing empty refcount table with valid L1 and L2 tables ==="
 echo

tests/qemu-iotests/060.out

@@ -182,6 +182,12 @@ discard 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read failed: Input/output error
 
+=== Testing empty refcount table ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
+qcow2: Marking image as corrupt: Preventing invalid write on metadata (overlaps with refcount table); further corruption events will be suppressed
+write failed: Input/output error
+
 === Testing empty refcount table with valid L1 and L2 tables ===
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864