qcow2: Fix qcow2_make_empty() with external data file

make_completely_empty() is an optimisated path for bdrv_make_empty()
where completely new metadata is created inside the image file instead
of going through all clusters and discarding them. For an external data
file, however, we actually need to do discard operations on the data
file; just overwriting the qcow2 file doesn't get rid of the data.

The necessary slow path with an explicit discard operation already
exists for other cases. Use it for external data files, too.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 7fbef97aab..840f289a48 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4384,14 +4384,17 @@ static int qcow2_make_empty(BlockDriverState *bs)
 
     if (s->qcow_version >= 3 && !s->snapshots && !s->nb_bitmaps &&
         3 + l1_clusters <= s->refcount_block_size &&
-        s->crypt_method_header != QCOW_CRYPT_LUKS) {
+        s->crypt_method_header != QCOW_CRYPT_LUKS &&
+        !has_data_file(bs)) {
         /* The following function only works for qcow2 v3 images (it
          * requires the dirty flag) and only as long as there are no
          * features that reserve extra clusters (such as snapshots,
          * LUKS header, or persistent bitmaps), because it completely
          * empties the image.  Furthermore, the L1 table and three
          * additional clusters (image header, refcount table, one
-         * refcount block) have to fit inside one refcount block. */
+         * refcount block) have to fit inside one refcount block. It
+         * only resets the image file, i.e. does not work with an
+         * external data file. */
         return make_completely_empty(bs);
     }