qcow2: Fix order in qcow2_snapshot_delete

First the snapshot must be deleted and only then the refcounts can be
decreased.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/qcow2-snapshot.c

@@ -489,32 +489,50 @@ fail:
 int qcow2_snapshot_delete(BlockDriverState *bs, const char *snapshot_id)
 {
     BDRVQcowState *s = bs->opaque;
-    QCowSnapshot *sn;
+    QCowSnapshot sn;
     int snapshot_index, ret;
 
+    /* Search the snapshot */
     snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
-    if (snapshot_index < 0)
+    if (snapshot_index < 0) {
         return -ENOENT;
-    sn = &s->snapshots[snapshot_index];
+    }
+    sn = s->snapshots[snapshot_index];
 
-    ret = qcow2_update_snapshot_refcount(bs, sn->l1_table_offset, sn->l1_size, -1);
-    if (ret < 0)
+    /* Remove it from the snapshot list */
+    memmove(s->snapshots + snapshot_index,
+            s->snapshots + snapshot_index + 1,
+            (s->nb_snapshots - snapshot_index - 1) * sizeof(sn));
+    s->nb_snapshots--;
+    ret = qcow2_write_snapshots(bs);
+    if (ret < 0) {
         return ret;
-    /* must update the copied flag on the current cluster offsets */
-    ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 0);
-    if (ret < 0)
+    }
+
+    /*
+     * The snapshot is now unused, clean up. If we fail after this point, we
+     * won't recover but just leak clusters.
+     */
+    g_free(sn.id_str);
+    g_free(sn.name);
+
+    /*
+     * Now decrease the refcounts of clusters referenced by the snapshot and
+     * free the L1 table.
+     */
+    ret = qcow2_update_snapshot_refcount(bs, sn.l1_table_offset,
+                                         sn.l1_size, -1);
+    if (ret < 0) {
         return ret;
-    qcow2_free_clusters(bs, sn->l1_table_offset, sn->l1_size * sizeof(uint64_t));
+    }
+    qcow2_free_clusters(bs, sn.l1_table_offset, sn.l1_size * sizeof(uint64_t));
 
-    g_free(sn->id_str);
-    g_free(sn->name);
-    memmove(sn, sn + 1, (s->nb_snapshots - snapshot_index - 1) * sizeof(*sn));
-    s->nb_snapshots--;
-    ret = qcow2_write_snapshots(bs);
+    /* must update the copied flag on the current cluster offsets */
+    ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 0);
     if (ret < 0) {
-        /* XXX: restore snapshot if error ? */
         return ret;
     }
+
 #ifdef DEBUG_ALLOC
     {
         BdrvCheckResult result = {0};