qcow2: Fix error handling in l2_allocate

l2_allocate has some intermediate states in which the image is inconsistent.
Change the order to write to the L1 table only after the new L2 table has
successfully been initialized.

Also reset the L2 cache in failure case, it's very likely wrong.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -239,14 +239,6 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
         return l2_offset;
     }
 
-    /* update the L1 entry */
-
-    s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
-    ret = write_l1_entry(bs, l1_index);
-    if (ret < 0) {
-        return ret;
-    }
-
     /* allocate a new entry in the l2 cache */
 
     min_index = l2_cache_new_entry(bs);
@@ -261,7 +253,7 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
         ret = bdrv_pread(bs->file, old_l2_offset, l2_table,
             s->l2_size * sizeof(uint64_t));
         if (ret < 0) {
-            return ret;
+            goto fail;
         }
     }
     /* write the l2 table to the file */
@@ -269,7 +261,14 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
     ret = bdrv_pwrite(bs->file, l2_offset, l2_table,
         s->l2_size * sizeof(uint64_t));
     if (ret < 0) {
-        return ret;
+        goto fail;
+    }
+
+    /* update the L1 entry */
+    s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
+    ret = write_l1_entry(bs, l1_index);
+    if (ret < 0) {
+        goto fail;
     }
 
     /* update the l2 cache entry */
@@ -279,6 +278,10 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
 
     *table = l2_table;
     return 0;
+
+fail:
+    qcow2_l2_cache_reset(bs);
+    return ret;
 }
 
 static int count_contiguous_clusters(uint64_t nb_clusters, int cluster_size,