qcow2: Return real error in qcow2_snapshot_goto

Besides fixing the return code, this adds some comments that make clear
how the code works and that it potentially breaks images if we fail in
the wrong place. Actually fixing this is left for the next patch.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/qcow2-snapshot.c

@@ -386,17 +386,32 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
     QCowSnapshot *sn;
     int i, snapshot_index;
     int cur_l1_bytes, sn_l1_bytes;
+    int 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];
 
-    if (qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, -1) < 0)
+    /* Decrease refcount of clusters of current L1 table.
+     * FIXME This is too early! */
+    ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset,
+                                         s->l1_size, -1);
+    if (ret < 0) {
         goto fail;
+    }
 
-    if (qcow2_grow_l1_table(bs, sn->l1_size, true) < 0)
+    /*
+     * Make sure that the current L1 table is big enough to contain the whole
+     * L1 table of the snapshot. If the snapshot L1 table is smaller, the
+     * current one must be padded with zeros.
+     */
+    ret = qcow2_grow_l1_table(bs, sn->l1_size, true);
+    if (ret < 0) {
         goto fail;
+    }
 
     cur_l1_bytes = s->l1_size * sizeof(uint64_t);
     sn_l1_bytes = sn->l1_size * sizeof(uint64_t);
@@ -405,19 +420,32 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
         memset(s->l1_table + sn->l1_size, 0, cur_l1_bytes - sn_l1_bytes);
     }
 
-    /* copy the snapshot l1 table to the current l1 table */
-    if (bdrv_pread(bs->file, sn->l1_table_offset,
-                   s->l1_table, sn_l1_bytes) < 0)
+    /*
+     * Copy the snapshot L1 table to the current L1 table.
+     *
+     * Before overwriting the old current L1 table on disk, make sure to
+     * increase all refcounts for the clusters referenced by the new one.
+     */
+    ret = bdrv_pread(bs->file, sn->l1_table_offset, s->l1_table, sn_l1_bytes);
+    if (ret < 0) {
         goto fail;
-    if (bdrv_pwrite_sync(bs->file, s->l1_table_offset,
-                    s->l1_table, cur_l1_bytes) < 0)
+    }
+
+    ret = bdrv_pwrite_sync(bs->file, s->l1_table_offset, s->l1_table,
+                           cur_l1_bytes);
+    if (ret < 0) {
         goto fail;
+    }
+
     for(i = 0;i < s->l1_size; i++) {
         be64_to_cpus(&s->l1_table[i]);
     }
 
-    if (qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 1) < 0)
+    /* FIXME This is too late! */
+    ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 1);
+    if (ret < 0) {
         goto fail;
+    }
 
 #ifdef DEBUG_ALLOC
     {
@@ -426,8 +454,9 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
     }
 #endif
     return 0;
- fail:
-    return -EIO;
+
+fail:
+    return ret;
 }
 
 int qcow2_snapshot_delete(BlockDriverState *bs, const char *snapshot_id)