block: Make bdrv_root_attach_child() unref child_bs on failure

A consequence of the previous patch is that bdrv_attach_child()
transfers the reference to child_bs from the caller to parent_bs,
which will drop it on bdrv_close() or when someone calls
bdrv_unref_child().

But this only happens when bdrv_attach_child() succeeds. If it fails
then the caller is responsible for dropping the reference to child_bs.

This patch makes bdrv_attach_child() take the reference also when
there is an error, freeing the caller for having to do it.

A similar situation happens with bdrv_root_attach_child(), so the
changes on this patch affect both functions.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 20dfb3d9ccec559cdd1a9690146abad5d204a186.1557754872.git.berto@igalia.com
[mreitz: Removed now superfluous BdrvChild * variable in
         bdrv_open_child()]
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -2243,6 +2243,13 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
     }
 }
 
+/*
+ * This function steals the reference to child_bs from the caller.
+ * That reference is later dropped by bdrv_root_unref_child().
+ *
+ * On failure NULL is returned, errp is set and the reference to
+ * child_bs is also dropped.
+ */
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const char *child_name,
                                   const BdrvChildRole *child_role,
@@ -2255,6 +2262,7 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
     ret = bdrv_check_update_perm(child_bs, NULL, perm, shared_perm, NULL, errp);
     if (ret < 0) {
         bdrv_abort_perm_update(child_bs);
+        bdrv_unref(child_bs);
         return NULL;
     }
 
@@ -2274,6 +2282,14 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
     return child;
 }
 
+/*
+ * This function transfers the reference to child_bs from the caller
+ * to parent_bs. That reference is later dropped by parent_bs on
+ * bdrv_close() or if someone calls bdrv_unref_child().
+ *
+ * On failure NULL is returned, errp is set and the reference to
+ * child_bs is also dropped.
+ */
 BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                              BlockDriverState *child_bs,
                              const char *child_name,
@@ -2401,12 +2417,9 @@ void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
     /* If backing_hd was already part of bs's backing chain, and
      * inherits_from pointed recursively to bs then let's update it to
      * point directly to bs (else it will become NULL). */
-    if (update_inherits_from) {
+    if (bs->backing && update_inherits_from) {
         backing_hd->inherits_from = bs;
     }
-    if (!bs->backing) {
-        bdrv_unref(backing_hd);
-    }
 
 out:
     bdrv_refresh_limits(bs, NULL);
@@ -2594,7 +2607,6 @@ BdrvChild *bdrv_open_child(const char *filename,
                            const BdrvChildRole *child_role,
                            bool allow_none, Error **errp)
 {
-    BdrvChild *c;
     BlockDriverState *bs;
 
     bs = bdrv_open_child_bs(filename, options, bdref_key, parent, child_role,
@@ -2603,13 +2615,7 @@ BdrvChild *bdrv_open_child(const char *filename,
         return NULL;
     }
 
-    c = bdrv_attach_child(parent, bs, bdref_key, child_role, errp);
-    if (!c) {
-        bdrv_unref(bs);
-        return NULL;
-    }
-
-    return c;
+    return bdrv_attach_child(parent, bs, bdref_key, child_role, errp);
 }
 
 /* TODO Future callers may need to specify parent/child_role in order for

block/block-backend.c

@@ -392,7 +392,6 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
     blk->root = bdrv_root_attach_child(bs, "root", &child_root,
                                        perm, BLK_PERM_ALL, blk, errp);
     if (!blk->root) {
-        bdrv_unref(bs);
         blk_unref(blk);
         return NULL;
     }
@@ -800,12 +799,12 @@ void blk_remove_bs(BlockBackend *blk)
 int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp)
 {
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
+    bdrv_ref(bs);
     blk->root = bdrv_root_attach_child(bs, "root", &child_root,
                                        blk->perm, blk->shared_perm, blk, errp);
     if (blk->root == NULL) {
         return -EPERM;
     }
-    bdrv_ref(bs);
 
     notifier_list_notify(&blk->insert_bs_notifiers, blk);
     if (tgm->throttle_state) {

block/quorum.c

@@ -1019,7 +1019,6 @@ static void quorum_add_child(BlockDriverState *bs, BlockDriverState *child_bs,
     child = bdrv_attach_child(bs, child_bs, indexstr, &child_format, errp);
     if (child == NULL) {
         s->next_child_index--;
-        bdrv_unref(child_bs);
         goto out;
     }
     s->children = g_renew(BdrvChild *, s->children, s->num_children + 1);

blockjob.c

@@ -204,6 +204,7 @@ int block_job_add_bdrv(BlockJob *job, const char *name, BlockDriverState *bs,
 {
     BdrvChild *c;
 
+    bdrv_ref(bs);
     c = bdrv_root_attach_child(bs, name, &child_job, perm, shared_perm,
                                job, errp);
     if (c == NULL) {
@@ -211,7 +212,6 @@ int block_job_add_bdrv(BlockJob *job, const char *name, BlockDriverState *bs,
     }
 
     job->nodes = g_slist_prepend(job->nodes, c);
-    bdrv_ref(bs);
     bdrv_op_block_all(bs, job->blocker);
 
     return 0;