block: Use bdrv_unref_child() for all children in bdrv_close()

bdrv_unref_child() does the following things:

  - Updates the child->bs->inherits_from pointer.
  - Calls bdrv_detach_child() to remove the BdrvChild from bs->children.
  - Calls bdrv_unref() to unref the child BlockDriverState.

When bdrv_unref_child() was introduced in commit 33a604075c it was not
used in bdrv_close() because the drivers that had additional children
(like quorum or blkverify) had already called bdrv_unref() on their
children during their own close functions.

This was changed later (in 0bd6e91a7e for quorum, in 3e586be0b2 for
blkverify) so there's no reason not to use bdrv_unref_child() in
bdrv_close() anymore.

After this there's also no need to remove bs->backing and bs->file
separately from the rest of the children, so bdrv_close() can be
simplified.

Now bdrv_close() unrefs all children (before this patch it was only
bs->file and bs->backing). As a result, none of the callers of
brvd_attach_child() should remove their reference to child_bs (because
this function effectively steals that reference). This patch updates a
couple of tests that were doing their own bdrv_unref().

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 6d1d5feaa53aa1ab127adb73d605dc4503e3abd5.1557754872.git.berto@igalia.com
[mreitz: s/where/were/]
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -3877,22 +3877,12 @@ static void bdrv_close(BlockDriverState *bs)
         bs->drv = NULL;
     }
 
-    bdrv_set_backing_hd(bs, NULL, &error_abort);
-
-    if (bs->file != NULL) {
-        bdrv_unref_child(bs, bs->file);
-        bs->file = NULL;
-    }
-
     QLIST_FOREACH_SAFE(child, &bs->children, next, next) {
-        /* TODO Remove bdrv_unref() from drivers' close function and use
+        bdrv_unref_child(bs, child);
-         * bdrv_unref_child() here */
-        if (child->bs->inherits_from == bs) {
-            child->bs->inherits_from = NULL;
-        }
-        bdrv_detach_child(child);
     }
 
+    bs->backing = NULL;
+    bs->file = NULL;
     g_free(bs->opaque);
     bs->opaque = NULL;
     atomic_set(&bs->copy_on_read, 0);

tests/test-bdrv-drain.c

@@ -1436,12 +1436,6 @@ static void test_detach_indirect(bool by_parent_cb)
     bdrv_unref(parent_b);
     blk_unref(blk);
 
-    /* XXX Once bdrv_close() unref's children instead of just detaching them,
-     * this won't be necessary any more. */
-    bdrv_unref(a);
-    bdrv_unref(a);
-    bdrv_unref(c);
-
     g_assert_cmpint(a->refcnt, ==, 1);
     g_assert_cmpint(b->refcnt, ==, 1);
     g_assert_cmpint(c->refcnt, ==, 1);

tests/test-bdrv-graph-mod.c

@@ -116,7 +116,6 @@ static void test_update_perm_tree(void)
     g_assert_nonnull(local_err);
     error_free(local_err);
 
-    bdrv_unref(bs);
     blk_unref(root);
 }