block: Add child access functions

There are BDS children that the general block layer code can access,
namely bs->file and bs->backing.  Since the introduction of filters and
external data files, their meaning is not quite clear.  bs->backing can
be a COW source, or it can be a filtered child; bs->file can be a
filtered child, it can be data and metadata storage, or it can be just
metadata storage.

This overloading really is not helpful.  This patch adds functions that
retrieve the correct child for each exact purpose.  Later patches in
this series will make use of them.  Doing so will allow us to handle
filter nodes in a meaningful way.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

1 files changed, 41 insertions, 3 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 9da7a42927..f280a95b26 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -92,9 +92,17 @@ struct BlockDriver {
     int instance_size;
 
     /* set to true if the BlockDriver is a block filter. Block filters pass
-     * certain callbacks that refer to data (see block.c) to their bs->file if
-     * the driver doesn't implement them. Drivers that do not wish to forward
-     * must implement them and return -ENOTSUP.
+     * certain callbacks that refer to data (see block.c) to their bs->file
+     * or bs->backing (whichever one exists) if the driver doesn't implement
+     * them. Drivers that do not wish to forward must implement them and return
+     * -ENOTSUP.
+     * Note that filters are not allowed to modify data.
+     *
+     * Filters generally cannot have more than a single filtered child,
+     * because the data they present must at all times be the same as
+     * that on their filtered child.  That would be impossible to
+     * achieve for multiple filtered children.
+     * (And this filtered child must then be bs->file or bs->backing.)
      */
     bool is_filter;
     /*
@@ -1382,4 +1390,34 @@ BdrvDirtyBitmap *block_dirty_bitmap_remove(const char *node, const char *name,
                                            BlockDriverState **bitmap_bs,
                                            Error **errp);
 
+BdrvChild *bdrv_cow_child(BlockDriverState *bs);
+BdrvChild *bdrv_filter_child(BlockDriverState *bs);
+BdrvChild *bdrv_filter_or_cow_child(BlockDriverState *bs);
+BdrvChild *bdrv_primary_child(BlockDriverState *bs);
+
+static inline BlockDriverState *child_bs(BdrvChild *child)
+{
+    return child ? child->bs : NULL;
+}
+
+static inline BlockDriverState *bdrv_cow_bs(BlockDriverState *bs)
+{
+    return child_bs(bdrv_cow_child(bs));
+}
+
+static inline BlockDriverState *bdrv_filter_bs(BlockDriverState *bs)
+{
+    return child_bs(bdrv_filter_child(bs));
+}
+
+static inline BlockDriverState *bdrv_filter_or_cow_bs(BlockDriverState *bs)
+{
+    return child_bs(bdrv_filter_or_cow_child(bs));
+}
+
+static inline BlockDriverState *bdrv_primary_bs(BlockDriverState *bs)
+{
+    return child_bs(bdrv_primary_child(bs));
+}
+
 #endif /* BLOCK_INT_H */