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>

2 files changed, 132 insertions, 3 deletions

diff --git a/block.c b/block.c
index b204b93a2e..9f866e8676 100644
--- a/block.c
+++ b/block.c
@@ -6943,3 +6943,94 @@ int bdrv_make_empty(BdrvChild *c, Error **errp)
 
     return 0;
 }
+
+/*
+ * Return the child that @bs acts as an overlay for, and from which data may be
+ * copied in COW or COR operations.  Usually this is the backing file.
+ */
+BdrvChild *bdrv_cow_child(BlockDriverState *bs)
+{
+    if (!bs || !bs->drv) {
+        return NULL;
+    }
+
+    if (bs->drv->is_filter) {
+        return NULL;
+    }
+
+    if (!bs->backing) {
+        return NULL;
+    }
+
+    assert(bs->backing->role & BDRV_CHILD_COW);
+    return bs->backing;
+}
+
+/*
+ * If @bs acts as a filter for exactly one of its children, return
+ * that child.
+ */
+BdrvChild *bdrv_filter_child(BlockDriverState *bs)
+{
+    BdrvChild *c;
+
+    if (!bs || !bs->drv) {
+        return NULL;
+    }
+
+    if (!bs->drv->is_filter) {
+        return NULL;
+    }
+
+    /* Only one of @backing or @file may be used */
+    assert(!(bs->backing && bs->file));
+
+    c = bs->backing ?: bs->file;
+    if (!c) {
+        return NULL;
+    }
+
+    assert(c->role & BDRV_CHILD_FILTERED);
+    return c;
+}
+
+/*
+ * Return either the result of bdrv_cow_child() or bdrv_filter_child(),
+ * whichever is non-NULL.
+ *
+ * Return NULL if both are NULL.
+ */
+BdrvChild *bdrv_filter_or_cow_child(BlockDriverState *bs)
+{
+    BdrvChild *cow_child = bdrv_cow_child(bs);
+    BdrvChild *filter_child = bdrv_filter_child(bs);
+
+    /* Filter nodes cannot have COW backing files */
+    assert(!(cow_child && filter_child));
+
+    return cow_child ?: filter_child;
+}
+
+/*
+ * Return the primary child of this node: For filters, that is the
+ * filtered child.  For other nodes, that is usually the child storing
+ * metadata.
+ * (A generally more helpful description is that this is (usually) the
+ * child that has the same filename as @bs.)
+ *
+ * Drivers do not necessarily have a primary child; for example quorum
+ * does not.
+ */
+BdrvChild *bdrv_primary_child(BlockDriverState *bs)
+{
+    BdrvChild *c, *found = NULL;
+
+    QLIST_FOREACH(c, &bs->children, next) {
+        if (c->role & BDRV_CHILD_PRIMARY) {
+            assert(!found);
+            found = c;
+        }
+    }
+
+    return found;
+}
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 */