block: User BdrvChild callback for device name

In order to get rid of bs->blk for bdrv_get_device_name() and
bdrv_get_device_or_node_name(), ask all parents for their name and
simply pick the first one.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>

block.c

@@ -2894,10 +2894,28 @@ const char *bdrv_get_node_name(const BlockDriverState *bs)
     return bs->node_name;
 }
 
+static const char *bdrv_get_parent_name(const BlockDriverState *bs)
+{
+    BdrvChild *c;
+    const char *name;
+
+    /* If multiple parents have a name, just pick the first one. */
+    QLIST_FOREACH(c, &bs->parents, next_parent) {
+        if (c->role->get_name) {
+            name = c->role->get_name(c);
+            if (name && *name) {
+                return name;
+            }
+        }
+    }
+
+    return NULL;
+}
+
 /* TODO check what callers really want: bs->node_name or blk_name() */
 const char *bdrv_get_device_name(const BlockDriverState *bs)
 {
-    return bs->blk ? blk_name(bs->blk) : "";
+    return bdrv_get_parent_name(bs) ?: "";
 }
 
 /* This can be used to identify nodes that might not have a device
@@ -2906,7 +2924,7 @@ const char *bdrv_get_device_name(const BlockDriverState *bs)
  * absent, then this returns an empty (non-null) string. */
 const char *bdrv_get_device_or_node_name(const BlockDriverState *bs)
 {
-    return bs->blk ? blk_name(bs->blk) : bs->node_name;
+    return bdrv_get_parent_name(bs) ?: bs->node_name;
 }
 
 int bdrv_get_flags(BlockDriverState *bs)

block/block-backend.c

@@ -97,11 +97,17 @@ static void blk_root_drained_end(BdrvChild *child);
 static void blk_root_change_media(BdrvChild *child, bool load);
 static void blk_root_resize(BdrvChild *child);
 
+static const char *blk_root_get_name(BdrvChild *child)
+{
+    return blk_name(child->opaque);
+}
+
 static const BdrvChildRole child_root = {
     .inherit_options    = blk_root_inherit_options,
 
     .change_media       = blk_root_change_media,
     .resize             = blk_root_resize,
+    .get_name           = blk_root_get_name,
 
     .drained_begin      = blk_root_drained_begin,
     .drained_end        = blk_root_drained_end,

include/block/block_int.h

@@ -368,6 +368,11 @@ struct BdrvChildRole {
     void (*change_media)(BdrvChild *child, bool load);
     void (*resize)(BdrvChild *child);
 
+    /* Returns a name that is supposedly more useful for human users than the
+     * node name for identifying the node in question (in particular, a BB
+     * name), or NULL if the parent can't provide a better name. */
+    const char* (*get_name)(BdrvChild *child);
+
     /*
      * If this pair of functions is implemented, the parent doesn't issue new
      * requests after returning from .drained_begin() until .drained_end() is