migration: fix bitmaps pre-blockdev migration with mirror job

Important thing for bitmap migration is to select destination block
node to obtain the migrated bitmap.

Prepatch, on source we use bdrv_get_device_or_node_name() to identify
the node, and on target we do bdrv_lookup_bs.
bdrv_get_device_or_node_name() returns blk name only for direct
children of blk. So, bitmaps of direct children of blks are migrated by
blk name and others - by node name.

Old libvirt is unprepared to bitmap migration by node-name,
node-names are mostly auto-generated. So actually only migration by blk
name works for it.

Newer libvirt will use new interface (which will be added soon) to
specify node-mapping for bitmaps migration explicitly. Still, let's
improve the current behavior a bit.

Now, consider classic libvirt migrations assisted by mirror block job:
mirror block job inserts filter, so our source is not a direct child of
blk, and bitmaps are migrated by node-names. And this just doesn't work
with auto-generated node names.

Let's fix it by using blk-name even if some implicit filters are
inserted.

Note2: we, of course, can't skip filters and use blk name to migrate
bitmaps in filtered node by blk name for this blk if these filters have
named bitmaps which should be migrated.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1652424
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200521220648.3255-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: comment typo fix, shorter subject line]
Signed-off-by: Eric Blake <eblake@redhat.com>

migration/block-dirty-bitmap.c

@@ -319,14 +319,54 @@ static int init_dirty_bitmap_migration(void)
 {
     BlockDriverState *bs;
     DirtyBitmapMigBitmapState *dbms;
+    GHashTable *handled_by_blk = g_hash_table_new(NULL, NULL);
+    BlockBackend *blk;
 
     dirty_bitmap_mig_state.bulk_completed = false;
     dirty_bitmap_mig_state.prev_bs = NULL;
     dirty_bitmap_mig_state.prev_bitmap = NULL;
     dirty_bitmap_mig_state.no_bitmaps = false;
 
+    /*
+     * Use blockdevice name for direct (or filtered) children of named block
+     * backends.
+     */
+    for (blk = blk_next(NULL); blk; blk = blk_next(blk)) {
+        const char *name = blk_name(blk);
+
+        if (!name || strcmp(name, "") == 0) {
+            continue;
+        }
+
+        bs = blk_bs(blk);
+
+        /* Skip filters without bitmaps */
+        while (bs && bs->drv && bs->drv->is_filter &&
+               !bdrv_has_named_bitmaps(bs))
+        {
+            if (bs->backing) {
+                bs = bs->backing->bs;
+            } else if (bs->file) {
+                bs = bs->file->bs;
+            } else {
+                bs = NULL;
+            }
+        }
+
+        if (bs && bs->drv && !bs->drv->is_filter) {
+            if (add_bitmaps_to_list(bs, name)) {
+                goto fail;
+            }
+            g_hash_table_add(handled_by_blk, bs);
+        }
+    }
+
     for (bs = bdrv_next_all_states(NULL); bs; bs = bdrv_next_all_states(bs)) {
-        if (add_bitmaps_to_list(bs, bdrv_get_device_or_node_name(bs))) {
+        if (g_hash_table_contains(handled_by_blk, bs)) {
+            continue;
+        }
+
+        if (add_bitmaps_to_list(bs, bdrv_get_node_name(bs))) {
             goto fail;
         }
     }
@@ -340,9 +380,12 @@ static int init_dirty_bitmap_migration(void)
         dirty_bitmap_mig_state.no_bitmaps = true;
     }
 
+    g_hash_table_destroy(handled_by_blk);
+
     return 0;
 
 fail:
+    g_hash_table_destroy(handled_by_blk);
     dirty_bitmap_mig_cleanup();
 
     return -1;