block: teach bdrv_debug_breakpoint skip filters with backing

Teach bdrv_debug_breakpoint and bdrv_debug_remove_breakpoint skip
filters with backing. This is needed to implement and use in backup job
it's own backup_top filter driver (like mirror already has one), and
without this improvement, breakpoint removal will fail at least in 55
iotest.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190920142056.12778-9-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -5164,14 +5164,35 @@ void bdrv_debug_event(BlockDriverState *bs, BlkdebugEvent event)
     bs->drv->bdrv_debug_event(bs, event);
 }
 
-int bdrv_debug_breakpoint(BlockDriverState *bs, const char *event,
-                          const char *tag)
+static BlockDriverState *bdrv_find_debug_node(BlockDriverState *bs)
 {
     while (bs && bs->drv && !bs->drv->bdrv_debug_breakpoint) {
-        bs = bs->file ? bs->file->bs : NULL;
+        if (bs->file) {
+            bs = bs->file->bs;
+            continue;
+        }
+
+        if (bs->drv->is_filter && bs->backing) {
+            bs = bs->backing->bs;
+            continue;
+        }
+
+        break;
     }
 
     if (bs && bs->drv && bs->drv->bdrv_debug_breakpoint) {
+        assert(bs->drv->bdrv_debug_remove_breakpoint);
+        return bs;
+    }
+
+    return NULL;
+}
+
+int bdrv_debug_breakpoint(BlockDriverState *bs, const char *event,
+                          const char *tag)
+{
+    bs = bdrv_find_debug_node(bs);
+    if (bs) {
         return bs->drv->bdrv_debug_breakpoint(bs, event, tag);
     }
 
@@ -5180,11 +5201,8 @@ int bdrv_debug_breakpoint(BlockDriverState *bs, const char *event,
 
 int bdrv_debug_remove_breakpoint(BlockDriverState *bs, const char *tag)
 {
-    while (bs && bs->drv && !bs->drv->bdrv_debug_remove_breakpoint) {
-        bs = bs->file ? bs->file->bs : NULL;
-    }
-
-    if (bs && bs->drv && bs->drv->bdrv_debug_remove_breakpoint) {
+    bs = bdrv_find_debug_node(bs);
+    if (bs) {
         return bs->drv->bdrv_debug_remove_breakpoint(bs, tag);
     }