block/copy-before-write: use file child instead of backing

We are going to publish copy-before-write filter, and there no public
backing-child-based filter in Qemu. No reason to create a precedent, so
let's refactor copy-before-write filter instead.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210824083856.17408-13-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

block/copy-before-write.c

@@ -43,7 +43,7 @@ static coroutine_fn int cbw_co_preadv(
         BlockDriverState *bs, uint64_t offset, uint64_t bytes,
         QEMUIOVector *qiov, int flags)
 {
-    return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
+    return bdrv_co_preadv(bs->file, offset, bytes, qiov, flags);
 }
 
 static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
@@ -71,7 +71,7 @@ static int coroutine_fn cbw_co_pdiscard(BlockDriverState *bs,
         return ret;
     }
 
-    return bdrv_co_pdiscard(bs->backing, offset, bytes);
+    return bdrv_co_pdiscard(bs->file, offset, bytes);
 }
 
 static int coroutine_fn cbw_co_pwrite_zeroes(BlockDriverState *bs,
@@ -82,7 +82,7 @@ static int coroutine_fn cbw_co_pwrite_zeroes(BlockDriverState *bs,
         return ret;
     }
 
-    return bdrv_co_pwrite_zeroes(bs->backing, offset, bytes, flags);
+    return bdrv_co_pwrite_zeroes(bs->file, offset, bytes, flags);
 }
 
 static coroutine_fn int cbw_co_pwritev(BlockDriverState *bs,
@@ -95,29 +95,22 @@ static coroutine_fn int cbw_co_pwritev(BlockDriverState *bs,
         return ret;
     }
 
-    return bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
+    return bdrv_co_pwritev(bs->file, offset, bytes, qiov, flags);
 }
 
 static int coroutine_fn cbw_co_flush(BlockDriverState *bs)
 {
-    if (!bs->backing) {
+    if (!bs->file) {
         return 0;
     }
 
-    return bdrv_co_flush(bs->backing->bs);
+    return bdrv_co_flush(bs->file->bs);
 }
 
 static void cbw_refresh_filename(BlockDriverState *bs)
 {
-    if (bs->backing == NULL) {
-        /*
-         * we can be here after failed bdrv_attach_child in
-         * bdrv_set_backing_hd
-         */
-        return;
-    }
     pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
-            bs->backing->bs->filename);
+            bs->file->bs->filename);
 }
 
 static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
@@ -186,6 +179,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     top = bdrv_new_open_driver(&bdrv_cbw_filter, filter_node_name,
                                BDRV_O_RDWR, errp);
     if (!top) {
+        error_prepend(errp, "Cannot open driver: ");
         return NULL;
     }
 
@@ -201,21 +195,32 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     state->target = bdrv_attach_child(top, target, "target", &child_of_bds,
                                       BDRV_CHILD_DATA, errp);
     if (!state->target) {
+        error_prepend(errp, "Cannot attach target child: ");
+        bdrv_unref(top);
+        return NULL;
+    }
+
+    bdrv_ref(source);
+    top->file = bdrv_attach_child(top, source, "file", &child_of_bds,
+                                  BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
+                                  errp);
+    if (!top->file) {
+        error_prepend(errp, "Cannot attach file child: ");
         bdrv_unref(top);
         return NULL;
     }
 
     bdrv_drained_begin(source);
 
-    ret = bdrv_append(top, source, errp);
+    ret = bdrv_replace_node(source, top, errp);
     if (ret < 0) {
         error_prepend(errp, "Cannot append copy-before-write filter: ");
         goto fail;
     }
     appended = true;
 
-    state->bcs = block_copy_state_new(top->backing, state->target,
-                                      false, compress, errp);
+    state->bcs = block_copy_state_new(top->file, state->target, false, compress,
+                                      errp);
     if (!state->bcs) {
         error_prepend(errp, "Cannot create block-copy-state: ");
         goto fail;