[PATCH v2 18/36] block: add bdrv_replace_child_safe() transaction action

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

To be used in the following commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/block.c b/block.c
index 6996aee1cf..f24bd60c2f 100644
--- a/block.c
+++ b/block.c
@@ -84,6 +84,8 @@ static BlockDriverState *bdrv_open_inherit(const char *filename,
 
 static void bdrv_parent_set_aio_context_ignore(BdrvChild *c, AioContext *ctx,
                                                GSList **ignore);
+static void bdrv_replace_child_noperm(BdrvChild *child,
+                                      BlockDriverState *new_bs);
 
 static int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue
                                *queue, Error **errp);
@@ -2164,6 +2166,57 @@ static int bdrv_drv_set_perm(BlockDriverState *bs, uint64_t perm,
     return 0;
 }
 
+typedef struct BdrvReplaceChildState {
+    BdrvChild *child;
+    BlockDriverState *old_bs;
+} BdrvReplaceChildState;
+
+static void bdrv_replace_child_commit(void *opaque)
+{
+    BdrvReplaceChildState *s = opaque;
+
+    bdrv_unref(s->old_bs);
+}
+
+static void bdrv_replace_child_abort(void *opaque)
+{
+    BdrvReplaceChildState *s = opaque;
+    BlockDriverState *new_bs = s->child->bs;
+
+    /* old_bs reference is transparently moved from @s to @s->child */
+    bdrv_replace_child_noperm(s->child, s->old_bs);
+    bdrv_unref(new_bs);
+}
+
+static TransactionActionDrv bdrv_replace_child_drv = {
+    .commit = bdrv_replace_child_commit,
+    .abort = bdrv_replace_child_abort,
+    .clean = g_free,
+};
+
+/*
+ * bdrv_replace_child_safe
+ *
+ * Note: real unref of old_bs is done only on commit.
+ */
+__attribute__((unused))
+static void bdrv_replace_child_safe(BdrvChild *child, BlockDriverState *new_bs,
+                                    GSList **tran)
+{
+    BdrvReplaceChildState *s = g_new(BdrvReplaceChildState, 1);
+    *s = (BdrvReplaceChildState) {
+        .child = child,
+        .old_bs = child->bs,
+    };
+    tran_prepend(tran, &bdrv_replace_child_drv, s);
+
+    if (new_bs) {
+        bdrv_ref(new_bs);
+    }
+    bdrv_replace_child_noperm(child, new_bs);
+    /* old_bs reference is transparently moved from @child to @s */
+}
+
 /*
  * Check whether permissions on this node can be changed in a way that
  * @cumulative_perms and @cumulative_shared_perms are the new cumulative
-- 
2.21.3