[v3,03/25] block: Add BDS.backing_overridden

diff --git a/block.c b/block.c
index 1615f5d..438201d 100644
--- a/block.c
+++ b/block.c
@@ -1495,6 +1495,11 @@  int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
     reference = qdict_get_try_str(parent_options, bdref_key);
     if (reference || qdict_haskey(options, "file.filename")) {
         backing_filename[0] = '\0';
+
+        /* FIXME: Should also be set to true if @options contains other runtime
+         *        options which control the data that is read from the backing
+         *        BDS */
+        bs->backing_overridden = true;
     } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
         QDECREF(options);
         goto free_exit;
@@ -1659,6 +1664,9 @@  static BlockDriverState *bdrv_append_temp_snapshot(BlockDriverState *bs,
     bdrv_ref(bs_snapshot);
     bdrv_append(bs_snapshot, bs);
 
+    bs_snapshot->backing_overridden = true;
+    bdrv_refresh_filename(bs_snapshot);
+
     g_free(tmp_filename);
     return bs_snapshot;
 
@@ -1786,6 +1794,7 @@  static BlockDriverState *bdrv_open_inherit(const char *filename,
     backing = qdict_get_try_str(options, "backing");
     if (backing && *backing == '\0') {
         flags |= BDRV_O_NO_BACKING;
+        bs->backing_overridden = true;
         qdict_del(options, "backing");
     }
 
@@ -3959,6 +3968,10 @@  void bdrv_refresh_filename(BlockDriverState *bs)
      * refresh those first */
     QLIST_FOREACH(child, &bs->children, next) {
         bdrv_refresh_filename(child->bs);
+
+        if (child->role == &child_backing && child->bs->backing_overridden) {
+            bs->backing_overridden = true;
+        }
     }
 
     if (drv->bdrv_refresh_filename) {
diff --git a/block/mirror.c b/block/mirror.c
index 4ece624..6cb6cbd 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -891,6 +891,10 @@  static void mirror_complete(BlockJob *job, Error **errp)
 
         assert(!target->backing);
         bdrv_set_backing_hd(target, backing);
+
+        /* The target image's file already has been created with the backing
+         * file we just set, so there is no need to set backing_overridden or
+         * call bdrv_refresh_filename(). */
     }
 
     s->should_complete = true;
diff --git a/blockdev.c b/blockdev.c
index 245e1e1..7889bab 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1771,6 +1771,8 @@  static void external_snapshot_commit(BlkActionState *common)
 {
     ExternalSnapshotState *state =
                              DO_UPCAST(ExternalSnapshotState, common, common);
+    TransactionAction *action = common->action;
+    bool image_was_existing = false;
 
     bdrv_set_aio_context(state->new_bs, state->aio_context);
 
@@ -1783,6 +1785,20 @@  static void external_snapshot_commit(BlkActionState *common)
         bdrv_reopen(state->old_bs, state->old_bs->open_flags & ~BDRV_O_RDWR,
                     NULL);
     }
+
+    if (action->type == TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC) {
+        BlockdevSnapshotSync *s = action->u.blockdev_snapshot_sync.data;
+        if (s->has_mode && s->mode == NEW_IMAGE_MODE_EXISTING) {
+            image_was_existing = true;
+        }
+    } else {
+        image_was_existing = true;
+    }
+
+    if (image_was_existing) {
+        state->new_bs->backing_overridden = true;
+        bdrv_refresh_filename(state->new_bs);
+    }
 }
 
 static void external_snapshot_abort(BlkActionState *common)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 83a423c..555f3c2 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -461,6 +461,7 @@  struct BlockDriverState {
     char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
                                     this file image */
     char backing_format[16]; /* if non-zero and backing_file exists */
+    bool backing_overridden; /* backing file has been specified by the user */
 
     QDict *full_open_options;
     char exact_filename[PATH_MAX];