[RFC] mirror: allow mirroring an intermediate image

diff --git a/block.c b/block.c
index e892cb4..7bbae5a 100644
--- a/block.c
+++ b/block.c
@@ -1245,6 +1245,8 @@  void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd)
                     bs->backing_blocker);
     bdrv_op_unblock(bs->backing_hd, BLOCK_OP_TYPE_STREAM,
                     bs->backing_blocker);
+    bdrv_op_unblock(bs->backing_hd, BLOCK_OP_TYPE_MIRROR,
+                    bs->backing_blocker);
 out:
     bdrv_refresh_limits(bs, NULL);
 }
diff --git a/block/mirror.c b/block/mirror.c
index 189e8f8..c05598f 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -340,6 +340,7 @@  static void mirror_exit(BlockJob *job, void *opaque)
     }
 
     if (s->should_complete && data->ret == 0) {
+        BlockDriverState *bdrv;
         BlockDriverState *to_replace = s->common.bs;
         if (s->to_replace) {
             to_replace = s->to_replace;
@@ -355,6 +356,39 @@  static void mirror_exit(BlockJob *job, void *opaque)
             bdrv_set_backing_hd(s->base, NULL);
             bdrv_unref(p);
         }
+
+        /* If a BlockDriverState was pointing to the image that we
+         * have just replaced, it must point now to the new one. */
+        for (bdrv = bdrv_next(NULL); bdrv; bdrv = bdrv_next(bdrv)) {
+            BlockDriverState *overlay;
+            AioContext *aio_context = bdrv_get_aio_context(bdrv);
+            aio_context_acquire(aio_context);
+            overlay = bdrv_find_overlay(bdrv, to_replace);
+            if (overlay) {
+                int ret, flags;
+                const char *filename, *format;
+
+                /* Change backing file in the header of the overlay image */
+                filename = to_replace->filename;
+                format = to_replace->drv ? to_replace->drv->format_name : "";
+                flags = bdrv_get_flags(overlay);
+                if (!(flags & BDRV_O_RDWR)) {
+                    bdrv_reopen(overlay, flags | BDRV_O_RDWR, NULL);
+                }
+                ret = bdrv_change_backing_file(overlay, filename, format);
+                if (!(flags & BDRV_O_RDWR)) {
+                    bdrv_reopen(overlay, flags, NULL);
+                }
+
+                /* If everything went well, update the BlockDriverState */
+                if (ret == 0) {
+                    bdrv_set_backing_hd(overlay, to_replace);
+                } else {
+                    data->ret = ret;
+                }
+            }
+            aio_context_release(aio_context);
+        }
     }
     if (s->to_replace) {
         bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
@@ -608,8 +642,8 @@  static void mirror_complete(BlockJob *job, Error **errp)
     }
     if (!s->synced) {
         error_setg(errp,
-                   "The active block job for device '%s' cannot be completed",
-                   bdrv_get_device_name(job->bs));
+                   "The active block job for node '%s' cannot be completed",
+                   bdrv_get_device_or_node_name(job->bs));
         return;
     }
 
diff --git a/blockdev.c b/blockdev.c
index f24cf2d..eb2e5c1 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2464,7 +2464,6 @@  void qmp_drive_mirror(const char *device, const char *target,
                       bool has_on_target_error, BlockdevOnError on_target_error,
                       Error **errp)
 {
-    BlockBackend *blk;
     BlockDriverState *bs;
     BlockDriverState *source, *target_bs;
     AioContext *aio_context;
@@ -2504,12 +2503,10 @@  void qmp_drive_mirror(const char *device, const char *target,
         return;
     }
 
-    blk = blk_by_name(device);
-    if (!blk) {
-        error_set(errp, QERR_DEVICE_NOT_FOUND, device);
+    bs = bdrv_lookup_bs(device, device, errp);
+    if (!bs) {
         return;
     }
-    bs = blk_bs(blk);
 
     aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(aio_context);
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 60b9664..3c10f89 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -903,7 +903,7 @@ 
 #
 # Start mirroring a block device's writes to a new destination.
 #
-# @device:  the name of the device whose writes should be mirrored.
+# @device: the device or node name of the image whose writes should be mirrored.
 #
 # @target: the target of the new image. If the file exists, or if it
 #          is a device, the existing file/device will be used as the new
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 3a42ad0..91902a1 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -1397,7 +1397,7 @@  of the source.
 
 Arguments:
 
-- "device": device name to operate on (json-string)
+- "device": device or node name to operate on (json-string)
 - "target": name of new image file (json-string)
 - "format": format of new image (json-string, optional)
 - "node-name": the name of the new block driver state in the node graph