[v3,4/8] add mode field to blockdev-snapshot-sync transaction item

diff --git a/blockdev.c b/blockdev.c
index b434cd4..c9bd25b 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -745,9 +745,10 @@  void qmp_transaction(BlockdevActionList *dev_list, Error **errp)
         BlockDriver *proto_drv;
         BlockDriver *drv;
         int flags;
+        enum NewImageMode mode;
+        const char *new_image_file;
         const char *device;
         const char *format = "qcow2";
-        const char *new_image_file = NULL;
 
         dev_info = dev_entry->value;
         dev_entry = dev_entry->next;
@@ -758,10 +759,14 @@  void qmp_transaction(BlockdevActionList *dev_list, Error **errp)
         switch (dev_info->kind) {
         case BLOCKDEV_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC:
             device = dev_info->blockdev_snapshot_sync->device;
+            if (!dev_info->blockdev_snapshot_sync->has_mode) {
+                dev_info->blockdev_snapshot_sync->mode = NEW_IMAGE_MODE_ABSOLUTE_PATHS;
+            }
+            new_image_file = dev_info->blockdev_snapshot_sync->snapshot_file;
             if (dev_info->blockdev_snapshot_sync->has_format) {
                 format = dev_info->blockdev_snapshot_sync->format;
             }
-            new_image_file = dev_info->blockdev_snapshot_sync->snapshot_file;
+            mode = dev_info->blockdev_snapshot_sync->mode;
             break;
         default:
             abort();
@@ -802,13 +807,15 @@  void qmp_transaction(BlockdevActionList *dev_list, Error **errp)
         }
 
         /* create new image w/backing file */
-        ret = bdrv_img_create(new_image_file, format,
-                              states->old_bs->filename,
-                              states->old_bs->drv->format_name,
-                              NULL, -1, flags);
-        if (ret) {
-            error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
-            goto delete_and_fail;
+        if (mode != NEW_IMAGE_MODE_EXISTING) {
+            ret = bdrv_img_create(new_image_file, format,
+                                  states->old_bs->filename,
+                                  states->old_bs->drv->format_name,
+                                  NULL, -1, flags);
+            if (ret) {
+                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
+                goto delete_and_fail;
+            }
         }
 
         /* We will manually add the backing_hd field to the bs later */
diff --git a/qapi-schema.json b/qapi-schema.json
index 1d1ffa6..b062d01 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1118,6 +1118,24 @@ 
 { 'command': 'block_resize', 'data': { 'device': 'str', 'size': 'int' }}
 
 ##
+# @NewImageMode
+#
+# An enumeration that tells QEMU how to set the backing file path in
+# a new image file.
+#
+# @existing: QEMU should look for an existing image file.
+#
+# @absolute-paths: QEMU should create a new image with absolute paths
+# for the backing file.
+#
+# @no-backing-file: QEMU should create a new image with no backing file.
+#
+# Since: 1.1
+##
+{ 'enum': 'NewImageMode'
+  'data': [ 'existing', 'absolute-paths' ] }
+
+##
 # @BlockdevSnapshot
 #
 # @device:  the name of the device to generate the snapshot from.
@@ -1127,7 +1145,8 @@ 
 # @format: #optional the format of the snapshot image, default is 'qcow2'.
 ##
 { 'type': 'BlockdevSnapshot',
-  'data': { 'device': 'str', 'snapshot-file': 'str', '*format': 'str' } }
+  'data': { 'device': 'str', 'snapshot-file': 'str', '*format': 'str',
+            '*mode': 'NewImageMode' } }
 
 ##
 # @BlockdevAction
diff --git a/qmp-commands.hx b/qmp-commands.hx
index fb4f1df..7c03b62 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -705,6 +705,13 @@  A list of dictionaries is accepted, that contains the actions to be performed.
 For snapshots this is the device, the file to use for the new snapshot,
 and the format.  The default format, if not specified, is qcow2.
 
+Each new snapshot defaults to being created by QEMU (wiping any
+contents if the file already exists), but it is also possible to reuse
+an externally-created file.  In the latter case, you should ensure that
+the new image file has the same contents as the current one; QEMU cannot
+perform any meaningful check.  Typically this is achieved by using the
+current image file as the backing file for the new image.
+
 Arguments:
 
 actions array:
@@ -715,6 +722,8 @@  actions array:
       - "device": device name to snapshot (json-string)
       - "snapshot-file": name of new image file (json-string)
       - "format": format of new image (json-string, optional)
+      - "mode": whether and how QEMU should create the snapshot file
+        (NewImageMode, optional, default "absolute-paths")
 
 Example:
 
@@ -725,6 +734,7 @@  Example:
                                          "format": "qcow2" } },
          { 'type': 'blockdev-snapshot-sync', 'data' : { "device": "ide-hd1",
                                          "snapshot-file": "/some/place/my-image2",
+                                         "mode": "existing",
                                          "format": "qcow2" } } ] } }
 <- { "return": {} }