[Qemu-devel] [PATCH 4/6] add reuse field

[Paolo Bonzini <pbonzini@redhat.com>]

In some cases it can be useful to use an existing file as the new image
in a snapshot.  Add this capability to blockdev-transaction.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 blockdev.c       |   18 +++++++++++-------
 qapi-schema.json |    3 ++-
 qmp-commands.hx  |    7 +++++++
 3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 3c35553..61e20f7 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -740,6 +740,7 @@ void qmp_blockdev_transaction(BlockdevActionList *dev_list,
     const char *device;
     const char *format;
     const char *new_image_file = NULL;
+    bool do_snapshot;
 
     QSIMPLEQ_HEAD(snap_bdrv_states, BlkTransactionStates) snap_bdrv_states;
     QSIMPLEQ_INIT(&snap_bdrv_states);
@@ -759,6 +760,7 @@ void qmp_blockdev_transaction(BlockdevActionList *dev_list,
         case BLOCKDEV_ACTION_KIND_SNAPSHOT:
             device = dev_info->snapshot->device;
             new_image_file = dev_info->snapshot->snapshot_file;
+            do_snapshot = !dev_info->snapshot->has_reuse || !dev_info->snapshot->reuse;
             format = dev_info->snapshot->format;
             assert(!(dev_info->snapshot->has_format && format));
             break;
@@ -805,13 +807,15 @@ void qmp_blockdev_transaction(BlockdevActionList *dev_list,
         }
 
         /* 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 (do_snapshot) {
+            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 d2fbee3..78df122 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1127,7 +1127,8 @@
 # @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',
+            '*reuse': 'bool' } }
 
 ##
 # @BlockdevAction
diff --git a/qmp-commands.hx b/qmp-commands.hx
index ee05ec2..6728495 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -704,6 +704,10 @@ the original disks pre-snapshot attempt are used.
 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.
+Image files can be created by QEMU, or it can be created externally.
+In the latter case, the contents of the image must be the same as the
+current contents of the device.  Typically this is achieved by using
+the current image file as the backing file for the image.
 
 Arguments:
 
@@ -715,6 +719,8 @@ 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)
+      - "reuse": whether QEMU should look for an existing image file
+        (json-bool, optional, default false)
 
 Example:
 
@@ -725,6 +731,7 @@ Example:
                                          "format": "qcow2" } },
          { 'type': 'snapshot, 'data' : { "device": "ide-hd1",
                                          "snapshot-file": "/some/place/my-image2",
+                                         "reuse": true,
                                          "format": "qcow2",
                                          "op": "snapshot" } } ] } }
 <- { "return": {} }
-- 
1.7.7.6