blockdev: add Abort transaction

The Abort action can be used to test QMP 'transaction' failure.  Add it
as the last action to exercise the .abort() and .cleanup() code paths
for all previous actions.
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -963,6 +963,16 @@ static void drive_backup_abort(BlkTransactionState *common)
     }
 }
 
+static void abort_prepare(BlkTransactionState *common, Error **errp)
+{
+    error_setg(errp, "Transaction aborted using Abort action");
+}
+
+static void abort_commit(BlkTransactionState *common)
+{
+    assert(false); /* this action never succeeds */
+}
+
 static const BdrvActionOps actions[] = {
     [TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC] = {
         .instance_size = sizeof(ExternalSnapshotState),
@@ -975,6 +985,11 @@ static const BdrvActionOps actions[] = {
         .prepare = drive_backup_prepare,
         .abort = drive_backup_abort,
     },
+    [TRANSACTION_ACTION_KIND_ABORT] = {
+        .instance_size = sizeof(BlkTransactionState),
+        .prepare = abort_prepare,
+        .commit = abort_commit,
+    },
 };
 
 /*

qapi-schema.json

@@ -1659,6 +1659,16 @@
             '*on-source-error': 'BlockdevOnError',
             '*on-target-error': 'BlockdevOnError' } }
 
+##
+# @Abort
+#
+# This action can be used to test transaction failure.
+#
+# Since: 1.6
+###
+{ 'type': 'Abort',
+  'data': { } }
+
 ##
 # @TransactionAction
 #
@@ -1668,7 +1678,8 @@
 { 'union': 'TransactionAction',
   'data': {
        'blockdev-snapshot-sync': 'BlockdevSnapshot',
-       'drive-backup': 'DriveBackup'
+       'drive-backup': 'DriveBackup',
+       'abort': 'Abort'
    } }
 
 ##