void qmp_transaction(TransactionActionList *dev_list, Error **errp)
{
    TransactionActionList *dev_entry = dev_list;
    BlkTransactionState *state, *next;
    Error *local_err = NULL;
    QSIMPLEQ_HEAD(snap_bdrv_states, BlkTransactionState) snap_bdrv_states;
    QSIMPLEQ_INIT(&snap_bdrv_states);
    /* drain all i/o before any snapshots */
    bdrv_drain_all();
    /* We don't do anything in this loop that commits us to the snapshot */
    while (NULL != dev_entry) {
        TransactionAction *dev_info = NULL;
        const BdrvActionOps *ops;
        dev_info = dev_entry->value;
        dev_entry = dev_entry->next;
        assert(dev_info->kind < ARRAY_SIZE(actions));
        ops = &actions[dev_info->kind];
        state = g_malloc0(ops->instance_size);
        state->ops = ops;
        state->action = dev_info;
        QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, state, entry);
        state->ops->prepare(state, &local_err);
        if (error_is_set(&local_err)) {
            error_propagate(errp, local_err);
            goto delete_and_fail;
        }
    }
    QSIMPLEQ_FOREACH(state, &snap_bdrv_states, entry) {
        if (state->ops->commit) {
            state->ops->commit(state);
        }
    }
    /* success */
    goto exit;
delete_and_fail:
    /*
    * failure, and it is all-or-none; abandon each new bs, and keep using
    * the original bs for all images
    */
    QSIMPLEQ_FOREACH(state, &snap_bdrv_states, entry) {
        if (state->ops->abort) {
            state->ops->abort(state);
        }
    }
exit:
    QSIMPLEQ_FOREACH_SAFE(state, &snap_bdrv_states, entry, next) {
        if (state->ops->clean) {
            state->ops->clean(state);
        }
        g_free(state);
    }
}