Add simple transaction API to use in further update of block graph
operations.

Supposed usage is:

- "prepare" is main function of the action and it should make the main
  effect of the action to be visible for the following actions, keeping
  possibility of roll-back, saving necessary things in action state,
  which is prepended to the list. So, driver struct doesn't include
  "prepare" field, as it is supposed to be called directly.

- commit/rollback is supposed to be called for the list of action
  states, to commit/rollback all the actions in reverse order

- When possible "commit" should not make visible effect for other
  actions, which make possible transparent logical interaction between
  actions.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/qemu/transactions.h | 46 +++++++++++++++++++++
 util/transactions.c         | 81 +++++++++++++++++++++++++++++++++++++
 util/meson.build            |  1 +
 3 files changed, 128 insertions(+)
 create mode 100644 include/qemu/transactions.h
 create mode 100644 util/transactions.c

diff --git a/include/qemu/transactions.h b/include/qemu/transactions.h
new file mode 100644
index 0000000000..db8fced65f
--- /dev/null
+++ b/include/qemu/transactions.h
@@ -0,0 +1,46 @@
+/*
+ * Simple transactions API
+ *
+ * Copyright (c) 2020 Virtuozzo International GmbH.
+ *
+ * Author:
+ *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef QEMU_TRANSACTIONS_H
+#define QEMU_TRANSACTIONS_H
+
+#include <gmodule.h>
+
+typedef struct BdrvActionDrv {
+    void (*abort)(void *opeque);
+    void (*commit)(void *opeque);
+    void (*clean)(void *opeque);
+} BdrvActionDrv;
+
+void tran_prepend(GSList **list, BdrvActionDrv *drv, void *opaque);
+void tran_abort(GSList *backup);
+void tran_commit(GSList *backup);
+static inline void tran_finalize(GSList *backup, int ret)
+{
+    if (ret < 0) {
+        tran_abort(backup);
+    } else {
+        tran_commit(backup);
+    }
+}
+
+#endif /* QEMU_TRANSACTIONS_H */
diff --git a/util/transactions.c b/util/transactions.c
new file mode 100644
index 0000000000..845d77fc16
--- /dev/null
+++ b/util/transactions.c
@@ -0,0 +1,81 @@
+/*
+ * Simple transactions API
+ *
+ * Copyright (c) 2020 Virtuozzo International GmbH.
+ *
+ * Author:
+ *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+
+#include "qemu/transactions.h"
+
+typedef struct BdrvAction {
+    BdrvActionDrv *drv;
+    void *opaque;
+} BdrvAction;
+
+void tran_prepend(GSList **list, BdrvActionDrv *drv, void *opaque)
+{
+    BdrvAction *act;
+
+    act = g_new(BdrvAction, 1);
+    *act = (BdrvAction) {
+        .drv = drv,
+        .opaque = opaque
+    };
+
+    *list = g_slist_prepend(*list, act);
+}
+
+void tran_abort(GSList *list)
+{
+    GSList *p;
+
+    for (p = list; p != NULL; p = p->next) {
+        BdrvAction *act = p->data;
+
+        if (act->drv->abort) {
+            act->drv->abort(act->opaque);
+        }
+
+        if (act->drv->clean) {
+            act->drv->clean(act->opaque);
+        }
+    }
+
+    g_slist_free_full(list, g_free);
+}
+
+void tran_commit(GSList *list)
+{
+    GSList *p;
+
+    for (p = list; p != NULL; p = p->next) {
+        BdrvAction *act = p->data;
+
+        if (act->drv->commit) {
+            act->drv->commit(act->opaque);
+        }
+
+        if (act->drv->clean) {
+            act->drv->clean(act->opaque);
+        }
+    }
+
+    g_slist_free_full(list, g_free);
+}
diff --git a/util/meson.build b/util/meson.build
index f359af0d46..8c7c28bd40 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -41,6 +41,7 @@ util_ss.add(files('qsp.c'))
 util_ss.add(files('range.c'))
 util_ss.add(files('stats64.c'))
 util_ss.add(files('systemd.c'))
+util_ss.add(files('transactions.c'))
 util_ss.add(when: 'CONFIG_POSIX', if_true: files('drm.c'))
 util_ss.add(files('guest-random.c'))
 
-- 
2.21.3