util: add transactions.c

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 action list (to do that, prepare func
  should call tran_add()). 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>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210428151804.439460-9-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

MAINTAINERS

@@ -2532,6 +2532,12 @@ M: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
 S: Maintained
 F: scripts/simplebench/
 
+Transactions helper
+M: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
+S: Maintained
+F: include/qemu/transactions.h
+F: util/transactions.c
+
 QAPI
 M: Markus Armbruster <armbru@redhat.com>
 M: Michael Roth <michael.roth@amd.com>

include/qemu/transactions.h

@@ -0,0 +1,63 @@
+/*
+ * Simple transactions API
+ *
+ * Copyright (c) 2021 Virtuozzo International GmbH.
+ *
+ * Author:
+ *  Vladimir Sementsov-Ogievskiy <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/>.
+ *
+ *
+ * = Generic transaction API =
+ *
+ * The intended usage is the following: you create "prepare" functions, which
+ * represents the actions. They will usually have Transaction* argument, and
+ * call tran_add() to register finalization callbacks. For finalization
+ * callbacks, prepare corresponding TransactionActionDrv structures.
+ *
+ * Then, when you need to make a transaction, create an empty Transaction by
+ * tran_create(), call your "prepare" functions on it, and finally call
+ * tran_abort() or tran_commit() to finalize the transaction by corresponding
+ * finalization actions in reverse order.
+ */
+
+#ifndef QEMU_TRANSACTIONS_H
+#define QEMU_TRANSACTIONS_H
+
+#include <gmodule.h>
+
+typedef struct TransactionActionDrv {
+    void (*abort)(void *opaque);
+    void (*commit)(void *opaque);
+    void (*clean)(void *opaque);
+} TransactionActionDrv;
+
+typedef struct Transaction Transaction;
+
+Transaction *tran_new(void);
+void tran_add(Transaction *tran, TransactionActionDrv *drv, void *opaque);
+void tran_abort(Transaction *tran);
+void tran_commit(Transaction *tran);
+
+static inline void tran_finalize(Transaction *tran, int ret)
+{
+    if (ret < 0) {
+        tran_abort(tran);
+    } else {
+        tran_commit(tran);
+    }
+}
+
+#endif /* QEMU_TRANSACTIONS_H */

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'))
 util_ss.add(files('yank.c'))

util/transactions.c

@@ -0,0 +1,96 @@
+/*
+ * Simple transactions API
+ *
+ * Copyright (c) 2021 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"
+#include "qemu/queue.h"
+
+typedef struct TransactionAction {
+    TransactionActionDrv *drv;
+    void *opaque;
+    QSLIST_ENTRY(TransactionAction) entry;
+} TransactionAction;
+
+struct Transaction {
+    QSLIST_HEAD(, TransactionAction) actions;
+};
+
+Transaction *tran_new(void)
+{
+    Transaction *tran = g_new(Transaction, 1);
+
+    QSLIST_INIT(&tran->actions);
+
+    return tran;
+}
+
+void tran_add(Transaction *tran, TransactionActionDrv *drv, void *opaque)
+{
+    TransactionAction *act;
+
+    act = g_new(TransactionAction, 1);
+    *act = (TransactionAction) {
+        .drv = drv,
+        .opaque = opaque
+    };
+
+    QSLIST_INSERT_HEAD(&tran->actions, act, entry);
+}
+
+void tran_abort(Transaction *tran)
+{
+    TransactionAction *act, *next;
+
+    QSLIST_FOREACH_SAFE(act, &tran->actions, entry, next) {
+        if (act->drv->abort) {
+            act->drv->abort(act->opaque);
+        }
+
+        if (act->drv->clean) {
+            act->drv->clean(act->opaque);
+        }
+
+        g_free(act);
+    }
+
+    g_free(tran);
+}
+
+void tran_commit(Transaction *tran)
+{
+    TransactionAction *act, *next;
+
+    QSLIST_FOREACH_SAFE(act, &tran->actions, entry, next) {
+        if (act->drv->commit) {
+            act->drv->commit(act->opaque);
+        }
+
+        if (act->drv->clean) {
+            act->drv->clean(act->opaque);
+        }
+
+        g_free(act);
+    }
+
+    g_free(tran);
+}