notify: add NotiferWithReturn so notifier list can abort

notifier_list_notify() has no return value.  This is fine when we just
want to invoke side-effects.

Sometimes it's useful for notifiers to produce a return value.  This
allows notifiers to "veto" an operation and will be used by the block
layer before-write notifier.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/qemu/notify.h

@@ -40,4 +40,33 @@ void notifier_remove(Notifier *notifier);
 
 void notifier_list_notify(NotifierList *list, void *data);
 
+/* Same as Notifier but allows .notify() to return errors */
+typedef struct NotifierWithReturn NotifierWithReturn;
+
+struct NotifierWithReturn {
+    /**
+     * Return 0 on success (next notifier will be invoked), otherwise
+     * notifier_with_return_list_notify() will stop and return the value.
+     */
+    int (*notify)(NotifierWithReturn *notifier, void *data);
+    QLIST_ENTRY(NotifierWithReturn) node;
+};
+
+typedef struct NotifierWithReturnList {
+    QLIST_HEAD(, NotifierWithReturn) notifiers;
+} NotifierWithReturnList;
+
+#define NOTIFIER_WITH_RETURN_LIST_INITIALIZER(head) \
+    { QLIST_HEAD_INITIALIZER((head).notifiers) }
+
+void notifier_with_return_list_init(NotifierWithReturnList *list);
+
+void notifier_with_return_list_add(NotifierWithReturnList *list,
+                                   NotifierWithReturn *notifier);
+
+void notifier_with_return_remove(NotifierWithReturn *notifier);
+
+int notifier_with_return_list_notify(NotifierWithReturnList *list,
+                                     void *data);
+
 #endif

util/notify.c

@@ -39,3 +39,33 @@ void notifier_list_notify(NotifierList *list, void *data)
         notifier->notify(notifier, data);
     }
 }
+
+void notifier_with_return_list_init(NotifierWithReturnList *list)
+{
+    QLIST_INIT(&list->notifiers);
+}
+
+void notifier_with_return_list_add(NotifierWithReturnList *list,
+                                   NotifierWithReturn *notifier)
+{
+    QLIST_INSERT_HEAD(&list->notifiers, notifier, node);
+}
+
+void notifier_with_return_remove(NotifierWithReturn *notifier)
+{
+    QLIST_REMOVE(notifier, node);
+}
+
+int notifier_with_return_list_notify(NotifierWithReturnList *list, void *data)
+{
+    NotifierWithReturn *notifier, *next;
+    int ret = 0;
+
+    QLIST_FOREACH_SAFE(notifier, &list->notifiers, node, next) {
+        ret = notifier->notify(notifier, data);
+        if (ret != 0) {
+            break;
+        }
+    }
+    return ret;
+}