db: handle transaction aborts properly

It turns out we don't properly handle transaction aborts as we should,
this likely broke when we implemented the shadow snapshots but as the
transaction concept was not exported via the API, and callers most
likely abandoned the filter on error this went unnoticed.

This patch ensures that transaction aborts are handled properly by
correctly managing the filter's transaction stack.

Signed-off-by: Paul Moore <paul@paul-moore.com>

src/db.c

@@ -2513,12 +2513,19 @@ void db_col_transaction_abort(struct db_filter_col *col)
 	struct db_filter **filters;
 	struct db_filter_snap *snap;
 
-	if (col->snapshots == NULL)
+	snap = col->snapshots;
+	if (snap == NULL)
 		return;
 
-	/* replace the current filter with the last snapshot */
-	snap = col->snapshots;
+	/* replace the current filter with the last snapshot, skipping shadow
+	 * snapshots are they are duplicates of the current snapshot */
+	if (snap->shadow) {
+		struct db_filter_snap *tmp = snap;
+		snap = snap->next;
+		_db_snap_release(tmp);
+	}
 	col->snapshots = snap->next;
+
 	filter_cnt = col->filter_cnt;
 	filters = col->filters;
 	col->filter_cnt = snap->filter_cnt;
@@ -2559,8 +2566,9 @@ void db_col_transaction_commit(struct db_filter_col *col)
 	if (snap->shadow) {
 		/* leave the shadow intact, but drop the next snapshot */
 		if (snap->next) {
-			snap->next = snap->next->next;
-			_db_snap_release(snap->next);
+			struct db_filter_snap *tmp = snap->next;
+			snap->next = tmp->next;
+			_db_snap_release(tmp);
 		}
 		return;
 	}