From 311de0b6358ba2b8eae20dcc5baf29472a273fc6 Mon Sep 17 00:00:00 2001
From: Simon Giesecke <sgiesecke@mozilla.com>
Date: Tue, 3 Dec 2019 15:25:49 +0000
Subject: [PATCH] Bug 1598164 - Added FlippedOnce class template to help
 reducing statefulness of boolean flags.
 r=dom-workers-and-storage-reviewers,ytausky

Differential Revision: https://phabricator.services.mozilla.com/D55080

--HG--
extra : moz-landing-system : lando
---
 dom/indexedDB/FlippedOnce.h | 39 +++++++++++++++++++++++++++++++++++++
 dom/indexedDB/moz.build     |  1 +
 2 files changed, 40 insertions(+)
 create mode 100644 dom/indexedDB/FlippedOnce.h

diff --git a/dom/indexedDB/FlippedOnce.h b/dom/indexedDB/FlippedOnce.h
new file mode 100644
index 0000000000000..94cd8570a4b6e
--- /dev/null
+++ b/dom/indexedDB/FlippedOnce.h
@@ -0,0 +1,39 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_indexeddb_flippedonce_h__
+#define mozilla_dom_indexeddb_flippedonce_h__
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+// A state-restricted bool, which can only be flipped once. It isn't required to
+// be flipped during its lifetime.
+template <bool Initial>
+class FlippedOnce {
+ public:
+  FlippedOnce(const FlippedOnce&) = delete;
+  FlippedOnce& operator=(const FlippedOnce&) = delete;
+
+  FlippedOnce() = default;
+
+  MOZ_IMPLICIT operator bool() const { return mValue; };
+
+  void Flip() {
+    MOZ_ASSERT(mValue == Initial);
+    EnsureFlipped();
+  }
+
+  void EnsureFlipped() { mValue = !Initial; }
+
+ private:
+  bool mValue = Initial;
+};
+}  // namespace mozilla
+
+#endif
diff --git a/dom/indexedDB/moz.build b/dom/indexedDB/moz.build
index 4302528e6847b..5237528963127 100644
--- a/dom/indexedDB/moz.build
+++ b/dom/indexedDB/moz.build
@@ -24,6 +24,7 @@ XPCSHELL_TESTS_MANIFESTS += [
 ]
 
 EXPORTS.mozilla.dom += [
+    'FlippedOnce.h',
     'IDBCursor.h',
     'IDBDatabase.h',
     'IDBEvents.h',