1 files changed, 126 insertions, 0 deletions
diff --git a/mfbt/ScopeExit.h b/mfbt/ScopeExit.h
new file mode 100644
index 0000000000..9ddcd4b8f0
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+/* -*- 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/. */
+
+/* RAII class for executing arbitrary actions at scope end. */
+
+#ifndef mozilla_ScopeExit_h
+#define mozilla_ScopeExit_h
+
+/*
+ * See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4189.pdf for a
+ * standards-track version of this.
+ *
+ * Error handling can be complex when various actions need to be performed that
+ * need to be undone if an error occurs midway. This can be handled with a
+ * collection of boolean state variables and gotos, which can get clunky and
+ * error-prone:
+ *
+ * {
+ *   if (!a.setup())
+ *       goto fail;
+ *   isASetup = true;
+ *
+ *   if (!b.setup())
+ *       goto fail;
+ *   isBSetup = true;
+ *
+ *   ...
+ *   return true;
+ *
+ *   fail:
+ *     if (isASetup)
+ *         a.teardown();
+ *     if (isBSetup)
+ *         b.teardown();
+ *     return false;
+ *  }
+ *
+ * ScopeExit is a mechanism to simplify this pattern by keeping an RAII guard
+ * class that will perform the teardown on destruction, unless released. So the
+ * above would become:
+ *
+ * {
+ *   if (!a.setup()) {
+ *       return false;
+ *   }
+ *   auto guardA = MakeScopeExit([&] {
+ *       a.teardown();
+ *   });
+ *
+ *   if (!b.setup()) {
+ *       return false;
+ *   }
+ *   auto guardB = MakeScopeExit([&] {
+ *       b.teardown();
+ *   });
+ *
+ *   ...
+ *   guardA.release();
+ *   guardB.release();
+ *   return true;
+ * }
+ *
+ * This header provides:
+ *
+ * - |ScopeExit| - a container for a cleanup call, automically called at the
+ *   end of the scope;
+ * - |MakeScopeExit| - a convenience function for constructing a |ScopeExit|
+ *   with a given cleanup routine, commonly used with a lambda function.
+ *
+ * Note that the RAII classes defined in this header do _not_ perform any form
+ * of reference-counting or garbage-collection. These classes have exactly two
+ * behaviors:
+ *
+ * - if |release()| has not been called, the cleanup is always performed at
+ *   the end of the scope;
+ * - if |release()| has been called, nothing will happen at the end of the
+ *   scope.
+ */
+
+#include <utility>
+
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+template <typename ExitFunction>
+class MOZ_STACK_CLASS ScopeExit {
+  ExitFunction mExitFunction;
+  bool mExecuteOnDestruction;
+
+ public:
+  explicit ScopeExit(ExitFunction&& cleanup)
+      : mExitFunction(std::move(cleanup)), mExecuteOnDestruction(true) {}
+
+  ScopeExit(ScopeExit&& rhs)
+      : mExitFunction(std::move(rhs.mExitFunction)),
+        mExecuteOnDestruction(rhs.mExecuteOnDestruction) {
+    rhs.release();
+  }
+
+  ~ScopeExit() {
+    if (mExecuteOnDestruction) {
+      mExitFunction();
+    }
+  }
+
+  void release() { mExecuteOnDestruction = false; }
+
+ private:
+  explicit ScopeExit(const ScopeExit&) = delete;
+  ScopeExit& operator=(const ScopeExit&) = delete;
+  ScopeExit& operator=(ScopeExit&&) = delete;
+};
+
+template <typename ExitFunction>
+[[nodiscard]] ScopeExit<ExitFunction> MakeScopeExit(
+    ExitFunction&& exitFunction) {
+  return ScopeExit<ExitFunction>(std::move(exitFunction));
+}
+
+} /* namespace mozilla */
+
+#endif /* mozilla_ScopeExit_h */