src: add ExclusiveAccess class

Similar to Rust's `std::sync::Mutex` in that it requires one to unwrap
the object before accessing it, acquiring the mutex in the process.

PR-URL: #31717
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: David Carlier <devnexen@gmail.com>

Author: bnoordhuis

src/node_mutex.h

@@ -6,6 +6,9 @@
 #include "util.h"
 #include "uv.h"
 
+#include <memory>  // std::shared_ptr<T>
+#include <utility>  // std::forward<T>
+
 namespace node {
 
 template <typename Traits> class ConditionVariableBase;
@@ -15,6 +18,51 @@ struct LibuvMutexTraits;
 using ConditionVariable = ConditionVariableBase<LibuvMutexTraits>;
 using Mutex = MutexBase<LibuvMutexTraits>;
 
+template <typename T, typename MutexT = Mutex>
+class ExclusiveAccess {
+ public:
+  ExclusiveAccess() = default;
+
+  template <typename... Args>
+  explicit ExclusiveAccess(Args&&... args)
+      : item_(std::forward<Args>(args)...) {}
+
+  ExclusiveAccess(const ExclusiveAccess&) = delete;
+  ExclusiveAccess& operator=(const ExclusiveAccess&) = delete;
+
+  class Scoped {
+   public:
+    // ExclusiveAccess will commonly be used in conjuction with std::shared_ptr
+    // and without this constructor it's too easy to forget to keep a reference
+    // around to the shared_ptr while operating on the ExclusiveAccess object.
+    explicit Scoped(const std::shared_ptr<ExclusiveAccess>& shared)
+        : shared_(shared)
+        , scoped_lock_(shared->mutex_)
+        , pointer_(&shared->item_) {}
+
+    explicit Scoped(ExclusiveAccess* exclusive_access)
+        : shared_()
+        , scoped_lock_(exclusive_access->mutex_)
+        , pointer_(&exclusive_access->item_) {}
+
+    T& operator*() const { return *pointer_; }
+    T* operator->() const { return pointer_; }
+
+    Scoped(const Scoped&) = delete;
+    Scoped& operator=(const Scoped&) = delete;
+
+   private:
+    std::shared_ptr<ExclusiveAccess> shared_;
+    typename MutexT::ScopedLock scoped_lock_;
+    T* const pointer_;
+  };
+
+ private:
+  friend class ScopedLock;
+  MutexT mutex_;
+  T item_;
+};
+
 template <typename Traits>
 class MutexBase {
  public: