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diff --git a/dom/heapsnapshot/AutoMemMap.h b/dom/heapsnapshot/AutoMemMap.h
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+++ b/dom/heapsnapshot/AutoMemMap.h
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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/. */
+
+#ifndef mozilla_devtools_AutoMemMap_h
+#define mozilla_devtools_AutoMemMap_h
+
+#include <prio.h>
+#include "mozilla/GuardObjects.h"
+
+namespace mozilla {
+namespace devtools {
+
+// # AutoMemMap
+//
+// AutoMemMap is an RAII class to manage mapping a file to memory. It is a
+// wrapper aorund managing opening and closing a file and calling PR_MemMap and
+// PR_MemUnmap.
+//
+// Example usage:
+//
+//     {
+//       AutoMemMap mm;
+//       if (NS_FAILED(mm.init("/path/to/desired/file"))) {
+//          // Handle the error however you see fit.
+//         return false;
+//       }
+//
+//       doStuffWithMappedMemory(mm.address());
+//     }
+//     // The memory is automatically unmapped when the AutoMemMap leaves scope.
+class MOZ_RAII AutoMemMap
+{
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER;
+
+  PRFileInfo64 fileInfo;
+  PRFileDesc*  fd;
+  PRFileMap*   fileMap;
+  void*        addr;
+
+  AutoMemMap(const AutoMemMap& aOther) = delete;
+  void operator=(const AutoMemMap& aOther) = delete;
+
+public:
+  explicit AutoMemMap(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM)
+      : fd(nullptr)
+      , fileMap(nullptr)
+      , addr(nullptr)
+  {
+      MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+  };
+  ~AutoMemMap();
+
+  // Initialize this AutoMemMap.
+  nsresult init(const char* filePath, int flags = PR_RDONLY, int mode = 0,
+                PRFileMapProtect prot = PR_PROT_READONLY);
+
+  // Get the size of the memory mapped file.
+  uint32_t size() const {
+      MOZ_ASSERT(fileInfo.size <= UINT32_MAX,
+                 "Should only call size() if init() succeeded.");
+      return uint32_t(fileInfo.size);
+  }
+
+  // Get the mapped memory.
+  void* address() { MOZ_ASSERT(addr); return addr; }
+  const void* address() const { MOZ_ASSERT(addr); return addr; }
+};
+
+} // namespace devtools
+} // namespace mozilla
+
+#endif // mozilla_devtools_AutoMemMap_h