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diff --git a/js/xpconnect/src/XPCSelfHostedShmem.h b/js/xpconnect/src/XPCSelfHostedShmem.h
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+++ b/js/xpconnect/src/XPCSelfHostedShmem.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 xpcselfhostedshmem_h___
+#define xpcselfhostedshmem_h___
+
+#include "base/shared_memory.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/Span.h"
+#include "mozilla/StaticPtr.h"
+#include "nsIMemoryReporter.h"
+#include "nsIObserver.h"
+#include "nsIThread.h"
+
+namespace xpc {
+
+// This class is a singleton which holds a file-mapping of the Self Hosted
+// Stencil XDR, to be shared by the parent process with all the child processes.
+//
+// It is either initialized by the parent process by monitoring when the Self
+// hosted stencil is produced, or by the content process by reading a shared
+// memory-mapped file.
+class SelfHostedShmem final : public nsIMemoryReporter {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+
+  // NOTE: This type is identical to JS::SelfHostedCache, but we repeat it to
+  // avoid including JS engine API in ipc code.
+  using ContentType = mozilla::Span<const uint8_t>;
+
+  static SelfHostedShmem& GetSingleton();
+
+  // Initialize this singleton with the content of the Self-hosted Stencil XDR.
+  // This will be used to initialize the shared memory which would hold a copy.
+  //
+  // This function is not thread-safe and should be call at most once and from
+  // the main thread.
+  void InitFromParent(ContentType aXdr);
+
+  // Initialize this singleton with the content coming from the parent process,
+  // using a file handle which maps to the memory pages of the parent process.
+  //
+  // This function is not thread-safe and should be call at most once and from
+  // the main thread.
+  [[nodiscard]] bool InitFromChild(base::SharedMemoryHandle aHandle,
+                                   size_t aLen);
+
+  // Return a span over the read-only XDR content of the self-hosted stencil.
+  ContentType Content() const;
+
+  // Return the file handle which is under which the content is mapped.
+  const mozilla::UniqueFileHandle& Handle() const;
+
+  // Register this class to the memory reporter service.
+  void InitMemoryReporter();
+
+  // Unregister this class from the memory reporter service, and delete the
+  // memory associated with the shared memory. As the memory is borrowed by the
+  // JS engine, this function should be called after JS_Shutdown.
+  //
+  // NOTE: This is not using the Observer service which would call Shutdown
+  // while JS code might still be running during shutdown process.
+  static void Shutdown();
+
+ private:
+  SelfHostedShmem() = default;
+  ~SelfHostedShmem() = default;
+
+  static mozilla::StaticRefPtr<SelfHostedShmem> sSelfHostedXdr;
+
+  // read-only file Handle used to transfer from the parent process to content
+  // processes.
+  mozilla::UniqueFileHandle mHandle;
+
+  // Shared memory used by JS runtime initialization.
+  mozilla::UniquePtr<base::SharedMemory> mMem;
+
+  // Length of the content within the shared memory.
+  size_t mLen = 0;
+};
+
+}  // namespace xpc
+
+#endif  // !xpcselfhostedshmem_h___