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diff --git a/ipc/glue/Shmem.h b/ipc/glue/Shmem.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_ipc_Shmem_h
+#define mozilla_ipc_Shmem_h
+
+#include "mozilla/Attributes.h"
+
+#include "base/basictypes.h"
+#include "base/process.h"
+
+#include "nscore.h"
+#include "nsDebug.h"
+
+#include "mozilla/ipc/SharedMemory.h"
+#include "mozilla/Range.h"
+#include "mozilla/UniquePtr.h"
+
+/**
+ * |Shmem| is one agent in the IPDL shared memory scheme.  The way it
+    works is essentially
+ *
+ *  (1) C++ code calls, say, |parentActor->AllocShmem(size)|
+
+ *  (2) IPDL-generated code creates a |mozilla::ipc::SharedMemory|
+ *  wrapping the bare OS shmem primitives.  The code then adds the new
+ *  SharedMemory to the set of shmem segments being managed by IPDL.
+ *
+ *  (3) IPDL-generated code "shares" the new SharedMemory to the child
+ *  process, and then sends a special asynchronous IPC message to the
+ *  child notifying it of the creation of the segment.  (What this
+ *  means is OS specific.)
+ *
+ *  (4a) The child receives the special IPC message, and using the
+ *  |SharedMemory{Basic}::Handle| it was passed, creates a
+ *  |mozilla::ipc::SharedMemory| in the child
+ *  process.
+ *
+ *  (4b) After sending the "shmem-created" IPC message, IPDL-generated
+ *  code in the parent returns a |mozilla::ipc::Shmem| back to the C++
+ *  caller of |parentActor->AllocShmem()|.  The |Shmem| is a "weak
+ *  reference" to the underlying |SharedMemory|, which is managed by
+ *  IPDL-generated code.  C++ consumers of |Shmem| can't get at the
+ *  underlying |SharedMemory|.
+ *
+ * If parent code wants to give access rights to the Shmem to the
+ * child, it does so by sending its |Shmem| to the child, in an IPDL
+ * message.  The parent's |Shmem| then "dies", i.e. becomes
+ * inaccessible.  This process could be compared to passing a
+ * "shmem-access baton" between parent and child.
+ */
+
+namespace mozilla::ipc {
+
+class IProtocol;
+class IToplevelProtocol;
+
+template <typename P>
+struct IPDLParamTraits;
+
+class Shmem final {
+  friend struct IPDLParamTraits<mozilla::ipc::Shmem>;
+  friend class mozilla::ipc::IProtocol;
+  friend class mozilla::ipc::IToplevelProtocol;
+
+ public:
+  typedef int32_t id_t;
+  // Low-level wrapper around platform shmem primitives.
+  typedef mozilla::ipc::SharedMemory SharedMemory;
+
+  Shmem() : mSegment(nullptr), mData(nullptr), mSize(0), mId(0) {}
+
+  Shmem(const Shmem& aOther) = default;
+
+  ~Shmem() {
+    // Shmem only holds a "weak ref" to the actual segment, which is
+    // owned by IPDL. So there's nothing interesting to be done here
+    forget();
+  }
+
+  Shmem& operator=(const Shmem& aRhs) = default;
+
+  bool operator==(const Shmem& aRhs) const { return mSegment == aRhs.mSegment; }
+
+  // Returns whether this Shmem is writable by you, and thus whether you can
+  // transfer writability to another actor.
+  bool IsWritable() const { return mSegment != nullptr; }
+
+  // Returns whether this Shmem is readable by you, and thus whether you can
+  // transfer readability to another actor.
+  bool IsReadable() const { return mSegment != nullptr; }
+
+  // Return a pointer to the user-visible data segment.
+  template <typename T>
+  T* get() const {
+    AssertInvariants();
+    AssertAligned<T>();
+
+    return reinterpret_cast<T*>(mData);
+  }
+
+  // Return the size of the segment as requested when this shmem
+  // segment was allocated, in units of T.  The underlying mapping may
+  // actually be larger because of page alignment and private data,
+  // but this isn't exposed to clients.
+  template <typename T>
+  size_t Size() const {
+    AssertInvariants();
+    AssertAligned<T>();
+
+    return mSize / sizeof(T);
+  }
+
+  template <typename T>
+  Range<T> Range() const {
+    return {get<T>(), Size<T>()};
+  }
+
+ private:
+  // These shouldn't be used directly, use the IPDL interface instead.
+
+  Shmem(SharedMemory* aSegment, id_t aId, size_t aSize, bool aUnsafe);
+
+  id_t Id() const { return mId; }
+
+  SharedMemory* Segment() const { return mSegment; }
+
+#ifndef DEBUG
+  void RevokeRights() {}
+#else
+  void RevokeRights();
+#endif
+
+  void forget() {
+    mSegment = nullptr;
+    mData = nullptr;
+    mSize = 0;
+    mId = 0;
+#ifdef DEBUG
+    mUnsafe = false;
+#endif
+  }
+
+  static already_AddRefed<Shmem::SharedMemory> Alloc(size_t aNBytes);
+
+  // Prepare this to be shared with another process. Return an IPC message that
+  // contains enough information for the other process to map this segment in
+  // OpenExisting() below.  Return a new message if successful (owned by the
+  // caller), nullptr if not.
+  UniquePtr<IPC::Message> MkCreatedMessage(int32_t routingId);
+
+  // Stop sharing this with another process. Return an IPC message that
+  // contains enough information for the other process to unmap this
+  // segment.  Return a new message if successful (owned by the
+  // caller), nullptr if not.
+  UniquePtr<IPC::Message> MkDestroyedMessage(int32_t routingId);
+
+  // Return a SharedMemory instance in this process using the descriptor shared
+  // to us by the process that created the underlying OS shmem resource.  The
+  // contents of the descriptor depend on the type of SharedMemory that was
+  // passed to us.
+  static already_AddRefed<SharedMemory> OpenExisting(
+      const IPC::Message& aDescriptor, id_t* aId, bool aProtect = false);
+
+  template <typename T>
+  void AssertAligned() const {
+    if (0 != (mSize % sizeof(T))) MOZ_CRASH("shmem is not T-aligned");
+  }
+
+#if !defined(DEBUG)
+  void AssertInvariants() const {}
+#else
+  void AssertInvariants() const;
+#endif
+
+  RefPtr<SharedMemory> mSegment;
+  void* mData;
+  size_t mSize;
+  id_t mId;
+#ifdef DEBUG
+  bool mUnsafe = false;
+#endif
+};
+
+}  // namespace mozilla::ipc
+
+#endif  // ifndef mozilla_ipc_Shmem_h