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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 {
+namespace layers {
+class ShadowLayerForwarder;
+} // namespace layers
+
+namespace 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;
+#ifdef DEBUG
+ // For ShadowLayerForwarder::CheckSurfaceDescriptor
+ friend class mozilla::layers::ShadowLayerForwarder;
+#endif
+
+ 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);
+
+ 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;
+ }
+
+ static already_AddRefed<Shmem::SharedMemory> Alloc(size_t aNBytes,
+ bool aUnsafe,
+ bool aProtect = false);
+
+ // 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);
+
+ static void Dealloc(SharedMemory* aSegment);
+
+ template <typename T>
+ void AssertAligned() const {
+ if (0 != (mSize % sizeof(T))) MOZ_CRASH("shmem is not T-aligned");
+ }
+
+#if !defined(DEBUG)
+ void AssertInvariants() const {}
+
+ static uint32_t* PtrToSize(SharedMemory* aSegment) {
+ char* endOfSegment =
+ reinterpret_cast<char*>(aSegment->memory()) + aSegment->Size();
+ return reinterpret_cast<uint32_t*>(endOfSegment - sizeof(uint32_t));
+ }
+
+#else
+ void AssertInvariants() const;
+#endif
+
+ RefPtr<SharedMemory> mSegment;
+ void* mData;
+ size_t mSize;
+ id_t mId;
+};
+
+} // namespace ipc
+} // namespace mozilla
+
+#endif // ifndef mozilla_ipc_Shmem_h