chromium通信系统-ipcz系统(四)-ipcz-分层、和mojo的关系以及handle

在只有mojo的情况下， 进程间通信都是靠unix 域套接字来完成了，由于这种方式比较低效，并且不够灵活，后来引入了ipcz。 但是系统中基本上使用mojo做进程间通信，想要一步到位迁移到ipcz系统是比较困难的。 所以chrome团队采用了一种折中的方法，利用原来mojo的channel进行socket通信，作为控制消息和唤醒机制。 使用ipcz 来实现共享内存和路由机制。另外由于chrome是一个面向多操作系统的任务，对于不同操作系统使用不同的代码实现，这样就要求抽取出操作系统相关的实现。这样使得现有代码非常混乱。另外ipcz系统希望减少耦合，尽量少的暴露实现细节，以及方便序列化，使用handle（句柄）代理不同层的对象。

ipcz主要有四个模块：
1、ipcz上层：third_party/ipcz 目录。 handle 为IpczHandle。 实现Node、NodeLink、RouterLink、Portal、NodeConnector、Router、Parcle 等对象。架空mojo的Node 和Port
2、ipcz driver层：mojo/core/ipcz_driver目录。handle 为 IpczDriverHandle。 实现Transport，ipcz层和mojo层的粘合剂，利用mojo层的通信能力服务ipcz层。序列化传输能力。
3、mojo 层：mojo/core目录。handle 为 MojoHandle。 实现Channel，进程间通信能力。
4、Platform层/mojo/public/cpp/platform目录。 handle 为PlatformHandle。 系统层面的实现，主要是对Socket文件描述的包装。

下面我们具体分析一下每一层handle的实现。

ipcz 层 IpczHandle实现

third_party/ipcz/src/ipcz/api_object.h

class APIObject : public RefCounted {
 public:
 enum ObjectType {
    kNode,
    kPortal,
    kBox,
    kTransport,
    kParcel,
  };
  static APIObject* FromHandle(IpczHandle handle) {
    return reinterpret_cast<APIObject*>(static_cast<uintptr_t>(handle));
  }

  // Takes ownership of an APIObject from an existing `handle`.
  static Ref<APIObject> TakeFromHandle(IpczHandle handle) {
    return AdoptRef(
        reinterpret_cast<APIObject*>(static_cast<uintptr_t>(handle)));
  }

  // Returns an IpczHandle which can be used to reference this object. The
  // reference is not owned by the caller.
  IpczHandle handle() const { return reinterpret_cast<uintptr_t>(this); }

  // Releases ownership of a Ref to produce a new IpczHandle which
  // implicilty owns the released reference.
  static IpczHandle ReleaseAsHandle(Ref<APIObject> object) {
    return static_cast<IpczHandle>(
        reinterpret_cast<uintptr_t>(object.release()));
  }
......  
}

ipcz层对应的对象基类为APIObject，APIObject 提供四个方法，FromHandle() 和 TakeFromHandle() 方法用于将IpczHandle() 转化为具体对象。handle() 和 ReleaseAsHandle() 方法用于将对象转成IpczHandle句柄。这里我们还可以看到系统里有5种类型的APIObject，分别是：

• kNode 代表IPCZ Node（节点）对象
• kPortal 代表ipcz Portal（端口）对象
• kBox 用于其他可传输的ipcz对象
• kTransport 代表ipcz Transport（传输点）对象
• kParcel 代表ipcz Parcel(消息)对象

ipcz driver 层 IpczDriverHandle实现

typedef uintptr_t IpczDriverHandle;

mojo/core/ipcz_driver/object.h

// Common base class for objects managed by Mojo's ipcz driver.
class MOJO_SYSTEM_IMPL_EXPORT ObjectBase
    : public base::RefCountedThreadSafe<ObjectBase> {
 public:
  REQUIRE_ADOPTION_FOR_REFCOUNTED_TYPE();

  enum Type : uint32_t {
    // An ipcz transport endpoint.
    kTransport,

    // A wrapped shared memory region.
    kSharedBuffer,

    // An active mapping for a shared memory region. These objects are not
    // serializable and cannot be transmitted over a Transport.
    kSharedBufferMapping,

    // A PlatformHandle which can be transmitted as-is by the platform's Channel
    // implementation, out-of-band from message data. This is the only type of
    // driver object which can be emitted by the driver's Serialize(), and it's
    // the only type accepted by its Transmit(). This type is unused on Windows,
    // where all platform handles are encoded as inline message data during
    // serialization.
    kTransmissiblePlatformHandle,

    // A PlatformHandle which may or may not be transmissible by the platform's
    // Channel implementation, but which can at least be transformed into
    // something transmissible during serialization.
    kWrappedPlatformHandle,

    // A DataPipe instance used to emulate Mojo data pipes over ipcz portals.
    kDataPipe,

    // A MojoTrap instance used to emulate a Mojo trap. These objects are not
    // serializable and cannot be transmitted over a Transport.
    kMojoTrap,

    // An Invitation instance used to emulate Mojo process invitations. These
    // objects are not serializable and cannot be transmitted over a Transport.
    kInvitation,
  };

  explicit ObjectBase(Type type);

  Type type() const { return type_; }

  IpczDriverHandle handle() const {
    return reinterpret_cast<IpczDriverHandle>(this);
  }

  static ObjectBase* FromHandle(IpczDriverHandle handle) {
    return reinterpret_cast<ObjectBase*>(handle);
  }

  static IpczDriverHandle ReleaseAsHandle(scoped_refptr<ObjectBase> object) {
    return reinterpret_cast<IpczDriverHandle>(object.release());
  }

  static scoped_refptr<ObjectBase> TakeFromHandle(IpczDriverHandle handle) {
    scoped_refptr<ObjectBase> object(FromHandle(handle));
    if (object) {
      // We're inheriting a ref previously owned by `handle`, so drop the extra
      // ref we just added.
      object->Release();
    }
    return object;
  }

......
// Computes the number of bytes and platform handles required to serialize
  // this object for transmission through `transmitter`. Returns false if the
  // object cannot be serialized or transmitted as such.
  virtual bool GetSerializedDimensions(Transport& transmitter,
                                       size_t& num_bytes,
                                       size_t& num_handles);

  // Attempts to serialize this object into `data` and `handles` which are
  // already sufficiently sized according to GetSerializedDimensions(). Returns
  // false if serialization fails.
  virtual bool Serialize(Transport& transmitter,
                         base::span<uint8_t> data,
                         base::span<PlatformHandle> handles);

ipcz driver层对应的对象基类为ObjectBase，ObjectBase 提供的方法包括： type方法返回对象的类型，FromHandle() 和 TakeFromHandle() 方法用于将IpczHandle() 转化为具体对象。handle() 和 ReleaseAsHandle() 方法用于将对象转成IpczHandle句柄。GetSerializedDimensions方法用来序列化过程中返回需要的内存空间和hanlde空间。 Serialize函数用于跨进程序列化传输。

这里我们还可以看到系统里有8种类型的ObjectBase，分别是：

• kTransport 代表传输点对象
• kSharedBuffer 代表共享内存（端口）对象
• kSharedBufferMapping 映射后的共享内存对象
• kTransmissiblePlatformHandle 可传输的PlatformHandle对象
• kWrappedPlatformHandle 包装的PlatformHandle对象
• kDataPipe 数据管道
• kMojoTrap Trap监听对象
• kInvitation 链接邀请对象。

Mojo层 MojoHandle实现

typedef uintptr_t MojoHandle;

mojo层名没有对应的对象。可以和ipcz driver handle 和 ipcz handle强转。

Platform层 PlatformHandle实现

class COMPONENT_EXPORT(MOJO_CPP_PLATFORM) PlatformHandle {
 public:
  enum class Type {
    kNone,
#if BUILDFLAG(IS_WIN) || BUILDFLAG(IS_FUCHSIA)
    kHandle,
#elif BUILDFLAG(IS_APPLE)
    kMachSend,
    kMachReceive,
#endif
#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
    kFd,
#endif
  };

 private:
  Type type_ = Type::kNone;

#if BUILDFLAG(IS_WIN)
  base::win::ScopedHandle handle_;
#elif BUILDFLAG(IS_FUCHSIA)
  zx::handle handle_;
#elif BUILDFLAG(IS_APPLE)
  base::mac::ScopedMachSendRight mach_send_;
  base::mac::ScopedMachReceiveRight mach_receive_;
#endif

#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
  base::ScopedFD fd_;
#endif
};

对于linux系统， type为kFd， fd_变量保存文件描述符。