mojo/system/message_in_transit.h

// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef MOJO_SYSTEM_MESSAGE_IN_TRANSIT_H_
#define MOJO_SYSTEM_MESSAGE_IN_TRANSIT_H_

#include <stddef.h>
#include <stdint.h>

#include <vector>

#include "base/macros.h"
#include "base/memory/aligned_memory.h"
#include "base/memory/scoped_ptr.h"
#include "mojo/system/dispatcher.h"
#include "mojo/system/system_impl_export.h"

namespace mojo {
namespace system {

class Channel;
class TransportData;

// This class is used to represent data in transit. It is thread-unsafe.
//
// |MessageInTransit| buffers:
//
// A |MessageInTransit| can be serialized by writing the main buffer and then,
// if it has one, the transport data buffer. Both buffers are
// |kMessageAlignment|-byte aligned and a multiple of |kMessageAlignment| bytes
// in size.
//
// The main buffer consists of the header (of type |Header|, which is an
// internal detail of this class) followed immediately by the message data
// (accessed by |bytes()| and of size |num_bytes()|, and also
// |kMessageAlignment|-byte aligned), and then any padding needed to make the
// main buffer a multiple of |kMessageAlignment| bytes in size.
//
// See |TransportData| for a description of the (serialized) transport data
// buffer.
class MOJO_SYSTEM_IMPL_EXPORT MessageInTransit {
 public:
  typedef uint16_t Type;
  // Messages that are forwarded to |MessagePipeEndpoint|s.
  static const Type kTypeMessagePipeEndpoint = 0;
  // Messages that are forwarded to |MessagePipe|s.
  static const Type kTypeMessagePipe = 1;
  // Messages that are consumed by the |Channel|.
  static const Type kTypeChannel = 2;
  // Messages that are consumed by the |RawChannel| (implementation).
  static const Type kTypeRawChannel = 3;

  typedef uint16_t Subtype;
  // Subtypes for type |kTypeMessagePipeEndpoint|:
  static const Subtype kSubtypeMessagePipeEndpointData = 0;
  // Subtypes for type |kTypeMessagePipe|:
  // Nothing currently.
  // Subtypes for type |kTypeChannel|:
  static const Subtype kSubtypeChannelRunMessagePipeEndpoint = 0;
  static const Subtype kSubtypeChannelRemoveMessagePipeEndpoint = 1;
  static const Subtype kSubtypeChannelRemoveMessagePipeEndpointAck = 2;
  // Subtypes for type |kTypeRawChannel|:
  static const Subtype kSubtypeRawChannelPosixExtraPlatformHandles = 0;

  typedef uint32_t EndpointId;
  // Never a valid endpoint ID.
  static const EndpointId kInvalidEndpointId = 0;

  // Messages (the header and data) must always be aligned to a multiple of this
  // quantity (which must be a power of 2).
  static const size_t kMessageAlignment = 8;

  // Forward-declare |Header| so that |View| can use it:
 private:
  struct Header;
 public:
  // This represents a view of serialized message data in a raw buffer.
  class MOJO_SYSTEM_IMPL_EXPORT View {
   public:
    // Constructs a view from the given buffer of the given size. (The size must
    // be as provided by |MessageInTransit::GetNextMessageSize()|.) The buffer
    // must remain alive/unmodified through the lifetime of this object.
    // |buffer| should be |kMessageAlignment|-byte aligned.
    View(size_t message_size, const void* buffer);

    // Checks that the given |View| appears to be for a valid message, within
    // predetermined limits (e.g., |num_bytes()| and |main_buffer_size()|, that
    // |transport_data_buffer()|/|transport_data_buffer_size()| is for valid
    // transport data -- see |TransportData::ValidateBuffer()|).
    //
    // It returns true (and leaves |error_message| alone) if this object appears
    // to be a valid message (according to the above) and false, pointing
    // |*error_message| to a suitable error message, if not.
    bool IsValid(size_t serialized_platform_handle_size,
                 const char** error_message) const;

    // API parallel to that for |MessageInTransit| itself (mostly getters for
    // header data).
    const void* main_buffer() const { return buffer_; }
    size_t main_buffer_size() const {
      return RoundUpMessageAlignment(sizeof(Header) + header()->num_bytes);
    }
    const void* transport_data_buffer() const {
      return (total_size() > main_buffer_size()) ?
          static_cast<const char*>(buffer_) + main_buffer_size() : NULL;
    }
    size_t transport_data_buffer_size() const {
      return total_size() - main_buffer_size();
    }
    size_t total_size() const { return header()->total_size; }
    uint32_t num_bytes() const { return header()->num_bytes; }
    const void* bytes() const {
      return static_cast<const char*>(buffer_) + sizeof(Header);
    }
    Type type() const { return header()->type; }
    Subtype subtype() const { return header()->subtype; }
    EndpointId source_id() const { return header()->source_id; }
    EndpointId destination_id() const { return header()->destination_id; }

   private:
    const Header* header() const { return static_cast<const Header*>(buffer_); }

    const void* const buffer_;

    // Though this struct is trivial, disallow copy and assign, since it doesn't
    // own its data. (If you're copying/assigning this, you're probably doing
    // something wrong.)
    DISALLOW_COPY_AND_ASSIGN(View);
  };

  // |bytes| is optional; if null, the message data will be zero-initialized.
  MessageInTransit(Type type,
                   Subtype subtype,
                   uint32_t num_bytes,
                   const void* bytes);
  // Constructs a |MessageInTransit| from a |View|.
  explicit MessageInTransit(const View& message_view);

  ~MessageInTransit();

  // Gets the size of the next message from |buffer|, which has |buffer_size|
  // bytes currently available, returning true and setting |*next_message_size|
  // on success. |buffer| should be aligned on a |kMessageAlignment| boundary
  // (and on success, |*next_message_size| will be a multiple of
  // |kMessageAlignment|).
  // TODO(vtl): In |RawChannelPosix|, the alignment requirements are currently
  // satisified on a faith-based basis.
  static bool GetNextMessageSize(const void* buffer,
                                 size_t buffer_size,
                                 size_t* next_message_size);

  // Makes this message "own" the given set of dispatchers. The dispatchers must
  // not be referenced from anywhere else (in particular, not from the handle
  // table), i.e., each dispatcher must have a reference count of 1. This
  // message must not already have dispatchers.
  void SetDispatchers(scoped_ptr<DispatcherVector> dispatchers);

  // Sets the |TransportData| for this message. This should only be done when
  // there are no dispatchers and no existing |TransportData|.
  void SetTransportData(scoped_ptr<TransportData> transport_data);

  // Serializes any dispatchers to the secondary buffer. This message must not
  // already have a secondary buffer (so this must only be called once). The
  // caller must ensure (e.g., by holding on to a reference) that |channel|
  // stays alive through the call.
  void SerializeAndCloseDispatchers(Channel* channel);

  // Gets the main buffer and its size (in number of bytes), respectively.
  const void* main_buffer() const { return main_buffer_.get(); }
  size_t main_buffer_size() const { return main_buffer_size_; }

  // Gets the transport data buffer (if any).
  const TransportData* transport_data() const { return transport_data_.get(); }
  TransportData* transport_data() { return transport_data_.get(); }

  // Gets the total size of the message (see comment in |Header|, below).
  size_t total_size() const { return header()->total_size; }

  // Gets the size of the message data.
  uint32_t num_bytes() const { return header()->num_bytes; }

  // Gets the message data (of size |num_bytes()| bytes).
  const void* bytes() const { return main_buffer_.get() + sizeof(Header); }
  void* bytes() { return main_buffer_.get() + sizeof(Header); }

  Type type() const { return header()->type; }
  Subtype subtype() const { return header()->subtype; }
  EndpointId source_id() const { return header()->source_id; }
  EndpointId destination_id() const { return header()->destination_id; }

  void set_source_id(EndpointId source_id) { header()->source_id = source_id; }
  void set_destination_id(EndpointId destination_id) {
    header()->destination_id = destination_id;
  }

  // Gets the dispatchers attached to this message; this may return null if
  // there are none. Note that the caller may mutate the set of dispatchers
  // (e.g., take ownership of all the dispatchers, leaving the vector empty).
  DispatcherVector* dispatchers() { return dispatchers_.get(); }

  // Returns true if this message has dispatchers attached.
  bool has_dispatchers() const {
    return dispatchers_ && !dispatchers_->empty();
  }

  // Rounds |n| up to a multiple of |kMessageAlignment|.
  static inline size_t RoundUpMessageAlignment(size_t n) {
    return (n + kMessageAlignment - 1) & ~(kMessageAlignment - 1);
  }

 private:
  // To allow us to make compile-assertions about |Header| in the .cc file.
  struct PrivateStructForCompileAsserts;

  // Header for the data (main buffer). Must be a multiple of
  // |kMessageAlignment| bytes in size. Must be POD.
  struct Header {
    // Total size of the message, including the header, the message data
    // ("bytes") including padding (to make it a multiple of |kMessageAlignment|
    // bytes), and serialized handle information. Note that this may not be the
    // correct value if dispatchers are attached but
    // |SerializeAndCloseDispatchers()| has not been called.
    uint32_t total_size;
    Type type;  // 2 bytes.
    Subtype subtype;  // 2 bytes.
    EndpointId source_id;  // 4 bytes.
    EndpointId destination_id;  // 4 bytes.
    // Size of actual message data.
    uint32_t num_bytes;
    uint32_t unused;
  };

  const Header* header() const {
    return reinterpret_cast<const Header*>(main_buffer_.get());
  }
  Header* header() { return reinterpret_cast<Header*>(main_buffer_.get()); }

  void UpdateTotalSize();

  const size_t main_buffer_size_;
  const scoped_ptr<char, base::AlignedFreeDeleter> main_buffer_;  // Never null.

  scoped_ptr<TransportData> transport_data_;  // May be null.

  // Any dispatchers that may be attached to this message. These dispatchers
  // should be "owned" by this message, i.e., have a ref count of exactly 1. (We
  // allow a dispatcher entry to be null, in case it couldn't be duplicated for
  // some reason.)
  scoped_ptr<DispatcherVector> dispatchers_;

  DISALLOW_COPY_AND_ASSIGN(MessageInTransit);
};

}  // namespace system
}  // namespace mojo

#endif  // MOJO_SYSTEM_MESSAGE_IN_TRANSIT_H_