runtime/vm/snapshot.h

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#ifndef RUNTIME_VM_SNAPSHOT_H_
#define RUNTIME_VM_SNAPSHOT_H_

#include <memory>
#include <utility>

#include "platform/assert.h"
#include "vm/allocation.h"
#include "vm/bitfield.h"
#include "vm/datastream.h"
#include "vm/finalizable_data.h"
#include "vm/globals.h"
#include "vm/growable_array.h"
#include "vm/isolate.h"
#include "vm/message.h"
#include "vm/visitor.h"

namespace dart {

// Forward declarations.
class AbstractType;
class Array;
class Class;
class ClassTable;
class Closure;
class Code;
class ExternalTypedData;
class TypedDataBase;
class GrowableObjectArray;
class Heap;
class Instance;
class Instructions;
class LanguageError;
class Library;
class LinkedHashMap;
class Object;
class PassiveObject;
class ObjectStore;
class MegamorphicCache;
class PageSpace;
class TypedDataView;
class String;
class TypeArguments;
class TypedData;
class UnhandledException;

// Serialized object header encoding is as follows:
// - Smi: the Smi value is written as is (last bit is not tagged).
// - VM object (from VM isolate): (object id in vm isolate | 0x3)
//   This value is serialized as a negative number.
//   (note VM objects are never serialized, they are expected to be found
//    using ths unique ID assigned to them).
// - Reference to object that has already been written: (object id | 0x3)
//   This valus is serialized as a positive number.
// - Object that is seen for the first time (inlined in the stream):
//   (a unique id for this object | 0x1)
enum SerializedHeaderType {
  kInlined = 0x1,
  kObjectId = 0x3,
};
static const int8_t kHeaderTagBits = 2;
static const int8_t kObjectIdBits = (kBitsPerInt32 - (kHeaderTagBits + 1));
static const intptr_t kMaxObjectId = (kMaxUint32 >> (kHeaderTagBits + 1));
static const bool kAsReference = true;
static const bool kAsInlinedObject = false;
static const intptr_t kInvalidPatchIndex = -1;

class SerializedHeaderTag
    : public BitField<intptr_t, enum SerializedHeaderType, 0, kHeaderTagBits> {
};

class SerializedHeaderData
    : public BitField<intptr_t, intptr_t, kHeaderTagBits, kObjectIdBits> {};

enum DeserializeState {
  kIsDeserialized = 0,
  kIsNotDeserialized = 1,
};

enum SerializeState {
  kIsSerialized = 0,
  kIsNotSerialized = 1,
};

// Structure capturing the raw snapshot.
//
class Snapshot {
 public:
  enum Kind {
    kFull,     // Full snapshot of core libraries or an application.
    kFullJIT,  // Full + JIT code
    kFullAOT,  // Full + AOT code
    kMessage,  // A partial snapshot used only for isolate messaging.
    kNone,     // gen_snapshot
    kInvalid
  };
  static const char* KindToCString(Kind kind);

  static const Snapshot* SetupFromBuffer(const void* raw_memory);

  static const int32_t kMagicValue = 0xdcdcf5f5;
  static const intptr_t kMagicOffset = 0;
  static const intptr_t kMagicSize = sizeof(int32_t);
  static const intptr_t kLengthOffset = kMagicOffset + kMagicSize;
  static const intptr_t kLengthSize = sizeof(int64_t);
  static const intptr_t kKindOffset = kLengthOffset + kLengthSize;
  static const intptr_t kKindSize = sizeof(int64_t);
  static const intptr_t kHeaderSize = kKindOffset + kKindSize;

  // Accessors.
  bool check_magic() const {
    return Read<int32_t>(kMagicOffset) == kMagicValue;
  }
  void set_magic() { return Write<int32_t>(kMagicOffset, kMagicValue); }
  // Excluding the magic value from the size written in the buffer is needed
  // so we give a proper version mismatch error for snapshots create before
  // magic value was written by the VM instead of the embedder.
  int64_t large_length() const {
    return Read<int64_t>(kLengthOffset) + kMagicSize;
  }
  intptr_t length() const { return static_cast<intptr_t>(large_length()); }
  void set_length(intptr_t value) {
    return Write<int64_t>(kLengthOffset, value - kMagicSize);
  }
  Kind kind() const { return static_cast<Kind>(Read<int64_t>(kKindOffset)); }
  void set_kind(Kind value) { return Write<int64_t>(kKindOffset, value); }

  static bool IsFull(Kind kind) {
    return (kind == kFull) || (kind == kFullJIT) || (kind == kFullAOT);
  }
  static bool IncludesCode(Kind kind) {
    return (kind == kFullJIT) || (kind == kFullAOT);
  }
  static bool IncludesBytecode(Kind kind) {
    return (kind == kFull) || (kind == kFullJIT);
  }

  const uint8_t* Addr() const { return reinterpret_cast<const uint8_t*>(this); }

  const uint8_t* DataImage() const {
    if (!IncludesCode(kind())) {
      return NULL;
    }
    uword offset = Utils::RoundUp(length(), kMaxObjectAlignment);
    return Addr() + offset;
  }

 private:
  // Prevent Snapshot from ever being allocated directly.
  Snapshot();

  template <typename T>
  T Read(intptr_t offset) const {
    return LoadUnaligned(
        reinterpret_cast<const T*>(reinterpret_cast<uword>(this) + offset));
  }

  template <typename T>
  void Write(intptr_t offset, T value) {
    return StoreUnaligned(
        reinterpret_cast<T*>(reinterpret_cast<uword>(this) + offset), value);
  }

  DISALLOW_COPY_AND_ASSIGN(Snapshot);
};

inline static bool IsSnapshotCompatible(Snapshot::Kind vm_kind,
                                        Snapshot::Kind isolate_kind) {
  if (vm_kind == isolate_kind) return true;
  if (vm_kind == Snapshot::kFull && isolate_kind == Snapshot::kFullJIT)
    return true;
  return Snapshot::IsFull(isolate_kind);
}

class BaseReader {
 public:
  BaseReader(const uint8_t* buffer, intptr_t size) : stream_(buffer, size) {}
  // Reads raw data (for basic types).
  // sizeof(T) must be in {1,2,4,8}.
  template <typename T>
  T Read() {
    return ReadStream::Raw<sizeof(T), T>::Read(&stream_);
  }

  classid_t ReadClassIDValue() {
    uint32_t value = Read<uint32_t>();
    return static_cast<classid_t>(value);
  }
  COMPILE_ASSERT(sizeof(uint32_t) >= sizeof(classid_t));

  void ReadBytes(uint8_t* addr, intptr_t len) { stream_.ReadBytes(addr, len); }

  double ReadDouble() {
    double result;
    stream_.ReadBytes(reinterpret_cast<uint8_t*>(&result), sizeof(result));
    return result;
  }

  intptr_t ReadTags() {
    const intptr_t tags = static_cast<intptr_t>(Read<int8_t>()) & 0xff;
    return tags;
  }

  const uint8_t* CurrentBufferAddress() const {
    return stream_.AddressOfCurrentPosition();
  }

  void Align(intptr_t value) { stream_.Align(value); }
  void Advance(intptr_t value) { stream_.Advance(value); }

  intptr_t PendingBytes() const { return stream_.PendingBytes(); }

  SmiPtr ReadAsSmi();
  intptr_t ReadSmiValue();

  // Negative header value indicates VM isolate object id.
  bool IsVMIsolateObject(intptr_t header_value) { return (header_value < 0); }
  intptr_t GetVMIsolateObjectId(intptr_t header_val) {
    ASSERT(IsVMIsolateObject(header_val));
    intptr_t value = -header_val;  // Header is negative for VM isolate objects.
    ASSERT(SerializedHeaderTag::decode(value) == kObjectId);
    return SerializedHeaderData::decode(value);
  }

  uword ReadWordWith32BitReads() { return stream_.ReadWordWith32BitReads(); }

 private:
  ReadStream stream_;  // input stream.
};

class BackRefNode : public ValueObject {
 public:
  BackRefNode(Object* reference, DeserializeState state)
      : reference_(reference), state_(state) {}
  Object* reference() const { return reference_; }
  bool is_deserialized() const { return state_ == kIsDeserialized; }
  void set_state(DeserializeState state) { state_ = state; }

  BackRefNode& operator=(const BackRefNode& other) {
    reference_ = other.reference_;
    state_ = other.state_;
    return *this;
  }

 private:
  Object* reference_;
  DeserializeState state_;
};

// Reads a snapshot into objects.
class SnapshotReader : public BaseReader {
 public:
  Thread* thread() const { return thread_; }
  Zone* zone() const { return zone_; }
  Isolate* isolate() const { return thread_->isolate(); }
  Heap* heap() const { return heap_; }
  ObjectStore* object_store() const { return isolate()->object_store(); }
  ClassTable* class_table() const { return isolate()->class_table(); }
  PassiveObject* PassiveObjectHandle() { return &pobj_; }
  Array* ArrayHandle() { return &array_; }
  Class* ClassHandle() { return &cls_; }
  Code* CodeHandle() { return &code_; }
  Instance* InstanceHandle() { return &instance_; }
  Instructions* InstructionsHandle() { return &instructions_; }
  String* StringHandle() { return &str_; }
  AbstractType* TypeHandle() { return &type_; }
  TypeArguments* TypeArgumentsHandle() { return &type_arguments_; }
  GrowableObjectArray* TokensHandle() { return &tokens_; }
  ExternalTypedData* DataHandle() { return &data_; }
  TypedDataBase* TypedDataBaseHandle() { return &typed_data_base_; }
  TypedData* TypedDataHandle() { return &typed_data_; }
  TypedDataView* TypedDataViewHandle() { return &typed_data_view_; }
  Function* FunctionHandle() { return &function_; }
  Snapshot::Kind kind() const { return kind_; }

  // Reads an object.
  ObjectPtr ReadObject();

  // Add object to backward references.
  void AddBackRef(intptr_t id, Object* obj, DeserializeState state);

  // Get an object from the backward references list.
  Object* GetBackRef(intptr_t id);

  // Read version number of snapshot and verify.
  ApiErrorPtr VerifyVersionAndFeatures(Isolate* isolate);

  ObjectPtr NewInteger(int64_t value);

 protected:
  SnapshotReader(const uint8_t* buffer,
                 intptr_t size,
                 Snapshot::Kind kind,
                 ZoneGrowableArray<BackRefNode>* backward_references,
                 Thread* thread);
  ~SnapshotReader() {}

  ZoneGrowableArray<BackRefNode>* GetBackwardReferenceTable() const {
    return backward_references_;
  }
  void ResetBackwardReferenceTable() { backward_references_ = NULL; }
  PageSpace* old_space() const { return old_space_; }

 private:
  void EnqueueTypePostprocessing(const AbstractType& type);
  void RunDelayedTypePostprocessing();

  void EnqueueRehashingOfMap(const LinkedHashMap& map);
  void EnqueueRehashingOfSet(const Object& set);
  ObjectPtr RunDelayedRehashingOfMaps();

  ClassPtr ReadClassId(intptr_t object_id);
  ObjectPtr ReadStaticImplicitClosure(intptr_t object_id, intptr_t cls_header);

  // Implementation to read an object.
  ObjectPtr ReadObjectImpl(bool as_reference);
  ObjectPtr ReadObjectImpl(intptr_t header, bool as_reference);

  // Read a Dart Instance object.
  ObjectPtr ReadInstance(intptr_t object_id, intptr_t tags, bool as_reference);

  // Read a VM isolate object that was serialized as an Id.
  ObjectPtr ReadVMIsolateObject(intptr_t object_id);

  // Read an object that was serialized as an Id (singleton in object store,
  // or an object that was already serialized before).
  ObjectPtr ReadIndexedObject(intptr_t object_id);

  // Decode class id from the header field.
  intptr_t LookupInternalClass(intptr_t class_header);

  void ArrayReadFrom(intptr_t object_id,
                     const Array& result,
                     intptr_t len,
                     intptr_t tags);

  intptr_t NextAvailableObjectId() const;

  void SetReadException(const char* msg);

  ObjectPtr VmIsolateSnapshotObject(intptr_t index) const;

  bool is_vm_isolate() const;

  Snapshot::Kind kind_;            // Indicates type of the snapshot.
  Thread* thread_;                 // Current thread.
  Zone* zone_;                     // Zone for allocations while reading.
  Heap* heap_;                     // Heap of the current isolate.
  PageSpace* old_space_;           // Old space of the current isolate.
  Class& cls_;                     // Temporary Class handle.
  Code& code_;                     // Temporary Code handle.
  Instance& instance_;             // Temporary Instance handle
  Instructions& instructions_;     // Temporary Instructions handle
  Object& obj_;                    // Temporary Object handle.
  PassiveObject& pobj_;            // Temporary PassiveObject handle.
  Array& array_;                   // Temporary Array handle.
  Field& field_;                   // Temporary Field handle.
  String& str_;                    // Temporary String handle.
  Library& library_;               // Temporary library handle.
  AbstractType& type_;             // Temporary type handle.
  TypeArguments& type_arguments_;  // Temporary type argument handle.
  GrowableObjectArray& tokens_;    // Temporary tokens handle.
  ExternalTypedData& data_;        // Temporary stream data handle.
  TypedDataBase& typed_data_base_;  // Temporary typed data base handle.
  TypedData& typed_data_;          // Temporary typed data handle.
  TypedDataView& typed_data_view_;  // Temporary typed data view handle.
  Function& function_;             // Temporary function handle.
  UnhandledException& error_;      // Error handle.
  const Class& set_class_;         // The LinkedHashSet class.
  intptr_t max_vm_isolate_object_id_;
  ZoneGrowableArray<BackRefNode>* backward_references_;
  GrowableObjectArray& types_to_postprocess_;
  GrowableObjectArray& objects_to_rehash_;

  friend class ApiError;
  friend class Array;
  friend class Class;
  friend class Closure;
  friend class ClosureData;
  friend class Context;
  friend class ContextScope;
  friend class ExceptionHandlers;
  friend class Field;
  friend class Function;
  friend class GrowableObjectArray;
  friend class ICData;
  friend class ImmutableArray;
  friend class KernelProgramInfo;
  friend class LanguageError;
  friend class Library;
  friend class LibraryPrefix;
  friend class LinkedHashMap;
  friend class MirrorReference;
  friend class Namespace;
  friend class PatchClass;
  friend class RedirectionData;
  friend class RegExp;
  friend class Script;
  friend class SignatureData;
  friend class SubtypeTestCache;
  friend class TransferableTypedData;
  friend class Type;
  friend class TypedDataView;
  friend class TypeArguments;
  friend class TypeParameter;
  friend class TypeRef;
  friend class UnhandledException;
  friend class WeakProperty;
  friend class WeakSerializationReference;
  DISALLOW_COPY_AND_ASSIGN(SnapshotReader);
};

class MessageSnapshotReader : public SnapshotReader {
 public:
  MessageSnapshotReader(Message* message, Thread* thread);
  ~MessageSnapshotReader();

  MessageFinalizableData* finalizable_data() const { return finalizable_data_; }

 private:
  MessageFinalizableData* finalizable_data_;

  DISALLOW_COPY_AND_ASSIGN(MessageSnapshotReader);
};

class BaseWriter : public StackResource {
 public:
  uint8_t* Steal(intptr_t* length) { return stream_.Steal(length); }
  intptr_t BytesWritten() const { return stream_.bytes_written(); }

  // Writes raw data to the stream (basic type).
  // sizeof(T) must be in {1,2,4,8}.
  template <typename T>
  void Write(T value) {
    MallocWriteStream::Raw<sizeof(T), T>::Write(&stream_, value);
  }

  void WriteClassIDValue(classid_t value) { Write<uint32_t>(value); }
  COMPILE_ASSERT(sizeof(uint32_t) >= sizeof(classid_t));

  // Write an object that is serialized as an Id (singleton in object store,
  // or an object that was already serialized before).
  void WriteIndexedObject(intptr_t object_id) {
    ASSERT(object_id <= kMaxObjectId);
    intptr_t value = 0;
    value = SerializedHeaderTag::update(kObjectId, value);
    value = SerializedHeaderData::update(object_id, value);
    Write<int32_t>(value);
  }

  // Write a VM Isolateobject that is serialized as an Id.
  void WriteVMIsolateObject(intptr_t object_id) {
    ASSERT(object_id <= kMaxObjectId);
    intptr_t value = 0;
    value = SerializedHeaderTag::update(kObjectId, value);
    value = SerializedHeaderData::update(object_id, value);
    Write<int32_t>(-value);  // Write as a negative value.
  }

  // Write serialization header information for an object.
  void WriteInlinedObjectHeader(intptr_t id) {
    ASSERT(id <= kMaxObjectId);
    intptr_t value = 0;
    value = SerializedHeaderTag::update(kInlined, value);
    value = SerializedHeaderData::update(id, value);
    Write<int32_t>(value);
  }

  void WriteTags(intptr_t tags) {
    const intptr_t flags = tags & 0xff;
    Write<int8_t>(static_cast<int8_t>(flags));
  }

  void Align(intptr_t value) { stream_.Align(value); }

  // Write out a buffer of bytes.
  void WriteBytes(const uint8_t* addr, intptr_t len) {
    stream_.WriteBytes(addr, len);
  }

  void WriteDouble(double value) {
    stream_.WriteBytes(reinterpret_cast<const uint8_t*>(&value), sizeof(value));
  }

  void WriteWordWith32BitWrites(uword value) {
    stream_.WriteWordWith32BitWrites(value);
  }

 protected:
  explicit BaseWriter(intptr_t initial_size)
      : StackResource(Thread::Current()), stream_(initial_size) {}
  ~BaseWriter() {}

  void ReserveHeader() {
    // Make room for recording snapshot buffer size.
    stream_.SetPosition(Snapshot::kHeaderSize);
  }

  void FillHeader(Snapshot::Kind kind) {
    intptr_t length;
    Snapshot* header = reinterpret_cast<Snapshot*>(Steal(&length));
    header->set_magic();
    header->set_length(length);
    header->set_kind(kind);
  }

  void FreeBuffer() {
    intptr_t unused;
    free(Steal(&unused));
  }

 private:
  MallocWriteStream stream_;

  DISALLOW_IMPLICIT_CONSTRUCTORS(BaseWriter);
};

class ForwardList {
 public:
  explicit ForwardList(Thread* thread, intptr_t first_object_id);
  ~ForwardList();

  class Node : public ZoneAllocated {
   public:
    Node(const Object* obj, SerializeState state) : obj_(obj), state_(state) {}
    const Object* obj() const { return obj_; }
    bool is_serialized() const { return state_ == kIsSerialized; }

   private:
    // Private to ensure the invariant of first_unprocessed_object_id_.
    void set_state(SerializeState value) { state_ = value; }

    const Object* obj_;
    SerializeState state_;

    friend class ForwardList;
    DISALLOW_COPY_AND_ASSIGN(Node);
  };

  Node* NodeForObjectId(intptr_t object_id) const {
    return nodes_[object_id - first_object_id_];
  }

  // Returns the id for the added object.
  intptr_t AddObject(Zone* zone, ObjectPtr raw, SerializeState state);

  // Returns the id for the object it it exists in the list.
  intptr_t FindObject(ObjectPtr raw);

  // Exhaustively processes all unserialized objects in this list. 'writer' may
  // concurrently add more objects.
  void SerializeAll(ObjectVisitor* writer);

  // Set state of object in forward list.
  void SetState(intptr_t object_id, SerializeState state) {
    NodeForObjectId(object_id)->set_state(state);
  }

 private:
  intptr_t first_object_id() const { return first_object_id_; }
  intptr_t next_object_id() const { return nodes_.length() + first_object_id_; }
  Isolate* isolate() const { return thread_->isolate(); }

  Thread* thread_;
  const intptr_t first_object_id_;
  GrowableArray<Node*> nodes_;
  intptr_t first_unprocessed_object_id_;

  void SetObjectId(ObjectPtr object, intptr_t id);
  intptr_t GetObjectId(ObjectPtr object);

  DISALLOW_COPY_AND_ASSIGN(ForwardList);
};

class SnapshotWriter : public BaseWriter {
 protected:
  SnapshotWriter(Thread* thread,
                 Snapshot::Kind kind,
                 intptr_t initial_size,
                 ForwardList* forward_list,
                 bool can_send_any_object);

 public:
  // Snapshot kind.
  Snapshot::Kind kind() const { return kind_; }
  Thread* thread() const { return thread_; }
  Zone* zone() const { return thread_->zone(); }
  Isolate* isolate() const { return thread_->isolate(); }
  Heap* heap() const { return isolate()->heap(); }

  // Serialize an object into the buffer.
  void WriteObject(ObjectPtr raw);

  static uint32_t GetObjectTags(ObjectPtr raw);
  static uint32_t GetObjectTags(ObjectLayout* raw);
  static uword GetObjectTagsAndHash(ObjectPtr raw);

  Exceptions::ExceptionType exception_type() const { return exception_type_; }
  void set_exception_type(Exceptions::ExceptionType type) {
    exception_type_ = type;
  }
  const char* exception_msg() const { return exception_msg_; }
  void set_exception_msg(const char* msg) { exception_msg_ = msg; }
  bool can_send_any_object() const { return can_send_any_object_; }
  void ThrowException(Exceptions::ExceptionType type, const char* msg);

  // Write a version string for the snapshot.
  void WriteVersionAndFeatures();

  FunctionPtr IsSerializableClosure(ClosurePtr closure);

  void WriteStaticImplicitClosure(intptr_t object_id,
                                  FunctionPtr func,
                                  intptr_t tags,
                                  TypeArgumentsPtr delayed_type_arguments);

 protected:
  bool CheckAndWritePredefinedObject(ObjectPtr raw);
  bool HandleVMIsolateObject(ObjectPtr raw);

  void WriteClassId(ClassLayout* cls);
  void WriteObjectImpl(ObjectPtr raw, bool as_reference);
  void WriteMarkedObjectImpl(ObjectPtr raw,
                             intptr_t tags,
                             intptr_t object_id,
                             bool as_reference);
  void WriteForwardedObjects();
  void ArrayWriteTo(intptr_t object_id,
                    intptr_t array_kind,
                    intptr_t tags,
                    SmiPtr length,
                    TypeArgumentsPtr type_arguments,
                    ObjectPtr data[],
                    bool as_reference);
  ClassPtr GetFunctionOwner(FunctionPtr func);
  void CheckForNativeFields(ClassPtr cls);
  void SetWriteException(Exceptions::ExceptionType type, const char* msg);
  void WriteInstance(ObjectPtr raw,
                     ClassPtr cls,
                     intptr_t tags,
                     intptr_t object_id,
                     bool as_reference);
  bool AllowObjectsInDartLibrary(LibraryPtr library);
  intptr_t FindVmSnapshotObject(ObjectPtr rawobj);

  ObjectStore* object_store() const { return object_store_; }

 private:
  Thread* thread_;
  Snapshot::Kind kind_;
  ObjectStore* object_store_;  // Object store for common classes.
  ClassTable* class_table_;  // Class table for the class index to class lookup.
  ForwardList* forward_list_;
  Exceptions::ExceptionType exception_type_;  // Exception type.
  const char* exception_msg_;  // Message associated with exception.
  bool can_send_any_object_;   // True if any Dart instance can be sent.

  friend class ArrayLayout;
  friend class ClassLayout;
  friend class CodeLayout;
  friend class ContextScopeLayout;
  friend class DynamicLibraryLayout;
  friend class ExceptionHandlersLayout;
  friend class FieldLayout;
  friend class FunctionLayout;
  friend class GrowableObjectArrayLayout;
  friend class ImmutableArrayLayout;
  friend class InstructionsLayout;
  friend class LibraryLayout;
  friend class LinkedHashMapLayout;
  friend class LocalVarDescriptorsLayout;
  friend class MirrorReferenceLayout;
  friend class ObjectPoolLayout;
  friend class PointerLayout;
  friend class ReceivePortLayout;
  friend class RegExpLayout;
  friend class ScriptLayout;
  friend class StackTraceLayout;
  friend class SubtypeTestCacheLayout;
  friend class TransferableTypedDataLayout;
  friend class TypeLayout;
  friend class TypeArgumentsLayout;
  friend class TypeParameterLayout;
  friend class TypeRefLayout;
  friend class TypedDataViewLayout;
  friend class UserTagLayout;
  friend class WeakSerializationReferenceLayout;
  friend class SnapshotWriterVisitor;
  friend class WriteInlinedObjectVisitor;
  DISALLOW_COPY_AND_ASSIGN(SnapshotWriter);
};

class SerializedObjectBuffer : public StackResource {
 public:
  SerializedObjectBuffer()
      : StackResource(Thread::Current()), message_(nullptr) {}

  void set_message(std::unique_ptr<Message> message) {
    ASSERT(message_ == nullptr);
    message_ = std::move(message);
  }
  std::unique_ptr<Message> StealMessage() { return std::move(message_); }

 private:
  std::unique_ptr<Message> message_;
};

class MessageWriter : public SnapshotWriter {
 public:
  static const intptr_t kInitialSize = 512;
  explicit MessageWriter(bool can_send_any_object);
  ~MessageWriter();

  std::unique_ptr<Message> WriteMessage(const Object& obj,
                                        Dart_Port dest_port,
                                        Message::Priority priority);

  MessageFinalizableData* finalizable_data() const { return finalizable_data_; }

 private:
  ForwardList forward_list_;
  MessageFinalizableData* finalizable_data_;

  DISALLOW_COPY_AND_ASSIGN(MessageWriter);
};

// An object pointer visitor implementation which writes out
// objects to a snapshot.
class SnapshotWriterVisitor : public ObjectPointerVisitor {
 public:
  SnapshotWriterVisitor(SnapshotWriter* writer, bool as_references)
      : ObjectPointerVisitor(Isolate::Current()->group()),
        writer_(writer),
        as_references_(as_references) {}

  virtual void VisitPointers(ObjectPtr* first, ObjectPtr* last);

 private:
  SnapshotWriter* writer_;
  bool as_references_;

  DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
};

}  // namespace dart

#endif  // RUNTIME_VM_SNAPSHOT_H_