Blink-compatible serialization of oddball values.

src/value-serializer.cc

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/value-serializer.h"
+
+#include <type_traits>
+
+#include "src/base/logging.h"
+#include "src/factory.h"
+#include "src/handles-inl.h"
+#include "src/isolate.h"
+#include "src/objects-inl.h"
+#include "src/objects.h"
+
+namespace v8 {
+namespace internal {
+
+static const uint32_t kLatestVersion = 9;
+
+enum class SerializationTag : uint8_t {
+  kVersion = 0xFF,
+  kPadding = '\0',
+  kVerifyObjectCount = '?',
+  kUndefined = '_',
+  kNull = '0',
+  kTrue = 'T',
+  kFalse = 'F',
+};
+
+ValueSerializer::ValueSerializer() {}
+
+ValueSerializer::~ValueSerializer() {}
+
+void ValueSerializer::WriteHeader() {
+  WriteTag(SerializationTag::kVersion);
+  WriteVarint(kLatestVersion);
+}
+
+void ValueSerializer::WriteTag(SerializationTag tag) {
+  buffer_.push_back(static_cast<uint8_t>(tag));
+}
+
+template <typename T>
+void ValueSerializer::WriteVarint(T value) {
+  // Writes an unsigned integer as a base-128 varint.
+  // The number is written, 7 bits at a time, from the least significant to the
+  // most significant 7 bits. Each byte, except the last, has the MSB set.
+  // See also https://developers.google.com/protocol-buffers/docs/encoding
+  static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
+                "Only unsigned integer types can be written as varints.");
+  uint8_t stack_buffer[sizeof(T) * 8 / 7 + 1];
+  uint8_t* next_byte = &stack_buffer[0];
+  do {
+    *next_byte = (value & 0x7f) | 0x80;
+    next_byte++;
+    value >>= 7;
+  } while (value);
+  *(next_byte - 1) &= 0x7f;
+  buffer_.insert(buffer_.end(), stack_buffer, next_byte);
+}
+
+Maybe<bool> ValueSerializer::WriteObject(Handle<Object> object) {
+  if (object->IsSmi()) UNIMPLEMENTED();
+
+  DCHECK(object->IsHeapObject());
+  switch (HeapObject::cast(*object)->map()->instance_type()) {
+    case ODDBALL_TYPE:
+      WriteOddball(Oddball::cast(*object));
+      return Just(true);
+    default:
+      UNIMPLEMENTED();
+      return Nothing<bool>();
+  }
+}
+
+void ValueSerializer::WriteOddball(Oddball* oddball) {
+  SerializationTag tag = SerializationTag::kUndefined;
+  switch (oddball->kind()) {
+    case Oddball::kUndefined:
+      tag = SerializationTag::kUndefined;
+      break;
+    case Oddball::kFalse:
+      tag = SerializationTag::kFalse;
+      break;
+    case Oddball::kTrue:
+      tag = SerializationTag::kTrue;
+      break;
+    case Oddball::kNull:
+      tag = SerializationTag::kNull;
+      break;
+    default:
+      UNREACHABLE();
+      break;
+  }
+  WriteTag(tag);
+}
+
+ValueDeserializer::ValueDeserializer(Isolate* isolate,
+                                     Vector<const uint8_t> data)
+    : isolate_(isolate),
+      position_(data.start()),
+      end_(data.start() + data.length()) {}
+
+ValueDeserializer::~ValueDeserializer() {}
+
+Maybe<bool> ValueDeserializer::ReadHeader() {
+  if (position_ < end_ &&
+      *position_ == static_cast<uint8_t>(SerializationTag::kVersion)) {
+    ReadTag().ToChecked();
+    if (!ReadVarint<uint32_t>().To(&version_)) return Nothing<bool>();
+    if (version_ > kLatestVersion) return Nothing<bool>();
+  }
+  return Just(true);
+}
+
+Maybe<SerializationTag> ValueDeserializer::ReadTag() {
+  SerializationTag tag;
+  do {
+    if (position_ >= end_) return Nothing<SerializationTag>();
+    tag = static_cast<SerializationTag>(*position_);
+    position_++;
+  } while (tag == SerializationTag::kPadding);
+  return Just(tag);
+}
+
+template <typename T>
+Maybe<T> ValueDeserializer::ReadVarint() {
+  // Reads an unsigned integer as a base-128 varint.
+  // The number is written, 7 bits at a time, from the least significant to the
+  // most significant 7 bits. Each byte, except the last, has the MSB set.
+  // If the varint is larger than T, any more significant bits are discarded.
+  // See also https://developers.google.com/protocol-buffers/docs/encoding
+  static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
+                "Only unsigned integer types can be read as varints.");
+  T value = 0;
+  unsigned shift = 0;
+  bool has_another_byte;
+  do {
+    if (position_ >= end_) return Nothing<T>();
+    uint8_t byte = *position_;
+    if (V8_LIKELY(shift < sizeof(T) * 8)) {
+      value |= (byte & 0x7f) << shift;
+      shift += 7;
+    }
+    has_another_byte = byte & 0x80;
+    position_++;
+  } while (has_another_byte);
+  return Just(value);
+}
+
+MaybeHandle<Object> ValueDeserializer::ReadObject() {
+  SerializationTag tag;
+  if (!ReadTag().To(&tag)) return MaybeHandle<Object>();
+  switch (tag) {
+    case SerializationTag::kVerifyObjectCount:
+      // Read the count and ignore it.
+      if (ReadVarint<uint32_t>().IsNothing()) return MaybeHandle<Object>();
+      return ReadObject();
+    case SerializationTag::kUndefined:
+      return isolate_->factory()->undefined_value();
+    case SerializationTag::kNull:
+      return isolate_->factory()->null_value();
+    case SerializationTag::kTrue:
+      return isolate_->factory()->true_value();
+    case SerializationTag::kFalse:
+      return isolate_->factory()->false_value();
+    default:
+      return MaybeHandle<Object>();
+  }
+}
+
+}  // namespace internal
+}  // namespace v8