从零构造现代语言编译器(5): 缓冲区

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class SourceBuffer {
 public:
  static auto CreateFromText(llvm::Twine text,
                             llvm::StringRef filename = "/text")
      -> llvm::Expected<SourceBuffer>;
  static auto CreateFromFile(llvm::StringRef filename)
      -> llvm::Expected<SourceBuffer>;

  SourceBuffer() = delete;

  SourceBuffer(const SourceBuffer&) = delete;

  SourceBuffer(SourceBuffer&& arg) noexcept;

  ~SourceBuffer();

  [[nodiscard]] auto filename() const -> llvm::StringRef { return filename_; }

  [[nodiscard]] auto text() const -> llvm::StringRef { return text_; }

 private:
  enum class ContentMode {
    Uninitialized,
    MMapped,
    Owned,
  };

  // Constructor for mmapped content.
  explicit SourceBuffer(std::string filename, llvm::StringRef text);
  // Constructor for owned content.
  explicit SourceBuffer(std::string filename, std::string text);

  ContentMode content_mode_;
  std::string filename_;
  std::string text_storage_;
  llvm::StringRef text_;
};

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static auto CheckContentSize(int64_t size) -> llvm::Error {
  if (size < std::numeric_limits<int32_t>::max()) {
    return llvm::Error::success();
  }
  return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                 "Input too large!");
}

auto SourceBuffer::CreateFromText(llvm::Twine text, llvm::StringRef filename)
    -> llvm::Expected<SourceBuffer> {
  std::string buffer = text.str();
  auto size_check = CheckContentSize(buffer.size());
  if (size_check) {
    return std::move(size_check);
  }
  return SourceBuffer(filename.str(), std::move(buffer));
}

static auto ErrnoToError(int errno_value) -> llvm::Error {
  return llvm::errorCodeToError(
      std::error_code(errno_value, std::generic_category()));
}

auto SourceBuffer::CreateFromFile(llvm::StringRef filename)
    -> llvm::Expected<SourceBuffer> {
  std::string filename_str = filename.str();

  errno = 0;
  int file_descriptor = open(filename_str.c_str(), O_RDONLY);
  if (file_descriptor == -1) {
    return ErrnoToError(errno);
  }

  auto closer =
      llvm::make_scope_exit([file_descriptor] { close(file_descriptor); });

  struct stat stat_buffer = {};
  errno = 0;
  if (fstat(file_descriptor, &stat_buffer) == -1) {
    return ErrnoToError(errno);
  }

  int64_t size = stat_buffer.st_size;
  if (size == 0) {
    return SourceBuffer(std::move(filename_str), std::string());
  }
  auto size_check = CheckContentSize(size);
  if (size_check) {
    return std::move(size_check);
  }

  errno = 0;
  void* mapped_text = mmap(nullptr, size, PROT_READ, MAP_PRIVATE,
                           file_descriptor, /*offset=*/0);
  if (mapped_text == MAP_FAILED) {
    return ErrnoToError(errno);
  }

  errno = 0;
  closer.release();
  if (close(file_descriptor) == -1) {
    munmap(mapped_text, size);
    return ErrnoToError(errno);
  }

  return SourceBuffer(
      std::move(filename_str),
      llvm::StringRef(static_cast<const char*>(mapped_text), size));
}
SourceBuffer::SourceBuffer(SourceBuffer&& arg) noexcept
    : content_mode_(
          std::exchange(arg.content_mode_, ContentMode::Uninitialized)),
      filename_(std::move(arg.filename_)),
      text_storage_(std::move(arg.text_storage_)),
      text_(content_mode_ == ContentMode::Owned ? text_storage_ : arg.text_) {}

SourceBuffer::SourceBuffer(std::string filename, std::string text)
    : content_mode_(ContentMode::Owned),
      filename_(std::move(filename)),
      text_storage_(std::move(text)),
      text_(text_storage_) {}

SourceBuffer::SourceBuffer(std::string filename, llvm::StringRef text)
    : content_mode_(ContentMode::MMapped),
      filename_(std::move(filename)),
      text_(text) {
  COCKTAIL_CHECK(!text.empty())
      << "Must not have an empty text when we have mapped data from a file!";
}

SourceBuffer::~SourceBuffer() {
  if (content_mode_ == ContentMode::MMapped) {
    errno = 0;
    int result =
        munmap(const_cast<void*>(static_cast<const void*>(text_.data())),
               text_.size());
    COCKTAIL_CHECK(result != -1) << "Unmapping text failed!";
  }
}