LevelDB的BlockHandle和Footer和读取block

主要作用就是记录一个block在sstable文件中的偏移量和大小

代码：table/format.h和table/format.cc

class BlockHandle {
 public:
  // Maximum encoding length of a BlockHandle
  enum { kMaxEncodedLength = 10 + 10 };

  BlockHandle();

  // The offset of the block in the file.
  uint64_t offset() const { return offset_; }
  void set_offset(uint64_t offset) { offset_ = offset; }

  // The size of the stored block
  uint64_t size() const { return size_; }
  void set_size(uint64_t size) { size_ = size; }
  // 将size和offset编码到string里面
  void EncodeTo(std::string* dst) const;
  // 从Slice里面解码出size和offset
  Status DecodeFrom(Slice* input);

 private:
  uint64_t offset_;
  uint64_t size_;
};

void BlockHandle::EncodeTo(std::string* dst) const {
  // Sanity check that all fields have been set
  assert(offset_ != ~static_cast<uint64_t>(0));
  assert(size_ != ~static_cast<uint64_t>(0));
  PutVarint64(dst, offset_);
  PutVarint64(dst, size_);
}

Status BlockHandle::DecodeFrom(Slice* input) {
  if (GetVarint64(input, &offset_) && GetVarint64(input, &size_)) {
    return Status::OK();
  } else {
    return Status::Corruption("bad block handle");
  }
}

class Footer {
 public:
  // Encoded length of a Footer.  Note that the serialization of a
  // Footer will always occupy exactly this many bytes.  It consists
  // of two block handles and a magic number.
  enum { kEncodedLength = 2 * BlockHandle::kMaxEncodedLength + 8 };

  Footer() = default;

  // The block handle for the metaindex block of the table
  const BlockHandle& metaindex_handle() const { return metaindex_handle_; }
  void set_metaindex_handle(const BlockHandle& h) { metaindex_handle_ = h; }

  // The block handle for the index block of the table
  const BlockHandle& index_handle() const { return index_handle_; }
  void set_index_handle(const BlockHandle& h) { index_handle_ = h; }

  void EncodeTo(std::string* dst) const;
  Status DecodeFrom(Slice* input);

 private:
  BlockHandle metaindex_handle_;
  BlockHandle index_handle_;
};

void Footer::EncodeTo(std::string* dst) const {
  const size_t original_size = dst->size();
  metaindex_handle_.EncodeTo(dst);
  index_handle_.EncodeTo(dst);
  dst->resize(2 * BlockHandle::kMaxEncodedLength);  // Padding
  PutFixed32(dst, static_cast<uint32_t>(kTableMagicNumber & 0xffffffffu));
  PutFixed32(dst, static_cast<uint32_t>(kTableMagicNumber >> 32));
  assert(dst->size() == original_size + kEncodedLength);
  (void)original_size;  // Disable unused variable warning.
}

Status Footer::DecodeFrom(Slice* input) {
  const char* magic_ptr = input->data() + kEncodedLength - 8;
  const uint32_t magic_lo = DecodeFixed32(magic_ptr);
  const uint32_t magic_hi = DecodeFixed32(magic_ptr + 4);
  const uint64_t magic = ((static_cast<uint64_t>(magic_hi) << 32) |
                          (static_cast<uint64_t>(magic_lo)));
  if (magic != kTableMagicNumber) {
    return Status::Corruption("not an sstable (bad magic number)");
  }

  Status result = metaindex_handle_.DecodeFrom(input);
  if (result.ok()) {
    result = index_handle_.DecodeFrom(input);
  }
  if (result.ok()) {
    // We skip over any leftover data (just padding for now) in "input"
    const char* end = magic_ptr + 8;
    *input = Slice(end, input->data() + input->size() - end);
  }
  return result;
}

下面就是读取block的代码

struct BlockContents {
  // 保存block的内容
  Slice data;           // Actual contents of data
  bool cachable;        // True iff data can be cached
  bool heap_allocated;  // True iff caller should delete[] data.data()
};

// Read the block identified by "handle" from "file".  On failure
// return non-OK.  On success fill *result and return OK.
Status ReadBlock(RandomAccessFile* file, const ReadOptions& options,
                 const BlockHandle& handle, BlockContents* result);

Status ReadBlock(RandomAccessFile* file, const ReadOptions& options,
                 const BlockHandle& handle, BlockContents* result) {
  result->data = Slice();
  result->cachable = false;
  result->heap_allocated = false;

  // Read the block contents as well as the type/crc footer.
  // See table_builder.cc for the code that built this structure.
  // 获取block->size()
  size_t n = static_cast<size_t>(handle.size());
  // block.size()+5(就是是否压缩和crc校验码，这两个没有算在size里面)
  char* buf = new char[n + kBlockTrailerSize];
  Slice contents;
  // 读取，主要是将block的数据读到buf里面，然后用buf构建content
  Status s = file->Read(handle.offset(), n + kBlockTrailerSize, &contents, buf);
  if (!s.ok()) {
    delete[] buf;
    return s;
  }
  if (contents.size() != n + kBlockTrailerSize) {
    delete[] buf;
    return Status::Corruption("truncated block read");
  }

  // Check the crc of the type and the block contents
  const char* data = contents.data();  // Pointer to where Read put the data
  if (options.verify_checksums) {
    // 校验crc
    const uint32_t crc = crc32c::Unmask(DecodeFixed32(data + n + 1));
    const uint32_t actual = crc32c::Value(data, n + 1);
    if (actual != crc) {
      delete[] buf;
      s = Status::Corruption("block checksum mismatch");
      return s;
    }
  }

  switch (data[n]) {
    case kNoCompression:
      // 没有使用压缩，将数据导出到BlockContent
      if (data != buf) {
        // File implementation gave us pointer to some other data.
        // Use it directly under the assumption that it will be live
        // while the file is open.
        delete[] buf;
        result->data = Slice(data, n);
        result->heap_allocated = false;
        result->cachable = false;  // Do not double-cache
      } else {
        result->data = Slice(buf, n);
        result->heap_allocated = true;
        result->cachable = true;
      }

      // Ok
      break;
    case kSnappyCompression: {
      // 使用了压缩，就得先解压
      size_t ulength = 0;
      if (!port::Snappy_GetUncompressedLength(data, n, &ulength)) {
        delete[] buf;
        return Status::Corruption("corrupted compressed block contents");
      }
      char* ubuf = new char[ulength];
      if (!port::Snappy_Uncompress(data, n, ubuf)) {
        delete[] buf;
        delete[] ubuf;
        return Status::Corruption("corrupted compressed block contents");
      }
      delete[] buf;
      result->data = Slice(ubuf, ulength);
      result->heap_allocated = true;
      result->cachable = true;
      break;
    }
    default:
      delete[] buf;
      return Status::Corruption("bad block type");
  }

  return Status::OK();
}