code: public void copyFrom(UnsafeRow row) {
      // copyFrom is only available for UnsafeRow created from byte array.
      assert (baseObject instanceof byte[]) && baseOffset == Platform.BYTE_ARRAY_OFFSET;
      if (row.sizeInBytes > this.sizeInBytes) {
        // resize the underlying byte[] if it's not large enough.
        this.baseObject = new byte[row.sizeInBytes];
      }
      Platform.copyMemory(
        row.baseObject, row.baseOffset, this.baseObject, this.baseOffset, row.sizeInBytes);
      // update the sizeInBytes.
      this.sizeInBytes = row.sizeInBytes;
    }
docs: Copies the input UnsafeRow to this UnsafeRow, and resize the underlying byte[] when the
  input row is larger than this row.