public long readRawVarInt64() throws IOException {
        int shift = 0;
        long result = 0;
        while (shift < 64) {
            final byte b = nioBuffer.get();
            result |= (long) (b & 0x7F) << shift;
            if ((b & 0x80) == 0) {
                return result;
            }
            shift += 7;
        }
        throw ProtocolException.malformedVarInt();
    }
Reads a var int 64 from the internal byte buffer.
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/serialization/impl/protostuff/io/NioBufInput.java