types: Fix encoding of negative varint

We would sometimes produce an unnecessary extra 0xff prefix byte.

The new encoding matches what cassandra does.

This was both a efficiency and correctness issue, as using varint in a
key could produce different tokens.

Fixes #5656

Signed-off-by: Rafael Ávila de Espíndola <espindola@scylladb.com>
(cherry picked from commit c89c90d)

test/boost/types_test.cc

@@ -415,6 +415,8 @@ BOOST_AUTO_TEST_CASE(test_varint) {
     BOOST_CHECK_EQUAL(value_cast<boost::multiprecision::cpp_int>(varint_type->deserialize(from_hex("00deadbeef"))), boost::multiprecision::cpp_int("0xdeadbeef"));
     BOOST_CHECK_EQUAL(value_cast<boost::multiprecision::cpp_int>(varint_type->deserialize(from_hex("00ffffffffffffffffffffffffffffffff"))), boost::multiprecision::cpp_int("340282366920938463463374607431768211455"));
 
+    BOOST_REQUIRE_EQUAL(from_hex("80000000"), varint_type->decompose(boost::multiprecision::cpp_int(-2147483648)));
+
     test_parsing_fails(varint_type, "1A");
 }
 

types.cc

@@ -2082,12 +2082,19 @@ static size_t concrete_serialized_size(const string_type_impl::native_type& v) {
 static size_t concrete_serialized_size(const bytes_type_impl::native_type& v) { return v.size(); }
 static size_t concrete_serialized_size(const inet_addr_type_impl::native_type& v) { return v.get().size(); }
 
-static size_t concrete_serialized_size(const boost::multiprecision::cpp_int& num) {
-    if (!num) {
+static size_t concrete_serialized_size_aux(const boost::multiprecision::cpp_int& num) {
+    if (num) {
+        return align_up(boost::multiprecision::msb(num) + 2, 8u) / 8;
+    } else {
         return 1;
     }
-    auto pnum = abs(num);
-    return align_up(boost::multiprecision::msb(pnum) + 2, 8u) / 8;
+}
+
+static size_t concrete_serialized_size(const boost::multiprecision::cpp_int& num) {
+    if (num < 0) {
+        return concrete_serialized_size_aux(-num - 1);
+    }
+    return concrete_serialized_size_aux(num);
 }
 
 static size_t concrete_serialized_size(const varint_type_impl::native_type& v) {