MDEV-37504 MemorySanitizer: use-of-uninitialized-value myrocks::Rdb_key_def::pack_field

m_charset_codec is uninitalized when calling m_make_unpack_info_func.

In the cases where m_make_unpack_info_func is one of:
* Rdb_key_def::make_unpack_unknown_varchar
* Rdb_key_def::make_unpack_unknown
* Rdb_key_def::dummy_make_unpack_info

the m_charset_coded that forms the first argument to this function
is unused.

In these limited cases we initialize the m_charset_codec member
as the only use is to pass though to the m_make_unpack_info_func

Ultimately MemorySanitizer shouldn't error on this as all
of these 3 functions clearly have the attribute
__unused__ on their first argument where the m_charset_coded is
passed.

Author: grooverdan

storage/rocksdb/rdb_datadic.cc

@@ -3380,6 +3380,11 @@ bool Rdb_field_packing::setup(const Rdb_key_def *const key_descr,
         m_skip_func = Rdb_key_def::skip_variable_space_pad;
         m_pack_func = Rdb_key_def::pack_with_varchar_space_pad;
         m_make_unpack_info_func = Rdb_key_def::dummy_make_unpack_info;
+#if __has_feature(memory_sanitizer)
+        // dummy_make_unpack_info doesn't use arguments but MSAN expects
+        // them to be initialized.
+        m_charset_codec = nullptr;
+#endif
         m_segment_size = get_segment_size_from_collation(cs);
         m_max_image_len =
             (max_image_len_before_chunks / (m_segment_size - 1) + 1) *
@@ -3453,6 +3458,15 @@ bool Rdb_field_packing::setup(const Rdb_key_def *const key_descr,
                                       : Rdb_key_def::make_unpack_unknown;
         m_unpack_func = is_varchar ? Rdb_key_def::unpack_unknown_varchar
                                    : Rdb_key_def::unpack_unknown;
+#if __has_feature(memory_sanitizer)
+       // Rdb_key_def::make_unpack_info_unknown and
+       // Rdb_key_def::make_unpack_unknown_varchar when called
+       // via m_make_unpack_info_func do not make use of the m_charset_codec
+       // provided as an argument. MemorySanitizer doesn't make the logical
+       // there is no risk in m_charset_codec being uninitialized. Therefore we
+       // initialize to make MemorySanitizer satisified.
+       m_charset_codec = nullptr;
+#endif
       } else {
         // Same as above: we don't know how to restore the value from its
         // mem-comparable form.