[CASSANDRA-21685] Size calculation mismatch in EncodingStats and SerializationHeader local deletion time

[koo-taejin]

Description

While working on CASSANDRA-21535, CompactionsCQLTest failed (testCompactionInvalidRowDeletion and testIndexedReaderRowDeletion). The cause is that some serializers calculate the size in a different way from how they write the value.

Problem

In EncodingStats.Serializer, the write path casts the value to int, but the size path keeps it as long:

// serialize()
out.writeUnsignedVInt32((int)(stats.minLocalDeletionTime - DELETION_TIME_EPOCH));
// serializedSize()
TypeSizes.sizeofUnsignedVInt(stats.minLocalDeletionTime - DELETION_TIME_EPOCH)

SerializationHeader has the same pattern:

// writeLocalDeletionTime()
out.writeUnsignedVInt32((int) (localDeletionTime - stats.minLocalDeletionTime));
// localDeletionTimeSerializedSize()
TypeSizes.sizeofUnsignedVInt(localDeletionTime - stats.minLocalDeletionTime);

If the value fits in a signed int, both give the same result. But when minLocalDeletionTime holds an invalid or capped deletion time, the delta is around 2.85e9 and does not fit in a signed int:

size path: sizeofUnsignedVInt(2852087294L) -> 5 bytes
write path: the (int) cast wraps to -1442880002, and writeUnsignedVInt32 sign-extends it -> 9 bytes
So serializedSize() returns a smaller value than the bytes actually written.

Proposed fix

Apply the same (int) cast in the size calculation, in both EncodingStats.Serializer.serializedSize() and SerializationHeader.localDeletionTimeSerializedSize(). This makes the size match the written bytes. It does not change any serialized byte, so there is no compatibility impact.

This change does not fix the root problem described below. But I think it is still clearly useful on its own, because the calculated size should always match the actual bytes written. This is why I am opening this ticket separately.

Out of scope

The written value itself does not round-trip. deserialize() reads it back with readUnsignedVInt32(), so the original value is not restored (minLocalDeletionTime comes back as -2).

Fixing this needs a change to the serialized format, which is used by both SSTables and messaging. So we would also need to handle compatibility with older versions and with data that is already written. I think the people who maintain the project over the long term should decide the policy for this, so I am leaving it out of this ticket.

patch by koo.taejin; reviewed by for CASSANDRA-21685

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