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30027d94a2
Whenever we see an XLOG_MULTIXACT_CREATE_ID WAL record, we need to update the nextMulti and NextMultiOffset fields in the pageserver's copy of the CheckPoint struct, to cover the new multi-XID. In PostgreSQL, this is done by updating an in-memory struct during WAL replay, but because in Neon you can start a compute node at any LSN, we need to have an up-to-date value pre-calculated in the pageserver at all times. We do the same for nextXid. However, we had a bug in WAL ingestion code that does that: the multi-XIDs will wrap around at 2^32, just like XIDs, so we need to do the comparisons in a wraparound-aware fashion. Fix that, and add tests. Fixes issue #6520 Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>