After a bit of thought, I've realized that to be conservative here is the way to go.
We can reuse a maximum of existing logic. I mean, we can remove offset wraparound "error logic" and reuse "warning logic". But set the threshold for "warning logic" to a much higher value. For now, I choose 2^32-1. In other world, legit logic, in my view, here would be to trigger autovacuum if the number of offsets (i.e. difference nextOffset - oldestOffset) exceeds 2^32-1. PFA patch set. -- Best regards, Maxim Orlov.
v5-0002-Use-64-bit-multixact-offsets.patch
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v5-0003-Make-pg_upgrade-convert-multixact-offsets.patch
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v5-0004-Get-rid-of-MultiXactMemberFreezeThreshold-call.patch
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v5-0001-Use-64-bit-format-output-for-multixact-offsets.patch
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