Previously we accessed offsets in shared MultiXactState without locks as 32-bit read is always atomic. But I'm not sure it's so when offset become 64-bit.

E.g. GetNewMultiXactId():

nextOffset = MultiXactState->nextOffset;

is outside lock. 

There might be other places we do the same as well. 

Regards,

Pavel Borisov

Supabase