IPC/MultixactCreation on the Standby server

In GetNewMultiXactId() this code may lead to error

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ExtendMultiXactOffset(MultiXactState->nextMXact + 1);

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If MultiXactState->nextMXact = MaxMultiXactId (0xFFFFFFFF)

we will not extend MultiXactOffset as we should

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ExtendMultiXactOffset(0);

   MultiXactIdToOffsetEntry(0)

        multi % MULTIXACT_OFFSETS_PER_PAGE  = 0

   return; /* skip SLRU extension */

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Perhaps we should introduce a simple function to handle next MultiXact

calculation

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static inline MultiXactId

NextMultiXactId(MultiXactId multi)

{

    return multi == MaxMultiXactId ? FirstMultiXactId : multi + 1;

}

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I've attached a patch that fixes this issue, although it seems I've discovered

another overflow bug in multixact_redo().

We might call:

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multixact_redo()

   MultiXactAdvanceNextMXact(0xFFFFFFFF + 1, ...);

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And if MultiXactState->nextMXact != InvalidMultiXactId (0), we will have

MultiXactState->nextMXact = 0.

This appears to cause problems in code that assumes MultiXactState->nextMXact

holds a valid MultiXactId.

For instance, in GetMultiXactId(), we would return an incorrect number

of MultiXacts.

Although, spreading MultiXact overflow handling throughout multixact.c code

seems error-prone.

Maybe we should use a macro instead (which would also allow us to modify this

check and add compiler hints):

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#define MultiXactAdjustOverflow(mxact) \

   if (unlikely((mxact) < FirstMultiXactId)) \

      mxact = FirstMultiXactId;

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