Re: IPC/MultixactCreation on the Standby server

Hi!

It seems my previous email was sent only to Andrey directly and didn't pass moderation
because it had a patch attached. I've now resent it without patch another email address.

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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 */
---

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):

---
#define MultiXactAdjustOverflow(mxact) \
   if (unlikely((mxact) < FirstMultiXactId)) \
      mxact = FirstMultiXactId;
---