Multixid wraparound handling

[hlinnaka]

In walingest.rs, we have this:

        if xlrec.mid >= self.checkpoint.nextMulti {
            self.checkpoint.nextMulti = xlrec.mid + 1;
            self.checkpoint_modified = true;
        }
        if xlrec.moff + xlrec.nmembers > self.checkpoint.nextMultiOffset {
            self.checkpoint.nextMultiOffset = xlrec.moff + xlrec.nmembers;
            self.checkpoint_modified = true;
        }

That doesn't do the right thing if multixid has wrapped around. Need to do wraparound aware comparison, as iin Postgres' MultiXactAdvanceNextMXact function, or as we do for XIDs in update_next_xid.

[ololobus]

@hlinnaka I see a fix merged recently, is it done? Is there any way to verify that this fix was enough?

[hlinnaka]

@hlinnaka I see a fix merged recently, is it done?

Yes. I'll close this.

Is there any way to verify that this fix was enough?

Well, the fix included tests. But that doesn't guarantee we didn't miss something of course. I think it's as good as it gets.