Re: Fix race condition in SSI when reading PredXact->SxactGlobalXmin

Hello!

Josh Curtis <jcurtis825@gmail.com>:
> This is definitely a bit more complex. It requires that SetNewSxactGlobalXmin is never called when SxactGlobalXmin is
invalidto prevent readers from seeing an invalid transaction ID when they should see a valid one -- I think this is the
casenow since before SetNewSxactGlobalXmin is called postgres checks that PredXact->SxactGlobalXminCount > 0. I assume
thisentails that SxactGlobalXmin is valid, but I have not checked every place the two variables are modified. 

Such logic feels fragile to me. Maybe add a special flag like
'PredXact->SxactGlobalSkipAllowed' which will be updated to 'true' by
SetNewSxactGlobalXmin, and dropped to 'false' by functions affecting
PredXact->activeList?
But I prefer the first solution anyway.

Best regards,
Mikhail.