Re: Make relfile tombstone files conditional on WAL level

On Thu, Sep 30, 2021 at 11:32 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> I managed to produce a case where live data is written to an unlinked
> file and lost

I guess this must have been broken since release 9.2 moved checkpoints
out of here[1].  The connection between checkpoints, tombstone files
and file descriptor cache invalidation in auxiliary (non-sinval)
backends was not documented as far as I can see (or at least not
anywhere near the load-bearing parts).

How could it be fixed, simply and backpatchably?  If BgSyncBuffer()
did if-FirstCallSinceLastCheckpoint()-then-smgrcloseall() after
locking each individual buffer and before flushing, then I think it
might logically have the correct interlocking against relfilenode
wraparound, but that sounds a tad expensive :-(  I guess it could be
made cheaper by using atomics for the checkpoint counter instead of
spinlocks.  Better ideas?

[1]
https://www.postgresql.org/message-id/flat/CA%2BU5nMLv2ah-HNHaQ%3D2rxhp_hDJ9jcf-LL2kW3sE4msfnUw9gA%40mail.gmail.com