Re: [patch] Fix checksum verification in base backups for zero page headers

On Thu, Oct 22, 2020 at 03:11:45PM +0300, Anastasia Lubennikova wrote:
> Most of such pages are valid and already in memory, because they were
> changed just recently, so no need for pg_prewarm here. If such LSN appeared
> because of a data corruption, page verification from inside ReadBuffer()
> will report an error first. In proposed function, we can handle this error
> in any fashion we want. Something like:
>
> if (PageGetLSN(page) > startptr)
> {
>     if (!read_page_via_buffercache())
>
>         //throw a warning about corrupted page
>         //handle checksum error as needed
>     else
>         //page is valid. No worries
> }

Yeah, we could try to make the logic a bit more complicated like
that. However, for any code path relying on a page read without any
locking insurance, we cannot really have a lot of trust in any of the
fields assigned to the page as this could just be random corruption
garbage, and the only thing I am ready to trust here a checksum
mismatch check, because that's the only field on the page that's
linked to its full contents on the 8k page. This also keeps the code
simpler.
--
Michael