Re: Improve WALRead() to suck data directly from WAL buffers when possible

Hi,

On 2024-01-10 19:59:29 +0530, Bharath Rupireddy wrote:
> +        /*
> +         * Typically, we must not read a WAL buffer page that just got
> +         * initialized. Because we waited enough for the in-progress WAL
> +         * insertions to finish above. However, there can exist a slight
> +         * window after the above wait finishes in which the read buffer page
> +         * can get replaced especially under high WAL generation rates. After
> +         * all, we are reading from WAL buffers without any locks here. So,
> +         * let's not count such a page in.
> +         */
> +        phdr = (XLogPageHeader) page;
> +        if (!(phdr->xlp_magic == XLOG_PAGE_MAGIC &&
> +              phdr->xlp_pageaddr == (ptr - (ptr % XLOG_BLCKSZ)) &&
> +              phdr->xlp_tli == tli))
> +            break;

I still think that anything that requires such checks shouldn't be
merged. It's completely bogus to check page contents for validity when we
should have metadata telling us which range of the buffers is valid and which
not.

Greetings,

Andres Freund