Re: Confine vacuum skip logic to lazy_scan_skip

On Sat, Feb 15, 2025 at 10:50 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Sat, Feb 15, 2025 at 7:30 AM Melanie Plageman
> <melanieplageman(at)gmail(dot)com> wrote:
> > Seems valgrind doesn't like this [1]. I'm looking into it.
>
> Melanie was able to reproduce this on her local valgrind and
> eventually we figured out that it's my fault. I put code into
> read_stream.c that calls wipe_mem(), thinking that that was our
> standard way of scribbling 0x7f on memory that you shouldn't access
> again until it's reused. I didn't realise that wipe_mem() also tells
> valgrind that the memory is now "no access". That makes sense for
> palloc/pfree because when that range is allocated again it'll clear
> that. The point is to help people discover that they have a dangling
> reference to per-buffer data after they advance to the next buffer,
> which wouldn't work because it's in a circular queue and could be
> overwritten any time after that.

Here's a patch. Is there a tidier way to write this?

It should probably be back-patched to 17, because external code might
use per-buffer data (obviously v17 core doesn't or skink would have
told us this sooner). It's not a good time to push to 17 today,
though. Push to master now to cheer skink up and 17 some time later
when the coast is clear, or just wait?