Re: refactoring relation extension and BufferAlloc(), faster COPY

Hi,

On 2023-03-01 11:12:35 +0200, Heikki Linnakangas wrote:
> On 27/02/2023 23:45, Andres Freund wrote:
> > But, uh, isn't this code racy? Because this doesn't go through shared buffers,
> > there's no IO_IN_PROGRESS interlocking against a concurrent reader. We know
> > that writing pages isn't atomic vs readers. So another connection could
> > connection could see the new relation size, but a read might return a
> > partially written state of the page. Which then would cause checksum
> > failures. And even worse, I think it could lead to loosing a write, if the
> > concurrent connection writes out a page.
>
> fsm_readbuf and vm_readbuf check the relation size first, with
> smgrnblocks(), before trying to read the page. So to have a problem, the
> smgrnblocks() would have to already return the new size, but the smgrread()
> would not return the new contents. I don't think that's possible, but not
> sure.

I hacked Thomas' program to test torn reads to ftruncate the file on the write
side.

It frequently observes a file size that's not the write size (e.g. reading 4k
when writing an 8k block).

After extending the test to more than one reader, I indeed also see torn
reads. So far all the tears have been at a 4k block boundary. However so far
it always has been *prior* page contents, not 0s.

Greetings,

Andres Freund