Re: ReadRecentBuffer() is broken for local buffer

Nice catch, LGTM.



> On Jul 25, 2022, at 02:22, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> 
> ReadRecentBuffer() doesn't work for local buffers, i.e. for temp tables. The bug is pretty clear if you look at the code:
> 
> 	if (BufferIsLocal(recent_buffer))
> 	{
> -		bufHdr = GetBufferDescriptor(-recent_buffer - 1);
> +		bufHdr = GetLocalBufferDescriptor(-recent_buffer - 1);
> 
> The code after that looks suspicious, too. It increases the usage count even if the buffer was already pinned. That's different from what it does for a shared buffer, and different from LocalBufferAlloc(). That's pretty harmless, just causes the usage count to be bumped more frequently, but I don't think it was intentional. The ordering of bumping the usage count, the local ref count, and registration in the resource owner are different too. As far as I can see, that makes no difference, but I think we should keep this code as close as possible to similar code used elsewhere, unless there's a particular reason to differ.
> 
> I propose the attached to fix those things.
> 
> I tested this by adding this little snippet to a random place where we have just read a page with ReadBuffer:
> 
> diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
> index aab8d6fa4e5..c4abdbc96dd 100644
> --- a/src/backend/access/heap/heapam.c
> +++ b/src/backend/access/heap/heapam.c
> @@ -403,6 +403,14 @@ heapgetpage(TableScanDesc sscan, BlockNumber page)
>   RBM_NORMAL, scan->rs_strategy);
>        scan->rs_cblock = page;
> 
> +       {
> +               bool still_ok;
> +
> +               still_ok = ReadRecentBuffer(scan->rs_base.rs_rd->rd_locator, MAIN_FORKNUM, page, scan->rs_cbuf);
> +               Assert(still_ok);
> +               ReleaseBuffer(scan->rs_cbuf);
> +       }
> +
>        if (!(scan->rs_base.rs_flags & SO_ALLOW_PAGEMODE))
>                return;
> 
> Without the fix, the assertion is fails quickly on "make check".
> 
> - Heikki<0001-Fix-ReadRecentBuffer-for-local-buffers.patch>