Re: BgBufferSync(): clarification about reusable_buffers variable

Hi, tks for working on this. I had a chance to look at this while
googling BgBufferSync
function.

>
> I also think tha reusable_buffers keep track of the number of reusable
> buffers. BgBufferSync() calls SyncOneBuffer() with skip_recently_used
> = true. In that case, if SyncOneBuffer() finds the buffer with
> refcount or usage_count non-zero, it just unlocks the header and
> returns. Hence when called from BgBufferSync(), SyncOneBuffer() would
> write a buffer only when it is not used. Hence the result would be 0
> or BUF_REUSABLE or BUF_REUSABLE | BUF_WRITTEN. It can never be just
> BUF_WRITTEN.
>

Agrees. For this call stack, if skip_recently_used is set to be true,
sync_state cannot be BUF_WRITTEN alone.

> I guess, a patch like the one attached will be more readable and clear.
>

I'm new to this part of code, and I found the patch version seems to be
more straightforward and less prone to misinterpretation.

> I ran pgbench for 5 minutes with this patch applied and didn't see the
> Assert failing. But I don't think that's a good enough test to cover
> all scenarios.
>

The patch LGTM.

--
> Best Wishes,
> Ashutosh Bapat
>