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

On Tue, Jan 30, 2024 at 11:01 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> Hmm, this looks quite nice and simple.

Thanks for looking at it.

> My only comment is that a
> sequence like this
>
>    /* Read from WAL buffers, if available. */
>    rbytes = XLogReadFromBuffers(&output_message.data[output_message.len],
>                                 startptr, nbytes, xlogreader->seg.ws_tli);
>    output_message.len += rbytes;
>    startptr += rbytes;
>    nbytes -= rbytes;
>
>    if (!WALRead(xlogreader,
>                 &output_message.data[output_message.len],
>                 startptr,
>
> leaves you wondering if WALRead() should be called at all or not, in the
> case when all bytes were read by XLogReadFromBuffers.  I think in many
> cases what's going to happen is that nbytes is going to be zero, and
> then WALRead is going to return having done nothing in its inner loop.
> I think this warrants a comment somewhere.  Alternatively, we could
> short-circuit the 'if' expression so that WALRead() is not called in
> that case (but I'm not sure it's worth the loss of code clarity).

It might help avoid a function call in case reading from WAL buffers
satisfies the read fully. And, it's not that clumsy with the change,
see following. I've changed it in the attached v22 patch set.

if (nbytes > 0 &&
    !WALRead(xlogreader,

> Also, but this is really quite minor, it seems sad to add more functions
> with the prefix XLog, when we have renamed things to use the prefix WAL,
> and we have kept the old names only to avoid backpatchability issues.
> I mean, if we have WALRead() already, wouldn't it make perfect sense to
> name the new routine WALReadFromBuffers?

WALReadFromBuffers looks better. Used that in v22 patch.

Please see the attached v22 patch set.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com