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

Hmm, this looks quite nice and simple.  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).

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?

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Tiene valor aquel que admite que es un cobarde" (Fernandel)