Re: Remove page-read callback from XLogReaderState.

Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:

> Hello. As mentioned before [1], read_page callback in
> XLogReaderState is a cause of headaches. Adding another
> remote-controlling stuff to xlog readers makes things messier [2].

The patch I posted in thread [2] tries to solve another problem: it tries to
merge xlogutils.c:XLogRead(), walsender.c:XLogRead() and
pg_waldump.c:XLogDumpXLogRead() into a single function,
xlogutils.c:XLogRead().

> [2]
> https://www.postgresql.org/message-id/20190412.122711.158276916.horiguchi.kyotaro@lab.ntt.co.jp

> I refactored XLOG reading functions so that we don't need the
> callback.

I was curious about the patch, so I reviewed it:

* xlogreader.c

** Comments mention "opcode", "op" and "expression step" - probably leftover
from the executor, which seems to have inspired you.

** XLR_DISPATCH() seems to be unused

** Comment: "duplicatedly" -> "repeatedly" ?

** XLogReadRecord(): comment "All internal state need ..." -> "needs"

** XLogNeedData()

*** shouldn't only the minimum amount of data needed (SizeOfXLogLongPHD)
be requested here?

state->loadLen = XLOG_BLCKSZ;
XLR_LEAVE(XLND_STATE_SEGHEAD, true);

Note that ->loadLen is also set only to the minimum amount of data needed
elsewhere.

*** you still mention "read_page callback" in a comment.

*** state->readLen is checked before one of the calls of XLR_LEAVE(), but
I think it should happen before *each* call. Otherwise data can be read
from the page even if it's already in the buffer.

* xlogreader.h

** XLND_STATE_PAGEFULLHEAD - maybe LONG rather than FULL? And perhaps HEAD
-> HDR, so it's clear that it's about (page) header, not e.g. list head.

** XLogReaderState.loadLen - why not reqLen? loadLen sounds to me like "loaded"
as opposed to "requested". And assignemnt like this

int reqLen = xlogreader->loadLen;

will also be less confusing with ->reqLen.

Maybe also ->loadPagePtr should be renamed to ->targetPagePtr.

* trailing whitespace: xlogreader.h:130, xlogreader.c:1058

* The 2nd argument of SimpleXLogPageRead() is "private", which seems too
generic given that the function is no longer used as a callback. Since the
XLogPageReadPrivate structure only has two fields, I think it'd be o.k. to
pass them to the function directly.

* I haven't found CF entry for this patch.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com