Re: pg_basebackup may fail to send feedbacks.

Hello,

The attached patch is fix for walreciever not using gettimeofday,
and fix for receivelog using it.

> > XLogWalRcvProcessMsg doesn't send feedback when processing
> > 'w'=WAL record packet. So the same thing as pg_basebackup and
> > pg_receivexlog will occur on walsender.
> >
> > Exiting the for(;;) loop by TimestampDifferenceExceeds just
> > before line 439 is an equivalent measure to I poposed for
> > receivelog.c, but calling XLogWalRcvSendHSFeedback(false) there
> > is seemingly simpler (but I feel a bit uncomfortable for the
> > latter)
>
> I'm concerned about the performance degradation by calling
> getimeofday() per a record.
>
> > Or other measures?
>
> Introduce the maximum number of records that we can receive and
> process between feedbacks? For example, we can change
> XLogWalRcvSendHSFeedback() so that it's called per at least
> 8 records. I'm not sure what number is good, though...

At the beginning of the "while(len > 0){if (len > 0){" block,
last_recv_timestamp is always kept up to date (by using
gettimeotda():). So we can use the variable instead of
gettimeofday() in my previous proposal.

The start time of the timeout could be last_recv_timestamp at the
beginning of the while loop, since it is a bit earlier than the
actual time at the point.

The another solution would be using
RegisterTimeout/enable_timeout_after and it seemed to be work for
me. It can throw out the time counting stuff in the loop we are
talking about and that of XLogWalRcvSendHSFeedback and
XLogWalRcvSendReply, but it might be a bit too large for the
gain.

Considering pg_basebackup/receivexlog, the loop in receivelog.c
does not maintain the time value within it, so I think we are
forced to use feGetCurrentTimeStamp if not using SIGALRM. The wal
reading function simply gets the data from the buffer in memory
for most calls so the gettimeofday for each iteration could slow
the process significantly. SIGALRM seems to be valuable for the
case.

Thoughts?

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center