Re: streaming header too small

On 20.02.2013 02:11, Selena Deckelmann wrote:
> So, I just ran into a similar issue backing up a 9.2.1 server using
> pg_basebackup version 9.2.3:
>
> pg_basebackup: starting background WAL receiver
> pg_basebackup: streaming header too small: 25
>
>
> I've had it happen two times in a row. I'm going to try again...
>
> But -- what would be helpful here? I can recompile pg_basebackup with more
> debugging...

Hmm, 25 bytes would be the size of the WAL data packet, if it contains
just the header and no actual WAL data. I think pg_basebackup should
accept that - it's not unreasonable that the server might send such a
packet sometimes.

Looking at the walsender code, it's not supposed to ever send such a
packet. But I suspect there's one corner-case where it might: if the
current send location is at an xlogid boundary, so that we previously
sent the last byte from the last WAL segment in the previous logical
xlog file, and the WAL flush position points to byte 0 in the beginning
of the new WAL file. Both of those positions are in fact the same thing,
but we have two different ways to represent the same position. For
example, if we've already sent up to WAL position (sentPtr in walsender.c):

xlogid = 4
xrecoff = XLogFileSize

and GetFlushRecPtr() returns:

xlogid = 5
xrecoff = 0

Those both point to the same position. But the check in XLogSend that
decides if there is any work to do uses XLByteLE() to check if they are
equal, and XLByteLE() treats the latter to be greater than the former.
So, in that situation, XLogSend() would decide that it has work to do,
but there actually isn't, so it would send 0 bytes of WAL data.

I'm not sure how GetFlushRecPtr() could return such a position, though.
But I'm also not convinced that it can't happen.

It would be fairly easy to fix walsender to not send anything in that
situation. It would also be easy to fix pg_basebackup to not treat it as
an error. We probably should do both.

In 9.3, the XLogRecPtr representation changed so that there is only one
value for a boundary position like that, so this is a 9.2-only issue.

- Heikki