Re: [PATCH 3/8] Add support for a generic wal reading facility dubbed XLogReader

On 17.09.2012 17:08, Tom Lane wrote:
> Heikki Linnakangas<hlinnakangas(at)vmware(dot)com> writes:
>> On 17.09.2012 13:01, Andres Freund wrote:
>>> It seems we would need one additional callback for both approaches like:
>>> ->error(severity, format, ...)
>>> For both to avoid having to draw in elog.c.
>
>> Yeah. Another approach would be to return the error string from
>> ReadRecord. The caller could then do whatever it pleases with it, like
>> ereport() it to the log or PANIC. I think I'd like that better.
>
> I think it's basically insane to imagine that you can carve out a
> non-trivial piece of the backend that doesn't contain any elog calls.
> There's too much low-level infrastructure, such as palloc, that could
> call it. Even if you managed to make it safe at the instant the feature
> is committed, the odds it would stay safe over time are negligible.

I wasn't thinking that we'd completely eliminate all elog() calls from
ReadRecord and everything it calls, but only the "expected" ones that
mean we've reached the end of valid WAL. The ones that use
emode_for_corrupt_record(). Any unexpected errors like running out of
file descriptors would still use ereport() like usual.

That said, Andres' suggestion of making this facility completely
independent of any backend functions, making it usable in external
programs, doesn't actually seem that hard. ReadRecord() itself is fairly
small, as are the subroutines that validate the records. XLogReadPage(),
which goes out to fetch the right xlog page from archive or whatever, is
way more complicated. But that would live in the callback, so it would
be free to use all the normal backend facilities. However, it means that
external programs would need to supply their own (hopefully much
simpler) version of XLogReadPage(); I'm not sure how that goes with
Andres' plans on using xlogreader.

- Heikki