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

On Monday, September 17, 2012 04:08:01 PM 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.
If you start relying on palloc all hope is gone anyway. I "only" want a
standalone XLogReader because thats just too damn annoying/hard to duplicate in
standalone code. There are several very useful utilities out there that are
incomplete and/or unreliable for that reason. And loads of others that haven't
been written because of that.

That is one of the reasons - beside finding the respective xlog.c code very
hard to read/modify/extend - why I wrote a completely standalone xlogreader.
One other factor was just learning how the hell all that works ;)

I still think the interface that something plain as the proposed
XLogReadRecord() provides is too restrictive for many use-cases. I aggree that
a wrapper with exactly such an interface for xlog.c is useful, though.

> Furthermore, returning enough state for useful error messages back out
> of multiple layers of function call is going to be notationally messy,
> and will end up requiring complicated infrastructure barely simpler than
> elog anyway.
Hm. You mean because of file/function/location?

> It'd be a lot better for the wal-dumping program to supply a cut-down
> version of elog than to try to promise that all errors will be returned
> back from ReadRecord.
Well, I suggested a ->error() callback for that reason, that seems relatively
easy to wrap.

Greetings,

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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