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

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.

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.

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.

regards, tom lane