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

On 17.09.2012 14:42, Andres Freund wrote:
> On Monday, September 17, 2012 12:55:47 PM Heikki Linnakangas wrote:
>> On 17.09.2012 13:01, Andres Freund wrote:
>>> On Monday, September 17, 2012 11:07:28 AM Andres Freund wrote:
>>>> On Monday, September 17, 2012 10:30:35 AM Heikki Linnakangas wrote:
>>>>> On 17.09.2012 11:12, Andres Freund wrote:
>>>>>> On Monday, September 17, 2012 09:40:17 AM Heikki Linnakangas wrote:
>>>>>> If you don't want the capability to forward/filter the data and read
>>>>>> partial data without regard for record constraints/buffering your
>>>>>> patch seems to be quite a good start. It misses xlogreader.h
>>>>>> though...
>>>>>
>>>>> Ah sorry, patch with xlogreader.h attached.
>>>>
>>>> Will look at it in a second.
>>>
>>> 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.
> That seems a bit more complex from a memory management perspective as you
> probably would have to sprintf() into some buffer. We cannot rely on a backend
> environment with memory contexts around et al...

Hmm. I was thinking that making this work in a non-backend context would
be too hard, so I didn't give that much thought, but I guess there isn't
many dependencies to backend functions after all. palloc/pfree are
straightforward to replace with malloc/free. That's what we could easily
do with the error messages too, just malloc a suitably sized buffer.

How does a non-backend program get access to xlogreader.c? Copy
xlogreader.c from the source tree at build time and link into the
program? Or should we turn it into a shared library?

- Heikki