Re: Fdw cleanup

Hi, Albe,

Thank you.

The resource is not memory, I have something like a file descriptor or
network connection, which I believe is the common case for foreign table.
Using RegisterXactCallback exposes an API at a much deep level to foreign
table writer, and I need to find a place to anchor the context (fdw_state).
I took a look at postgres_fdw and found that xact callback is exactly
what is done. So I will take this approach.

Again, thank you Albe, for pointing me to it.
Feng

On Mon, Dec 14, 2015 at 12:30 AM, Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>
wrote:

> Feng Tian wrote:
> > I need some help to understand foreign table error handling.
> >
> > For a query on foreign table, ExecInitForeignScan is called, which in
> turn calls the BeginForeignScan
> > hook. Inside this hook, I allocated some resource,
> >
> >
> > node->fdw_state = allocate_resource(...);
> >
> > If everything goes well, ExecEndForeignScan will call call my
> EndForeignScan hook, inside the hook, I
> > free the resource.
> >
> > free_resource(node->fdw_state);
> >
> > However, if during the execution an error happened, seems to me that
> EndForeignScan will not be called
> > (traced using gdb). So my question is, is Begin/End the right place
> for allocate/free resources?
> > If it is not, what is the right way to do this?
>
> If the resource is memory, use "palloc" to allocate it, and PostgreSQL
> will take care of it automatically.
>
> Other than that, you could call RegisterXactCallback() and register a
> callback
> that will be called upon transaction end. In the callback function you can
> free the resource if it was not already freed by EndForeignScan.
>
> Yours,
> Laurenz Albe
>