Re: [HACKERS] Fdw cleanup

[Feng Tian]

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 
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
>

Re: [HACKERS] Fdw cleanup

[Albe Laurenz]

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

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[HACKERS] Fdw cleanup

[Feng Tian]

Hi, Hackers,

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?

Thank you very much,
Feng