Re: Hooks for session start and end, take two

Hi,

On 2019-10-03 11:19:58 -0300, Fabrízio de Royes Mello wrote:
> On Thu, Oct 3, 2019 at 1:35 AM Andres Freund <andres@anarazel.de> wrote:
> >
> > [...]
> >
> > In this state I think this patch should be flat out rejected.
> >
> 
> Ok.
> 
> > I'm seriously baffled at how this stuff is being pursued aggressively,
> > quite apparently without any serious design considerations. I mean this
> > stuff had to be backed out twice, yet it's being reproposed without much
> > consideration for concerns?
> >
> 
> And what if (again) for the first version of session start hook we do it
> just for:
> - client backends
> - background workers
> 
> For sure we'll need new design, but for now I'm can't imagine a use case
> for sessions different than listed above.

I think what would need to be designed is something more like
RegisterXactCallback():

/*
 *    start- and end-of-transaction callbacks for dynamically loaded modules
 */
typedef enum
{
    XACT_EVENT_COMMIT,
    XACT_EVENT_PARALLEL_COMMIT,
    XACT_EVENT_ABORT,
    XACT_EVENT_PARALLEL_ABORT,
    XACT_EVENT_PREPARE,
    XACT_EVENT_PRE_COMMIT,
    XACT_EVENT_PARALLEL_PRE_COMMIT,
    XACT_EVENT_PRE_PREPARE
} XactEvent;

typedef void (*XactCallback) (XactEvent event, void *arg);

extern void RegisterXactCallback(XactCallback callback, void *arg);
extern void UnregisterXactCallback(XactCallback callback, void *arg);


which would be called at various parts of a processes lifetime. Maybe
something like:

1) shortly after process creation
2) shortly after shared memory attach
3) after database connection has been established
4) after database writes are possible (this excludes database-less walsenders)
5a) before closing database connection due to fatal error
    (with a note saying that no writes are ever allowed)
5b) before closing database connection due to normal exit
    (with a note saying that errors better be handled, and that no
    transaction may escape)
6) before shared memory detach
7) before final exit

Greetings,

Andres Freund