tevent immediate events

[Stefan (metze) Metzmacher]

simo schrieb:
> On Fri, 2009-03-13 at 12:40 +0100, Stefan (metze) Metzmacher wrote:
>> Hi,
>>
>> We have some usecases for directly triggered timed events
>> where we can't do proper reporting of allocation errors.
>>
>> I'd like to propose to add new event class for immediately triggered
>> events:
>>
>> typedef void (*tevent_immediate_handler_t)(struct tevent_context *ctx,
>>                                            struct tevent_immediate *im,
>>                                            void *private_data);
>> struct tevent_immediate *tevent_allocate_immediate(TALLOC_CTX *mem_ctx);
>> void tevent_schedule_immediate(struct tevent_context *ctx,
>>                                struct tevent_immediate *im,
>>                                tevent_immediate_handler_t handler,
>>                                void *private_data);
>>
>> The caller allocated the tevent_immediate structure once and can
>> then schedule() it when needed. Once the event is triggered it's
>> detached from the tevent_context again. The caller can reuse it later.
>>
>> if the schedule function is called while the event was still scheduled,
>> it will be detached from the old tevent_context and attached to the new
>> context.
> 
> Can you please clarify better what problem are you trying to solve with
> this scheme?
> Maybe give an example that shows why tevent_add_timer does not work and
> how this solves the problem.

in tevent_queue, we use a timer event to trigger the next event.
currently we only do this for the first event, when it's added to the
queue, so we can report tevent_add_timer() returning NULL.
The bad thing is that we trigger the following events from deep inside
of the talloc destructor of the finished tevent_req. I'd like to avoid
that, as it would mean we have event nesting. I'd like to always
trigger the next request via an immediate event. That means the
talloc destructor of the finished request would just schedule
the immediate event and that should not fail as we have no way to
handle an error there.

I'd also like to use the immediate events in my tsocket code
for triggering read and write events on virtual sockets, which don't
have a fd. It simplifies the code a lot, if I can use call
a void function to schedule the event.

There were also a lot of other situations where I liked to have
a way to add immediate timers without adding a lot of logic to handle
possible errors in the past.

I also thought about adding a tevent_allocate_timer()/
tevent_schedule_timer(), but Volker proposed to add a separate
class of events, as it's to confusing to have to behavior for
tevent_timer objects. And I like the idea to split them.

metze

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