[erlang-questions] : : : : : driver_entry stop() and driver_async() interaction

[Raimo Niskanen]

On Fri, Dec 19, 2008 at 07:08:11AM -0800, Chris Newcombe wrote:
> > We plan while fixing the bug with async_free() mentioned below
> > to introduce a reference counter of async jobs so it will be
> > guaranteed that stop() is not called until all async jobs are done.
> 
> Excellent.  But please could you provide APIs to increment and
> decrement that reference count, so that drivers that implement their
> own private thread-pools can achieve the same behaviour?

Sounds a bit hackish, but why not; drivers tend to be hackish.
An even hackier hack would be to use the I/O queue for this,
and that even works today.

> 
> Also, I hate to cause feature creep, but if you do implement the above
> reference count/delayed 'stop', please could you also add an optional
> "prepare to stop" callback, which the VM calls when the Erlang code
> closes the port (e.g. the port owner process dies, or something
> explicitly closes the port)?  Some drivers often have fairly
> long-running operations (seconds/minutes) running in their own
> threads, and would like timely notification that they should force
> such jobs to terminate, to allow "stop" to be called as soon as
> possible (the berkeley_db driver has to do this).  i.e. It's not
> always the case that running async jobs should be left to complete in
> their own sweet time.  (Note that this is different from 'cancelling'
> pending-but-not-yet-started jobs.)

Was not this (later in the same mail from me) enough?
: 
> Raimo wrote:
> > While we are at it we plan to introduce a driver flag
> > indicating that the driver wants to be called on
> > flush() if there are pending async jobs (as well as
> > today if the IO queue is not empty).
:
Today the flush() callback is called when the port gets killed e.g
gets an exit signal from its linked owner when it dies, if there
is data in the I/O queue. Then the stop() callback is called when the
I/O queue is emptied.

> 
> Rationale:  It does actually matter a lot to some drivers that "stop"
:

-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB