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

Raimo Niskanen <>
Fri Jan 9 09:42:25 CET 2009

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

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