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Thu Nov 19 11:30:04 CET 2009

External filters

Make an abstraction to call an external data pipe filter as if it were
a scheme function.  This needs some select-based sequencing.

Actually, this only works if there is a clear RPC semantics.  Maybe
coroutines are better?  If unix pipes can be seen as synchronous
channels with well-defined messages this can be greatly simplified.

The original PF sequencer can probably be used.  This needs
non-blocking IO.

Is there an SRFI for this?

Anyway, in order to try this, it is necessary to create tasks and i/o
operations that can be retried.  As opposed to PF, it's probably best
to implement the basic functionality in C.

This needs:
  - pre-emptable parser (a C one-shot continuation).
  - output state machines
  - input state machines

Only the parser is a push-down automaton, not a state machine.  This
will be a significant amount of work.  Let's postpone it a couple of
weeks.

Rewriting the parser to use an explicit stack might be an interesting
approach.  However, it's probably better to use a proper coroutine
implementation.  Maybe this can be taken from lua?

Alternatively, pthread can be used.  It might be a lot simpler to
implement.  As long as the EX memory model can be separated from
read/write threads this should actually be quite straightforward.
However..  This does make them more expensive.

We can have both..  On the longer term, figure out how lua does it.
Switchable stacks is what is necessary.  On the short term: create a
channel abstraction that can produce/consume leaf objects.  Malloc can
still be used to allocate data that's passed between threads, and the
leaf code is completely separate from any graph memory model.

So, a pchannel can be used to synchronize with system threads.


Implementing channels using condition variables (semaphores are not
enough?) There are two blocking conditions.

GET blocks when C = NULL
PUT blocks when C = OBJ

Signalling  consumer -> producer:  write_ok
            producer -> consumer:  read_ok



leaf_object *channel_get(channel *x) {
    leaf_object *ob;
    pthread_mutex_lock(&x->mut);
        while(!x->object) { pthread_cond_wait(&x->get_ok, &x->mut); }
        ob = x->object;
        x->object = NULL;
        pthread_cond_signal(&x->put_ok);  // wake up producer
    pthread_mutex_unlock(&x->mut);
    return ob;
}
int channel_put(channel *x, leaf_object *ob) {
    pthread_mutex_lock(&x->mut);
        while(x->object) { pthread_cond_wait(&x->put_ok, &x->mut); }
        x->object = ob;
        pthread_cond_signal(&x->get_ok); // wake up consumer
    pthread_mutex_unlock(&x->mut);
    return 0;
}


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