Re: [Vala] Documenting 'async' / WAS: Further speculations on couroutines, generators and threads : Emulating Go's goroutines and channels in Vala

[Jim Peters <jim uazu net>]

Luca Bruno wrote:
> All correct.

Thanks.

> >   Question: At the C level, I guess this first calls forwards to
> >   'other_method' to start it before returning to the caller due to the
> >   'yield'.  If the 'other_method' also yields, then there is no
> >   problem, but if 'other_method' finishes without yielding (e.g. if it
> >   can return the result right away without doing any asynchronous
> >   work), then the 'method' callback would be called again, only a few
> >   stack frames lower.  In theory if 'method' called 'other_method'
> >   repeatedly like this, the stack could overflow.  Is this correct?
> >   If so maybe that needs documenting.
>
> Variables in async methods are allocated on the heap. By the way I
> think I didn't understand well the problem you're raising.

The generated C code sometimes uses 'g_simple_async_result_complete',
and sometimes 'g_simple_async_result_complete_in_idle'.  There are
cases where if it used 'g_simple_async_result_complete', then the C
stack could grow and grow.  However, I tried writing some code to
provoke this situation, and it used the *_idle call in this case and
didn't fail.  So perhaps Joerg already thought of this case.

Here was the test case:

  async void tester() {
     // Call read_item for values it returns right away, trying to
     // provoke stack overflow from the callbacks
     for (int a = 0; a<2000000; a+=2) {
        var val = yield read_item(a);
     }
  }
  
  async string read_item(int a) {
     if (a % 2 == 0) {
        // Pretend we have it in cache and can return right away
        return @"Value: $a";
     } else {
        // Pretend that this is a value that we need to fetch over the
        // network, so we yield and return the value some time later
        Idle.add(read_item.callback);
        yield;
        return @"Value: $a";
     }
  }
  
  void main(string[] args) {
     var loop = new MainLoop();
     tester.begin((obj, res) => {
           tester.end(res);
           loop.quit();
        });
     loop.run();
  }


> > - Call with .begin().  Question: Is this just like the 'method()' call
> >   but adding a callback request for when method() finishes?  If so
> >   will it also just run until the first 'yield' and then return?
> >
> >     method.begin(args, callback);
>
> Yes.
>
> > - Call with .end().  For use within the 'begin' callback, to get
> >   method return value and clean up.  Question: Is an .end() call
> >   required for plain 'method()' style invocation?
> >
> >     method.end(out_args);
>
> Can you rephrase?

We can call 'method' either like this:

   method();

or like this:

   method.begin();

or like this:

   method.begin((obj, res) => {
           method.end(res);
        });

In the first two cases, is the .end() handled automatically?  Looking
at the generated code, it doesn't seem so.  Could this be a problem?
For example, could this cause a leak?


> > Also, questions:
> >
> > - Are there any other special features associated with async methods?
> >   (Any other .<identifier> features, for example.)
>
> No.
>
> > - When it is necessary to have a main loop running?  Some of the
> >   generated C code requests an idle callback.  I guess this won't work
> >   without a main loop running.  But the Generator, for example, seems
> >   to run through fine without a main loop.
>
> Mainloop is required only for async methods that never use yield.
>
> > - The docs say "The .callback call is used to implicitly register a
> >   _finish method for the async method".  What does this mean?  Also:
> >   "end() is a syntax for the *_finish method".  I'm confused.
>
> Try looking at the generated C code.

I think the second phrase makes sense.  The first phrase doesn't seem
correct: "The .callback call is used to implicitly register a _finish
method for the async method".

Who owns the Vala Tutorial?  I could add more info to the Tutorial,
but if that isn't okay I will create a new wiki page.

Jim

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