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

[Jim Peters <jim uazu net>]

Alexandre Rosenfeld wrote:
> Async methods were created primarily for GIO to do non-blocking
> IO. Doing IO usually blocks the application and so freezes the UI,
> so the GLib answer is to provide callbacks to be called when the IO
> is done and the program can keep executing while IO is done in the
> background (note here that background does not necessarily mean
> another thread, the IO can be done by the kernel and notify the
> process when it's complete).
>
> Async methods have nothing to do with threads, so none of these
> points are valid. However, what async methods allow is to stop the
> processing of a function in the middle and resume it after the async
> method returns, by the means of the callbacks. So, what the async
> method can do is spawn a thread to do background processing and
> later call the callback to resume the operation of the method that
> called it.
>
> Which means async methods can be just as blocking as any other
> method if it does sequential processing and at the end calls the
> callback. But also means it can be designed to use any other way to
> provide background processing (including threads) without blocking
> the main program (especially the UI) and thus can take advantage of
> multiple cores.

This is helpful, thanks.

There seems to be special stuff associated with an async method which
I haven't found documented well anywhere.  For async method 'method',
there are all these ways to use it:

- Call 'method' from non-async code, starts it running until its first
  'yield', at which point it returns to the caller.  (Correct?)

    method(args);

- Get callback (from within method itself).  This is the callback to
  resume execution after the following 'yield'.  (Correct?)

    SourceFunc callback = method.callback;

- Use the resume callback from elsewhere.  'method' resumes and takes
  control again, running until its next 'yield', at which point it
  returns from this call.  (Correct?)

    callback();

- Give up control and return to the caller.  This doesn't guarantee in
  any way that the method will be resumed, i.e. callback() must be
  called somewhere else.  (Correct?)

    yield;

- Give up control but arrange for a resume callback when idle.  This
  requires the main loop to be running.  (Correct?)

    Idle.add(method.callback);
    yield;

- Call async method 'other_method' from 'method'.  This automatically
  sets up a callback for 'method' to resume itself and collect the
  return value when 'other_method' completes.  (Correct?)
  
    yield other_method(args);

  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.

- 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);

- 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);

Also, questions:

- Are there any other special features associated with async methods?
  (Any other .<identifier> features, for example.)

- 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.

- 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.

If someone can say RIGHT or WRONG to some of the points in this list
I'd be happy to document this on the wiki somewhere.

Jim

-- 
 Jim Peters                  (_)/=\~/_(_)                 jim uazu net
                          (_)  /=\  ~/_  (_)
 Uazú                  (_)    /=\    ~/_    (_)                http://
 in Peru            (_) ____ /=\ ____ ~/_ ____ (_)            uazu.net