Support added for exceptions with ASYNCIFY

The general scheme is not very complicated. Changes:

1. Rather than doing `dynCall_vi` on the *foo*__asnc_cb function, we use `invoke_vi`. This is essential if the callback functions are to throw exceptions.
1. If a callback did throw an exception, then we should reenter the parent callback function only if it checks for exceptions at the resume point. Otherwise, the exception should propagate up. We check for this using an extra boolean in the async context: `emscripten_alloc_async_context` now has an extra argument which the C++ code uses to indicate whether the async function is called with "invoke" or "call". (So the `invoke` argument to `emscripten_alloc_async_context` is set when `__THREW__` is checked immediately after the async call.)
1. If the exception isn't caught anywhere, we rethrow it finally so that an error message is provided (admittedly not a very clear one, but it's no worse than when an exception is throw out of a synchronous `main`).

This pull request works in conjunction with the pull request to the fastcomp compiler.
async_cb functions should never use alloca, they are part of the original
function, which should have already allocated necessary spaces

On Tue, Sep 9, 2014 at 10:58 PM, Nicholas Wilson <notifications@github.com>
wrote:

> In src/library_async.js:
>
> > -        ___async_unwind = 1;
> > -        continue;
> > +      if (!___async_cur_frame) {
> > +        // print ugly integer in console, same as if an exception is thrown out of main()
> > +        if (activeException) _emscripten_async_abort_with_exception();
> > +        return;
> > +      }
> > +      catchesExceptions = {{{ makeGetValueAsm('___async_cur_frame', 8, 'i8') }}};
> > +      // When an async function throws an exception, it can't just bubble up as
> > +      // normal (because there isn't a native JavaScript call stack, we're
> > +      // manually reconstructing it here).  We keep rewinding the stack until we
> > +      // find a function that can handle the exception.
> > +      if (!!catchesExceptions | !activeException) {
> > +        __THREW__ = activeException|0;
> > +        invoke_vi({{{ makeGetValueAsm('___async_cur_frame', 12, 'i32') }}},
> > +                  (___async_cur_frame + 12)|0);
>
> Yup, I saw what you meant after looking at the comment more closely! I've
> expanded the comment a bit, and discovered a little bug while thinking it
> through:
>
> If an __aysnc_cb function uses alloca (or anything that calls stackAlloc)
> then uses realloc_async_context to call another async function, then
> there's a problem! realloc_async_context tramples on the stack. I've added
> an assertion to catch this case at least; we should fail clearly rather
> than corrupting memory an application is using.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/kripken/emscripten/pull/2772/files#r17305558>.
>