Callbacks, simplified.

[Andreas Raab]

Hi Rob -

Rob Gayvert wrote:
> Andreas Raab wrote:
>>  > Do you have any thoughts on how this could be handled better?
>>
>> How about we just suspend the currently active process when we 
>> re-enter the interpreter? As long as the callback is synchronous (it 
>> better be) this should work just fine, right?
> 
> Yes, that should do the trick. I tried doing something like this awhile 
> back, but couldn't get it to work reliably. I suspect the problem was 
> that the process oop might move during a callback, in which case trying 
> to resume it would cause a crash. It sounds like your other changes to 
> keep oops across primitives would solve this.

Yes, although (if you look at the changes) I decided to keep this 
internal in the VM because it is also necessary to ensure proper 
resumption upon callback return (otherwise the return value will be 
inaccurate). So the ultimate solution goes like this:

Upon entering the callback, suspend the active process forcefully. Upon 
returning from the callback resume the active process forcefully(!) so 
that the "returning primitive" (e.g., the primitive which invoked the 
callback) can set the proper return value.

As far as I am aware that solves the trick completely (check out the 
code) at the cost of requiring that callbacks are only entered during 
the execution of primitives (meaning that the interpreter's state is 
externalized).

Cheers,
   - Andreas