>> It currently is difficult to write a correct implementation of
>> get-bytevector-all in pure Scheme, because ‘get-bytevector-all’ needs to
>> return a _fresh_ bytevector and could return twice (e.g. in case of
>> system-async-mark + call-with-prompt shenanigans). I think the proposed
>> implementation is incorrect in this way.
>Hmm I don’t see how it could return twice. If an async runs while
>‘get-bytevector-all’ is executed, it cannot cause ‘get-bytevector-all’
>to abort to a prompt. I think we’re fine, no?
I did not say that ‘get-bytevector-all’ aborts to a prompt. Rather, the async
itself does the abort-to-prompt, and since the async is run inside the
get-bytevector-all, as a result get-bytevector-all could return twice
(depending on what the handler of call-with-prompt is doing with the delimited
continuation).
I’m pretty sure it can do this, that’s how Fibers works (to be clear I’m
referring to the abort-to-prompt from an async, not the return twice, Fibers
doesn’t do return twice things) -- non-cooperative scheduling is implemented by
aborting from an async.
Example (assuming bad/good timing):
Thread #1:
;; Consider the situation where the handler invokes the delimited continuation
twice.
(call-with-prompt
[tag + handler things]
(lambda () (get-bytevector-all [...]))
[tag + handler things])
Thread #2:
(system-async-mark
(lambda ()
(abort-to-prompt [tag + more arguments]))
[Thread #1]))
Best regards,
Maxime Devos.
