Re: The Shepherd on Fibers

[Maxime Devos]

Philip McGrath schreef op ma 28-03-2022 om 20:14 [-0400]:
> Maybe it would be enough for this case for Fibers to provide variants of 
> `dynamic-wind` and/or `call-with-continuation-barrier` that cooperate 
> with the Fibers implementation. I don't know if that would be possible 
> of not—in addition to my limited familiarity with Fibers, I have not 
> read all of the related literature that Alexis, Matthew, and Matthias 
> Felleisen discuss in [5] and [6]—again, I am not an expert!

Fibers' context switching is based on continuations.  By definition,
‘call-with-continuation-barrier’ must create a continuation barrier
(and as a consequence, interfere with Fibers' context switching).

It can be important to let 'call-with-continuation-barrier' (*)
actually create a continuation barrier, even when fibers is used, e.g.
to not create deadlocks (or livelocks or whatever, I don't know the
terminology) when POSIX mutexes are used.  As such, as-is, I don't
think so.

Unless Guile could add some kind of '#:key "foobar"' argument to 'call-
wih-continuation-barrier' and 'call-with-prompt/abort-to-prompt' -- the
idea is that, by default, a continuation barrier cannot be passed,
unless a matching key is supplied.  So you could have a continuation
barrier that allows context switching, but not continuing with raise-
continuable or amb.

The unwind-protect defined at
<https://www.ccs.neu.edu/home/will/UWESC/uwesc.sch> looks interesting,
although it needs to be generalised to remove the global variables (for
multi-threading) and it needs to be verified whether it interacts well
with exeption handling.

Greetings,
Maxime.

(*) Actually, 'call-with-continuation-barrier' and 'dynamic-wind' are
already a bit of a lie, since the kernel ignores them when context
switching ... Maybe continuation barriers and {un,re}winding is a
matter of perspective?

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