[cffi-devel] protect C frames from Lisp non-local exits in callbacks

[Stephen Compall]

A brief discussion in #lisp pointed me to the EXIT-EXTENT:MINIMAL ANSI
issue[1].  The salient point is that there is no way to portably stop
all unwinding for a non-local exit at a particular point, due to this
case of undefined behavior:

;;returns 2 under MEDIUM, is error under MINIMAL
(block a    
  (block b
    (unwind-protect (return-from a 1)
      (return-from b 2))))

A CL implementation that implements MEDIUM should still be fully
compliant with the MINIMAL decision.  Therefore, I would like to hear
from those with particular interest in any particular implementation
running CFFI: does implementation X implement EXIT-EXTENT:MEDIUM, and/or
does its callback definition form as exploited by cffi-sys:%defcallback
have some "magic" unwind stopping?

As an aside: would it be better for the defcallback semantics to simply
say "don't exit non-locally through a C frame"?  It would probably make
callbacks (and perhaps callouts, for that matter) cheaper to call, on
those without "magic" unwind stopping anyway.

[1] http://www.lisp.org/HyperSpec/Issues/iss152.html

-- 
Stephen Compall
http://scompall.nocandysw.com/blog