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[Chicken-users] Threads and dynamic-wind still problematic

From: Peter Bex
Subject: [Chicken-users] Threads and dynamic-wind still problematic
Date: Mon, 2 Mar 2009 21:48:17 +0100
User-agent: Mutt/


I just got bitten by this problem:

I'm increasing and decreasing a mutex value based on how many threads
are doing some work.  When they stop working, the mutex is decreased
and when they start, the mutex is increased.  I decided to wrap the
thunk that does the work in a dynamic-wind to protect it from escape
continuations being called, and from exceptions.

However, it turns out that an exception thrown in a thread simply
terminates the thread instead of unwinding the stack.  The same does not
happen in the primordial thread:

$ csi
#;1> (use srfi-18)
; loading library srfi-18 ...
#;2> (dynamic-wind (lambda () (print "BEFORE")) (lambda () (error "die!")) 
(lambda () (print "AFTER")))
Error: die!

        Call history:

        <syntax>                (begin (error "die!"))
        <syntax>                (error "die!")
        <syntax>                (lambda () (print "AFTER"))
        <syntax>                (begin (print "AFTER"))
        <syntax>                (print "AFTER")
        <eval>          (dynamic-wind (lambda () (print "BEFORE")) (lambda () 
(error "die!")) (lambda () (print "AFTER")))
        <eval>          (print "BEFORE")
        <eval>          (error "die!")  <--
#;2> (thread-start! (lambda () (dynamic-wind (lambda () (print "BEFORE")) 
(lambda () (error "die!")) (lambda () (print "AFTER")))))
#<thread: thread0>
Warning (#<thread: thread0>): : die!

        Call history:

        <eval>          (dynamic-wind (lambda () (print "BEFORE")) (lambda () 
(error "die!")) (lambda () (print "AFTER")))
        <eval>          (print "BEFORE")
        <eval>          (error "die!")  <--

I'd say this is *extremely* counterintuitive; that raising an error
outside a thread (or rather; in the primordial thread) causes pending
dynamic-unwinds to unwind while doing the same in a thread does not.
In fact, I'd say it's outright dangerous; this can cause hard to track
race condition type bugs.

This bug is still there in Chicken 4.

"The process of preparing programs for a digital computer
 is especially attractive, not only because it can be economically
 and scientifically rewarding, but also because it can be an aesthetic
 experience much like composing poetry or music."
                                                        -- Donald Knuth

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