Re: [Chicken-users] EINTR with self-pipe signal trampoline

[Alan Post]

On Thu, Sep 29, 2011 at 03:26:22PM +0200, Jörg F. Wittenberger wrote:
> Attempt work around: defer the signal even more: until next schedule
> time.  I'm not yet satisfied with that solution.  (But at least it
> allows me to run arbitrary code in the signal handler.)
> (I intended so far to try whether it would be better to run it close
> to the finalizers; that is at the end of garbage collection.
> But that again is just an experiment TBD.)
> 
> Given your situation I hope we can devise a better way.
> 
> I'd need to understand your situation better.
> 
> >I need a way to deliver deferred signals after a syscall returns
> >EINTR, before restarting that syscall.
> 
> I see.  I understand you need "minimum latency" - right?
> 
> May I ask for more details.  I need to understand where this
> latency requirement is important.
> 

Let me try a demonstration showing just the main thread:

(define (restart-read fd buf isize)
  ; call read(2), on the self-pipe, which blocks
  (let ((r (file-read fd buf isize)))
    (if (= -1 r)
        ; ah, a signal was delivered.  In Chicken, the signal
        ; delivery to the prcoess causes this error, but the
        ; Scheme code signal handler might not have been
        ; delivered.  In this case, it means that I haven't
        ; written a byte to the self-pipe yet.
        ;
        (if (= errno EINTR)
            ; restart the read.  Normally, we would have written
            ; to our self-pipe already (as the signal is delivered
            ; the moment we return from our syscall.), but in
            ; Chicken, I'm not sure our deferred Scheme code has
            ; been called.  When does that happen?  If it hasn't
            ; happened yet, I'll block in this routine, and the
            ; deferred signal handler will never run.
            ;
            ; I need a way to guarantee that the deferred signal
            ; handler has already been run, before I call any other
            ; syscall.
            ;
            (restart-read fd buf isize))
        ; something else went wrong, die.
        (io-error "read"))))

Does that story make more sense?  Chicken may already behave this
way, I'm not sure how long it waits/when it delivers a deferred
signal.  The critically important thing is that it do so before
making another syscall.

-Alan
-- 
.i ma'a lo bradi cu penmi gi'e du