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

[Alan Post]

On Thu, Sep 29, 2011 at 01:11:49PM +0200, Jörg F. Wittenberger wrote:
> On Sep 29 2011, Alan Post wrote:
> 
> >Below is a test case for a problem I'm seeing in some multi-process
> >code I'm writing.  I'm getting the error:
> >
> > Error: (file-read) cannot read from file - Interrupted system call
> 
> There are two ways to fix that: either make the posix unit thread safe
> (recall my recent message how to avoid process-wait having a bad effect).
> 
> The other one is working around the problem.  That's what I'm doing based
> on some code Felix supplied ages ago.  It wraps the file descriptors
> into custom ports those are properly restarted on EINTR.
> 
> However I'd be rather interested to learn what exactly the problem is
> you observe.  Recently (maybe 4.7.3 or .4) I'm seeing missbehavior
> from formerly well working code.  I can't say that's chickens fault
> but neither I can say it's not.
> 

If I understand the part of the code below that wraps read/write, it
can't be used as-is for my problem.  Because chicken defers signals,
a signal is delivered, deferred, then read/write return with EINTR.

If I immediately restart these syscalls (all in the same C call), the
deferred signal has not be delivered, and the code deadlocks, as
read/write pauses--blocking the signal handler from ever being run.

I need a way to deliver deferred signals after a syscall returns
EINTR, before restarting that syscall.  You may well not notice this
in non-blocking code, as no data would be ready on the file
descriptor and the code would continue running--eventually to
deliver the deferred signal and unwedge everything.

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