Interrupting repl in Slime

[Alessio Stalla]

To me, this looks overly complicated with all these flags. I never
understood why ABCL does all this. Java threads already natively have an
interrupt flag. Is there a reason for not using the native Java mechanism?
https://docs.oracle.com/javase/tutorial/essential/concurrency/interrupt.html

On Thu, 7 Apr 2022 at 00:13, Alan Ruttenberg <alanruttenberg at gmail.com>
wrote:

> Hi folks,
>
> Maybe someone has a feel for how interrupts work in ABCL?
>
> Doing an interrupt in ABCL from Slime doesn't interrupt until the end of a
> function or sleep or wait is called.  This means whenever I accidentally
> have a too long or infinite loop by mistake the only thing I can do is quit
> lisp and start over. This is probably THE most irritating problem I have
> with ABCL. It doesn't happen often but when it does it completely disrupts
> work.
>
> Slime handles an interrupt by calling interrupt-thread. Interrupt-thread
> saves the function and sets a flag. When the interrupted process next
> checks the flag it will call the interrupt function. At first I thought
> lisp wasn't checking for an interrupt often enough, but that's not the
> problem. If I compile (loop for i = 1)  I get
>
>   public final LispObject execute() {
> ;         do {
> ;             if (!Lisp.interrupted) {
> ;                 continue;
> ;             }
> ;             Lisp.handleInterrupt();
> ;         } while (true);
> ;     }
>
> So why doesn't the interrupt work? Well it seems there are two paths to
> interrupt. One is via interrupt-thread. Here is the code that sets the flag.
>
>     final synchronized void interrupt(LispObject function, LispObject args)
>     {
>         pending = new Cons(args, pending);
>         pending = new Cons(function, pending);
>         threadInterrupted = true;
>         javaThread.interrupt();
>     }
>
> Note that it is setting thread.threadInterrupted rather than
> Lisp.interrupted. In fact, the only way to set Lisp.interrupted is via
> Lisp.setInterrupted() and the only function that calls that is
> ext:interrupt-lisp. Nothing calls ext:interrupt-lisp. So that nice check
> within the loop is checking for a signal that currently never arrives.
>
> The javathread.interrupt is only periodically detected by an explicit call
> or via an InterruptedException that Java will throw when the thread yields
> or via a check of thread.isInterrupted. The latter is checked in one place,
> inside a catch, which afaik isn't executed unless there's an error in the
> thread - it's the "Ignoring uncaught exception" path.
>
> Now, if I call ext:interrupt-lisp from the *inferior-lisp* a break comes
> up during the infinite loop and I can kill it! This is the desired
> behavior.
>
> The problem is, I can't figure out where to call (ext:interrupt-lisp)
> within slime. I tried to put it right after interrupt-thread when that is
> called in swank::queue-thread-interrupt, which is what is eventually called
> when you hit C-c in the repl.  But that causes an error and kills the slime
> session. And emacs, for that matter, which needs to be force quit.
>
> I'm thinking that in the case of a control-c we should not use
> thread-interrupt but rather lisp interrupted. threadInterrupted is thread
> local and Lisp.interrupted is a global. So technically the control c might
> not go to the right thread, since all threads are checking the global. But
> in the situation where it's locked in an infinite loop, there's no other
> thread doing anything, so the right thing happens. In any case I think
> that's probably easy to fix by setting another volatile to the thread to be
> interrupted. In general one would have to worry about a race condition if
> there were several places that might call interrupt-lisp, but in this case
> it would only be called with an explicit Control-c and perhaps on a request
> to kill a thread from the slime threads buffer.
>
> It's an open question as to whether interrupt-thread should take advantage
> of this as well, but that would require more care to avoid a race condition.
>
> Any help on sorting this would be much appreciated.
>
> Thanks,
> Alan
>
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