Re: Interrupting repl in Slime

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@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