[Openmcl-devel] debugging debugging

Arthur W Cater arthur.cater at ucd.ie
Fri May 8 07:21:19 PDT 2009

Thank you Gary.

I've now put into my ccl-init.lisp the line
   (defun cl-user::yield () (process-interrupt ccl::*initial-process* #'break))
which will give me an easily remembered workaround.

As I now understand it, perhaps wrongly, the Listener does run a REPL but not in the appropriate
thread. The break loop in *initial-process* responds to :Y by executing code.
Idea: could the Listener thread not respond to :Y by causing that code to be executed in the
initial process? Perhaps only when it is not itself in a break loop? (If that's daft, feel free to say so!)


----- Original Message -----
From: Gary Byers <gb at clozure.com>
Date: Friday, May 8, 2009 12:26 pm
Subject: Re: [Openmcl-devel] debugging debugging
To: Arthur W Cater <arthur.cater at ucd.ie>
Cc: openmcl-devel Devel <openmcl-devel at clozure.com>

> On Fri, 8 May 2009, Arthur W Cater wrote:
> > As I mentioned in March, I have a similar problem with errors 
> occurring on other threads,
> > in my case often threads other than the Event�thread.
> > I'm told to type (:Y 8) � eg � but neither the Listener nor 
> the AltConsole does anything
> > when I do so. A simple example of this is with
> >
> > (process-run-function "Breaker" #'(lambda nil (break "Talk to me")))
> >
> > The Event thread still runs, but there is no way afaik of 
> getting any
> > debugging info, not even a simple backtrace.
> >
> > Arthur
> >
> Yuck.  Let me see if I can explain what's going on here 
> (though I agree that
> what's going on and what should/could be going on are different 
> things.)
> When the lisp starts up, it initializes (global) values of the 
> standard input
> and output stream variables.  When run in a TTY environment 
> or under Emacs,
> these streams are (usefully) connected to the process-wide 
> standard input
> and output devices (pipes or sockets or terminal-like 
> devices).  When a GUI
> application is launched on OSX, the devices to which these 
> streams are connected
> aren't appropriate for interactive I/O; when the CCL IDE starts 
> up, it tries
> to reconnect these streams to devices that talk to the 
> AltConsole application
> (which at least provides a primitive way of doing interactive I/O.)
> When the (non-GUI) CCL starts up, a listener thread is created 
> that uses
> these standard streams (and can therefore talk to a Terminal 
> window, Emacs, etc.)
> The listener thread is marked as being the "owner" of the global 
> standard input
> stream (since it's expected to make heavy use of that stream, to 
> run the REPL,
> etc.)  If some background thread needs to do input from 
> that stream, it announces
> that fact and advises us to type (:y <n>); in the listener, 
> we have to ensure
> that we're talking to the REPL (either at top level or in a 
> break loop) and type
> that command in order to (temporarily) transfer the exclusive 
> right to use that
> global input stream to the background thread.
> If the listener thread isn't in the REPL, we generally have to 
> wait until it
> is or interrupt it to get it to that point before typing (:Y 
> <n>) will have
> the desired effect.
> When the IDE starts up, the initial thread is marked as the 
> owner of the global
> input stream (connected to AltConsole).  When listener 
> windows are created in
> the IDE, the listener threads that are created at the same time 
> arrange to use
> interactive streams connected to that listener window (and its 
> buffer.)  If
> a background thread is created, then by default it will try to 
> use the global
> (AltConsole-based) interactive streams.  (See below.)
> If that background thread needs to use the global input stream (owned
> by the initial/event thread), it'll print a message to that 
> effect on
> its standard error or output stream (I forget which, but they'd both
> be associated with AltConsole) and wait for the input stream's owner
> to transfer ownership (via a :y command in a REPL).  Of course,
> the input stream's owner is busy processing events isn't really 
> runninga REPL, so (as you noted) nothing really happens.
> If we were to keep this paradigm (background threads try to do 
> emergencyI/O using a shared input stream), then we'd presumably 
> want to have some
> other means of doing the equivalent of :Y (a menu command or 
> something)that wouldn're require the event thread to be at a 
> REPL prompt.  If you
> want to see what's going on here or need to handle this 
> situation in an
> emergency in the short term, you can do:
> - in an IDE listener, do:
> ? (process-run-function "background" #'break)
> This should cause output to the AltConsole window, announcing 
> that the background
> thread wants to enter a break loop and needs someone to type a 
> :Y command to let
> it use the global input stream.  (Of course, it's not the 
> act of typing that
> command that transfers ownership, it's the act of a REPL running 
> in the owning
> thread reading and processing that command that does the 
> transfer.)  The owning
> thread, of course, is busy processing events, so let's change that.
> - in the IDE listener, do:
> ? (process-interrupt ccl::*initial-process* #'break)
> There are several ways to refer to the initial process; sadly, I 
> don't know
> of a way that involves exported names.
> Event processing should stop, and the initial process should 
> have entered a
> break loop in the AltConsole window.  In that break loop 
> typing the appropriate
> :Y command will do the "ownership transfer" and let the 
> background thread use
> the input stream for its break loop; exiting the break loop will 
> restore control
> to the event thread (which will be in the break loop we forced 
> it into from the
> IDE), and exiting from that break loop will allow the event 
> thread to get back
> to processing events.
> That's all pretty horrible; it could be made slightly less 
> horrible if
> there were some GUI object (a menu item or button or something
> somewhere) that avoided the need to enter a break loop to do the
> ownership transfer of the input stream, but ... well, there's no
> reason for a background thread to have to use the global
> AltConsole-based stream for incidental or other I/O: it could 
> create a
> listener window ("on demand") if it needed to do I/O, or try to share
> an existing listener window, or (maybe) create its own AltConsole
> window, or any number or other things.  (It's very hard to 
> get the
> event thread to do event-driven I/O to and from a GUI window, 
> but shouldn't
> be any harder for a random thread to do that than it is for a 
> listener.)
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