About

clj-con defines a set of concurrency operations modeled after their Clojure counterparts. Sample operators include future, promise, deref, deliver, and atom. See the exported symbols in package.lisp for the full list.

Or, if you're familiar with the Clojure Cheatsheet, the project implements the following:

along with promise and deliver.

Usage

Mar-03-2024: Added to Ultralisp because quicklisp hasn't been updated in 5 months and there are updates I really wanted to get out.

If you didn't get this via quickload using a quicklisp/ultralisp repo, add it to your ~/quicklisp/localprojects/ directory and update/wipe the system-index.txt file accordingly, and then you can quickload it.

;; See 'local-projects' note in preceding paragraph
(ql:quickload :clj-con) ; to use the code

or

;; To run the tests
(ql:quickload :clj-con-test)
(clj-con-test:run-tests)

Supported Lisps

This package will wishfully run on any lisp supporting bordeaux-threads, which is most of them.

Lisps supporting the atomics package will use compare-and-swap behavior via atomics:cas for the atom implementation. At the time of this writing (DEC 2023), that includes:

• Allegro
• CCL
• ECL
• LispWorks
• Mezzano
• SBCL
• CMUCL
• CLASP (*)

*CLASP isn't on the atomics README but does seem to be supported in the code.

If the atomics-provided :atomics-cas-svref isn't in *features* then the implementation defaults to a locking behavior to emulate compare-and-swap in the various atom functions that require it.

Tested Lisps

Here are my experiences so far with Fedora 38 running on an Intel machine and tests run on some of the lisps. Note that I am primarily an SBCL user and am not particularly familiar with the other lisps, their heap configurations, or even how to debug them since I didn't bother to enable them for SLIME, I just ran them from the command line.

All tests were run with default memory configurations. I have no explanation for why some of them seem to be running out of memory, though some may be running with overly conservative heap sizes by default. The test suite allocates fewer than 50 threads, but the CLJ-CON package does expect threads to be (eventually) reclaimed when the lisp code running on them returns.

Given that the test suite does deliberately signal conditions in the bodies of many thread tests I suppose it's possible threads are hung and locks are not being released. There are caveats about unpredicable unwind behavior w.r.t. locks in some of the tools used.

All locking is done via bt2:with-lock-held. If you want lisps that seem to keep chugging along even with many allocations, look for the ones I've labelled "GOOD".

• SBCL GOOD

  RUN-TESTS passes. (dotimes (i 1000) (debug! 'test-suite)) passes.

• CCL GOOD

  RUN-TESTS passes. (dotimes (i 1000) (debug! 'test-suite)) passes.

  The test seemed to slow a bit toward the end of the 1000 iterations, as if perhaps a lot of gc activity were happening, but it did finish.

• ABCL 1.9.0, OpenJDK 17.0.9 GOOD (No CAS)

  RUN-TESTS passes. (dotimes (i 1000) (debug! 'test-suite)) passes.

  This now works but required special timeout logic because BT2:CONDITION-WAIT always returns T on ABCL. Unfortunatey the logic which fixes ABCL breaks tests on CCL, and perhaps others.

• LispWorks 8.0.1 Personal Edition. UNRELIABLE

  RUN-TESTS passes. (dotimes (i 1000) (debug! 'test-suite)) runs out of memory. Lots of messages on the console "Hanging Unknown thread 5612"

  On a personal note. No init file with personal edition. Really?

• Allegro CL Express 11.0 (alisp executable) UNRELIABLE

  RUN-TESTS passes. (dotimes (i 1000) (debug! 'test-suite)) gets the following error after a number of iterations:

  Running test suite TEST-SUITE Running test PROMISE-DELIVERY .... Running test NO-TIMEOUT-WAITS Allegro CL(pid 1082089): System Error (gsgc) Object already pointing to target newspace half: 0x1000c8a1a68 The internal data structures in the running Lisp image have been corrupted and execution cannot continue. Check all foreign functions and any Lisp code that was compiled with high speed and/or low safety, as these are two common sources of this failure. If you cannot find anything incorrect in your code you should contact technical support for Allegro Common Lisp, and we will try to help determine whether this is a coding error or an internal bug.

  The message suggests a gc bug, but maybe that's just a symptom of running out of memory.

• ECL 21.2.1 UNRELIABLE

  Works for the minimal (run once) clj-con-test:run-tests case, but runs out of memory if the test suite is run repeatedly.

V1.0.0, possible breaking changes

1. compare-and-set! now returns NIL and non-NIL, instead of strict NIL and T values.
2. deliver no longer returns the value delivered, it returns the input promise or nil according to clojure semantics, see the doc string for deliver.

Changelog

v1.0.0

Tested and fixed for multiple platforms.

See "Tested Lisps" above.

Add support for compare-and-swap

Added conditional use of the atomics package for a real compare-and-swap behavior in the atom implementation.

Eliminate use of recursive locks used with condition-variables (ECL fix)

Recent testing with ECL found that ECL doesn't like condition broadcasts with recursive locks. The recursive locks were changed to non-recursive locks, hopefully without loss of functionality or introduction of bugs.

Migration to Bordeaux-Threads APIV2

The motiviation was to use CONDITION-BROADCAST which is not in APIV1 and was forcing CLJ-CON code to loop on CONDITION-NOTIFY.

v0.1.0 - initial bordeaux-threads implementation

Only tested with SBCL and ABCL, known to be broken on ECL.

Differences from Clojure

Uses of multiple value return

reset-vals! and swap-vals! return vectors in Clojure but return multiple values here. I couldn't see the point of returning vectors when CL has no destructuring bind that works on vectors. At least you can use multiple-value-bind if you want, though it doesn't destructure either.

Tip: metabang-bind (available in quicklisp) provides a nice destructuring tool that also handles multiple values.

Character/number EQ is not identical to Java's == used by Clojure

YMMV if you use the atomics-enabled compare-and-swap behavior on Common Lisp characters and numbers, because it uses EQ semantics, not EQL, and EQ is not necessarily true for numbers and characters.

In SBCL, fixnums are usually EQ, and (eq #\a #\a) will likely return true, but have a care.

atom package conflict

If you're going to (use :clj-con) note that atom requires a (:shadowing-import-from #:clj-con #:atom).

Use of interrupt-thread by future-cancel

The Java Virtual Machine's threading tools are really a marvelous thing. If you've been in that ecosystem a long time, going back to pthreads with some of its limitations (or lisp oddities built on them), will feel fragile, and reading the various SBCL source comments on interrupt-thread doesn't do much to prevent that feeling.

The test suite does test future-cancel and other ways of unwinding the thread stack, and seems to work on all tested platforms. But it may still be a source of bugs, such as the memory problems noted on some lisps.

Have a care if you are repeatedly interrupting threads or using complicated mission critical handlers in the threads unless you have taken to heart the use of SBCL's WITHOUT-INTERRUPTS and other appropriate implementation dependent tools. I didn't hit any problems with my simple tests but that isn't saying much.

Non-Goals

There is no attempt here to bring clojure syntax or persistent data structures to Common Lisp. Fortunately neither of those things is particularly prevalent in Clojure's concurrency operator model, at least not in the clojure.core namespace.

Some enterprising person might want to make a readtable that maps @ to deref, assuming it doesn't conflict with ,@, but that hasn't been done here so you'll just have to call deref.

Blocking Queues?

If you're missing clojure.core.async and want some blocking queues for producer/consumer situations, take a look at the lparallel.queue package (ql:quickload :lparallel). Unlike clojure.core.async it has a peek operator which I find useful when I need to speculatively try something on a queue element without losing FIFO ordering.

The Atomics maintainers were considering adding some queue capabilities in 2023, so you may wish to check there as well. It isn't in the quicklisp distribution as of June 2023 though.

Cautionary note for Clojure devs new to Common Lisp

I recommend reading documentation on the bordeaux-threads make-thread function for cautions about interactions between threads and dynamic variables.

You also need to mentally prepare yourself for how values you've closed over in the body of your future can mutate. Consider this example:

(dotimes (i 20)
  (future ... (print i) ...))

You may be expecting the first value of i printed by the first future created would be zero because dotimes starts at zero. However depending on your lisp implementation it may actually print one, or some other value, depending on time of evaluation and whether the reference to the location/register holding 'i' has been incremented or not by the time the future body is executed on the new thread.

The clj-con-test package has a test case where this exact issue was encountered on SBCL, and the workaround was to use something like this:

(dotimes (i 20)
  (let ((i2 i))          ;of course I could have rebound 'i' as well
    (future ... (print i2) ...)))

This way the future is referencing a binding that won't change.

Binding semantics such as the above may vary by lisp implementation and has nothing to do with parallelism. E.g., you might get this:

(let ((funs nil)) 
  (dotimes (i 3) (push (lambda () i) funs))
  (dotimes (j 3) (print (funcall (elt funs j)))))
3
3
3

The behavior is related to closing over bindings for mutable data. The CL spec for dotimes says this:

"It is implementation-dependent whether dotimes establishes a new binding
of var on each iteration or whether it establishes a binding for var once
at the beginning and then assigns it on any subsequent iterations."

When in doubt, add a binding that won't change for use in your closed over future (or other) bodies.

Feedback welcome

(reverse "[email protected]")

This is a secondary address that isn't monitored every day. Feel free to submit Github issues if appropriate.