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https://github.com/dtenny/clj-con

Clojure-style concurrency operations in Common Lisp
https://github.com/dtenny/clj-con

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Clojure-style concurrency operations in Common Lisp

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README 

        
# 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:



![Cheatsheet Screenshot](https://github.com/dtenny/clj-con/blob/main/Screenshot_20210621_205249.png?raw=true)



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](https://github.com/hraban/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](https://sionescu.github.io/bordeaux-threads/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](http://clhs.lisp.se/Body/m_dotime.htm) 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.