Concurrency / variable locking woes

gistfile1.txt

>$ time raku julia-set.raku
MoarVM oops: MVM_str_hash_fetch_nocheck called with a stale hashtable pointer
MoarVM oops: MVM_str_hash_fetch_nocheck called with a stale hashtable pointer
   at julia-set.raku:26  (<ephemeral file>:hsv2rgb)
   at julia-set.raku:26  (<ephemeral file>:hsv2rgb)
 from julia-set.raku:17  (<ephemeral file>:)
 from julia-set.raku:16  (<ephemeral file>:)
 from julia-set.raku:17  (<ephemeral file>:)
 from SETTING::src/core.c/Rakudo/Internals/HyperRaceSharedImpl.pm6:63  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:process-batch)
 from julia-set.raku:16  (<ephemeral file>:)
 from SETTING::src/core.c/Rakudo/Internals/HyperPipeline.pm6:74  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Rakudo/Internals/HyperRaceSharedImpl.pm6:63  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:process-batch)
 from SETTING::src/core.c/Rakudo/Internals/HyperPipeline.pm6:73  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Rakudo/Internals/HyperPipeline.pm6:74  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Rakudo/Internals/HyperPipeline.pm6:70  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Rakudo/Internals/HyperPipeline.pm6:73  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Promise.pm6:263  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Rakudo/Internals/HyperPipeline.pm6:70  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:883  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Promise.pm6:263  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:251  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:883  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:239  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:run-one)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:251  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:281  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/ThreadPoolScheduler.pm6:242  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:run-one)
 from SETTING::src/core.c/Thread.pm6:54  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:THREAD-ENTRY)

 from SETTING::src/core.c/ThreadPoolScheduler.pm6:284  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:)
 from SETTING::src/core.c/Thread.pm6:54  (/home/steve/.perl6/share/perl6/runtime/CORE.c.setting.moarvm:THREAD-ENTRY)

julia-set.raku

#`[[

Trying to fix concurrency issues for a Rosettacode entry.

https://rosettacode.org/wiki/Julia_set#Raku

2020.09 and earlier worked flawlessly with no variable locking.
This has become unreliable under recent Raku. I've been attempting
to add appropriate locks to make it reliable again but have not been
succesful. The below works no better than than version with no variable
locking. Falts with:

MoarVM oops: MVM_str_hash_fetch_nocheck called with a stale hashtable pointer

Any hints or pointers?

$ raku -v
Welcome to 𝐑𝐚𝐤𝐮𝐝𝐨™ v2021.07.
Implementing the 𝐑𝐚𝐤𝐮™ programming language v6.d.
Built on MoarVM version 2021.07.

Incidentally, when it _does_ run, it is about 30% slower now than under 2020.09
~13 seconds vs ~10 seconds on my system.

]]

use Image::PNG::Portable;

my ($w, $h) = 800, 600;
my $out = Image::PNG::Portable.new: :width($w), :height($h);

my $maxIter = 300;
my $c = -0.7 + 0.27015i;

my $lock = Lock::Async.new; # Added for caching troubles

julia($out);

$out.write: 'Julia-set-raku.png';

sub julia ( $png ) {
    ^$w .race.map: -> $x {
        for ^$h -> $y {
            my $z = Complex.new(($x - $w / 2) / $w * 3, ($y - $h / 2) / $h * 2);
            my $i = $maxIter;
            $z = $z*$z + $c while abs($z) < 2 and --$i;
            $png.set: $x, $y, |hsv2rgb($i / $maxIter, 1, ?$i).reverse;
        }
    }
}

sub hsv2rgb ( $h, $s, $v ){
    state %cache;
    %cache{"$h|$s|$v"} //= do {
        $lock.protect: { # Added for caching troubles
            my $c = $v * $s;
            my $x = $c * (1 - abs( (($h*6) % 2) - 1 ) );
            my $m = $v - $c;
            [(do given $h {
                when   0..^1/6 { $c, $x, 0 }
                when 1/6..^1/3 { $x, $c, 0 }
                when 1/3..^1/2 { 0, $c, $x }
                when 1/2..^2/3 { 0, $x, $c }
                when 2/3..^5/6 { $x, 0, $c }
                when 5/6..1    { $c, 0, $x }
            } ).map: ((*+$m) * 255).Int ]
        } # Added for caching troubles
    }
}