[perl #127659] Possible race in Supply simple operations when closing Tap

[Brian S. Julin]

# New Ticket Created by  "Brian S. Julin" 
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# <URL: https://rt.perl.org/Ticket/Display.html?id=127659 >



In role SimpleOpTappable, the method !cleanup uses a Perl 6 int in a lock-like 
fashion:

method !cleanup(int $cleaned-up is rw, $source-tap) {
            if $source-tap && !$cleaned-up  {
                $cleaned-up = 1;
                $source-tap.close;
            }
        }

This seems to be primarily tasked with ensuring that the &on-closed closure of 
the
composed supply is not run more than once.  A lot of the places are protected by
serial supply locks, however that is not true in the case of the &on-closed of 
the
composite Supply, nor in the .on-closed simpleop, so an absolute guarantee of 
this
cannot be made between different closers of that Tap object, or between a single
closer and an emit/done/quit.

Maybe using Tap.new({ once self!cleanup($source-tap) }) instead would
prevent the need for $cleaned-up?