[perl #131740] [BUG] Concurrent threads report same Thread ID post-await

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# New Ticket Created by  Evan Miller 
# Please include the string:  [perl #131740]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=131740 >


I'm using the new scheduler behavior described here:

https://github.com/rakudo/rakudo/pull/1004

After an "await" on a managed (non-main) thread it appears that execution
may continue on a different thread than it began on; however, $*THREAD
continues to report the same ID, so that multiple concurrent threads are
reporting the same thread ID.

Here's a script that demonstrates the issue 50% of the time. This starts
three promises, which start work after receiving a message from the
channel. With RAKUDO_MAX_THREADS=2, Promise #1 and #2 will receive the
first 2 messages and start work. Promise #2 has less work and finishes
early. Promise #3 then receives a message from the channel and is scheduled
on the thread freed by Promise #2. Both Promise #1 and #3 are then working
concurrently, as demonstrated by CPU usage; however, about half the time
they report an identical $*THREAD.id.

Expected behavior: Each thread should report its current, post-await
$*THREAD.id

Actual behavior: Each thread continues to report its pre-await $*THREAD.id

Possibly related, per IRC conversation:
https://github.com/rakudo/rakudo/commit/bedf2a7073042c79b886aec9e39ad8be4b0af498

./perl6 --version
This is Rakudo version 2017.06-197-g83e157012 built on MoarVM version
2017.06-56-g161ec639
implementing Perl 6.c.




use v6.d.PREVIEW;

my $channel = Channel.new;

my $promise1 = start {
    say "1. Waiting ({$*THREAD.id})";
    await $channel;
    say "1. Got it ({$*THREAD.id})";
    say "1. Result ({$*THREAD.id}): " ~ [+] (1..^1e6).map: 1/*;
    say "1. Result ({$*THREAD.id}): " ~ [+] (1..^1e6).map: 1/*;
}

my $promise2 = start {
    say "2. Waiting ({$*THREAD.id})";
    await $channel;
    say "2. Got it ({$*THREAD.id})";
    say "2. Result ({$*THREAD.id}): " ~ [+] (1..^1e5).map: 1/*;
    say "2. Result ({$*THREAD.id}): " ~ [+] (1..^1e5).map: 1/*;
}

my $promise3 = start {
    say "3. Waiting ({$*THREAD.id})";
    await $channel;
    say "3. Got it ({$*THREAD.id})";
    say "3. Result ({$*THREAD.id}): " ~ [+] (1..^1e6).map: 1/*;
    say "3. Result ({$*THREAD.id}): " ~ [+] (1..^1e6).map: 1/*;
}

sleep 3;
$channel.send("Hello");
$channel.send("Hello");
$channel.send("Hello");

await $promise1, $promise2, $promise3;