From @zoffixznet

Seems something with decoding actually, since using char of different size or adding another char after the heart avoids the issue​:

<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥"; $f1.open.readchars(2).say
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «fo♥␤»
<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥z"; $f1.open.readchars(2).say
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «fo␤»
<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "foz"; $f1.open.readchars(2).say
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «fo␤»

Bisection results​:

<Zoffix___> bisect​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥"; $f1.open.readchars(2).say
<bisectable6> Zoffix___, Bisecting by output (old=2015.12 new=0c5fe56) because on both starting points the exit code is 0
<bisectable6> Zoffix___, bisect log​: https://gist.github.com/6417457868d7d3389761c1b1d1499ada
<bisectable6> Zoffix___, (2017-01-02) rakudo/rakudo@7c8b705
<Zoffix___> Geth, ver rakudo/rakudo@7c8b705
<Geth> Zoffix___, version bump brought in these changes​: Raku/nqp@2016.12-48-g5daf0e3...2016.12-55-gc782eb7c
<Zoffix___> Geth, ver Raku/nqp@c782eb7
<Geth> Zoffix___, version bump brought in these changes​: MoarVM/MoarVM@2016.12-35-g293bda71...2016.12-50-g9a9f2d46

<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥"; with $f1.open { .readchars(1).say xx 3 }
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «f␤o♥␤␤»
<Zoffix___> c​: 7c8b705b33755 my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥"; $f1.open.readchars(2).say
<committable6> Zoffix___, ¦7c8b705​: «fo♥»
<Zoffix___> c​: 7c8b705b33755~ my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥"; $f1.open.readchars(2).say
<committable6> Zoffix___, ¦7c8b705b33755~​: «fo»

From @samcv

I bisected MoarVM and the offending commit is here​:

MoarVM/MoarVM@c98634c

From what I see, this commit did not actually cause the root bug, it just exposed it.

The Unicode Database was rebuilt so that NFG_QC=False for Emoji characters, which the ♥ is.

Also interestingly​:

  <samcv> m​:my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥22"; $f1.open.readchars(2).say
  <evalable6> samcv, rakudo-moar 0c5fe56cc​: OUTPUT​: «fo♥22»

The RT System itself - Status changed from 'new' to 'open'

From @jnthn

On Sat, 27 May 2017 07​:06​:59 -0700, cpan@​zoffix.com wrote​:

Seems something with decoding actually, since using char of different
size or adding another char after the heart avoids the issue​:

<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥";
$f1.open.readchars(2).say
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «fo♥␤»
<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "fo♥z";
$f1.open.readchars(2).say
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «fo␤»
<Zoffix___> m​: my $f1 = "/tmp/foo121212".IO; $f1.spurt​: "foz";
$f1.open.readchars(2).say
<camelia> rakudo-moar 0c5fe5​: OUTPUT​: «fo␤»

Fixed and tested in S16-io/readchars.t.

@jnthn - Status changed from 'open' to 'resolved'