[perl #132885] .next-handle seems to change handles but doesn't

[brian d foy]

# New Ticket Created by  "brian d foy" 
# Please include the string:  [perl #132885]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=132885 >


#!/Users/brian/bin/perl6s/perl6-latest

I'm playing with .next-handle from IO::CatHandle. I'm
trying to create a situation where I can read just the first five
lines from each command line argument:

    quietly {
        my $limit = 5;
        for lines() {
            state $lines = 1;
            FIRST { $*ARGFILES.on-switch = { put "NEW FILE"; $lines = 1 } }
            if $lines > $limit {
                $*ARGFILES.next-handle;
                next;
                }
            put "{$*ARGFILES.path}:{$lines++} $_";
            }
        }

Here's a test file:

    First
    Second
    Third
    Fourth
    Fifth
    Six
    Seventh

With one or more command-line arguments I get this odd behavior (and
lots of warnings that I suppressed):

    test.txt:1 First
    test.txt:2 Second
    test.txt:3 Third
    test.txt:4 Fourth
    test.txt:5 Fifth
    NEW FILE
    :1 Seventh
    read bytes requires an object with REPR MVMOSHandle (got VMNull
with REPR Null)
      in block <unit> at lines-method.p6 line 5

It does "switch" after five lines, but then it keeps reading from the
same handle while losing a line. Then there's a strange error at the
end that kills the whole thing.

I expected that it would close the current handle, open a new one, and
continue. If it gets to the end, it would simply not provide any more
lines() and things would end normally.

The docs for .next-handle lets you keep changing it as long as you
like no matter how many

-----

This is Rakudo Star version 2018.01 built on MoarVM version 2018.01
implementing Perl 6.c.