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2016/09/01

Perl on Ubuntu on Windows: Finding The Right Test Case



I'm still hung on getting CPAN working for Ubuntu on Windows. In the comments, Chas. Owens gave me great advice for proceeding with this problem:
Write a minimal test case, run the test case with strace on Ubuntu on Windows and Ubuntu on Linux. The outputs are supposed be identical, if they aren't (and we expect them not to be because of the error), then you have a good shot at identifying the misbehaving syscall (which is the only difference between Ubuntu on Windows and Ubuntu on Linux).

Once you see the difference, look into what the syscall does and try to determine which system is implementing it correctly (probably Linux). If it is Windows post to the github tracker with the test case and the identified syscall. If it is Linux, then report it to Ubuntu.
My first thought was to do this within cpanm, but I thought sudo su then strace -o platform -ff strace cpanm YAML::XS was a bit much. In part because when platform was Linux, it generated one file and hundreds on Windows.

Then it struck me that instead, I should just focus on the tests themselves. I cloned Ingy döt Net's YAML repo and tried to run prove test/ in both places. Went fine with Linux, failed with Windows. Butrealized after a second, it succeeded while using my personal perl, not system perl. /usr/bin/prove test/ failed on Ubuntu. apt-get install libtest-base-perl on both systems helped a lot, but now it wants Test::More, (I know because I searched for what modules the tests are asking for.)

For all I know, there's a package that provides Test::More, but it isn't libtest-more-perl, and digging further into that seems like a waste.

So I'm thinking it through again, looking at a failing test in YAML::XS:

use t::TestYAMLTests tests => 2;
use utf8;

is Dump("1234567890\n1234567890\n1234567890\n"), "--- |
  1234567890
  1234567890
  1234567890
", 'Literal Scalar';

is Dump("A\nB\nC\n"), q{--- "A\nB\nC\n"} . "\n", 'Double Quoted Scalar';


By "failing test" I am saying it works in natural Linux but not Ubuntu on Windows. And it's striking me: I need to find Dump. Where is Dump? In the C. It is an XS module, is it not? So, it's striking me that the solution is in C.

Which means I have to write C.

More later.

I think there's only been one time when I coded C on the clock, and only one time when my C knowledge was required on the clock.

The former was at a former workplace, where I wrote and compiled some C to compare UltraEdit with another editor, so I could help decide which we were going to buy a site license for. As I can only remember UltraEdit, I can say that's the one I liked better. The code itself was scarcely better than Hello World.

The latter was at another former workplace, where there was a tool that allowed mechanical engineers to drag together components like traces, and then first turned those traces into C code and compiled them. There was an issue where it wouldn't work, and I found the error logs and worked back.

I'm looking at perl_libyaml.c. I'm looking at perl_libyaml.h. I don't have nearly enough C skills to start here.
/*
 * This is the main Dump function.
 * Take zero or more Perl objects and return a YAML stream (as a string)
 */
void
Dump(SV *dummy, ...)
{
    dXSARGS;
    perl_yaml_dumper_t dumper;
    yaml_event_t event_stream_start;
    yaml_event_t event_stream_end;
    int i;
    SV *yaml = sv_2mortal(newSVpvn("", 0));
    sp = mark;

    set_dumper_options(&dumper);

    /* Set up the emitter object and begin emitting */
    yaml_emitter_initialize(&dumper.emitter);
    yaml_emitter_set_unicode(&dumper.emitter, 1);
    yaml_emitter_set_width(&dumper.emitter, 2);
    yaml_emitter_set_output(
        &dumper.emitter,
        &append_output,
        (void *) yaml
    );
    yaml_stream_start_event_initialize(
        &event_stream_start,
        YAML_UTF8_ENCODING
    );
    yaml_emitter_emit(&dumper.emitter, &event_stream_start);

    dumper.anchors = newHV();
    dumper.shadows = newHV();

    sv_2mortal((SV *)dumper.anchors);
    sv_2mortal((SV *)dumper.shadows);

    for (i = 0; i < items; i++) {
        dumper.anchor = 0;

        dump_prewalk(&dumper, ST(i));
        dump_document(&dumper, ST(i));

        hv_clear(dumper.anchors);
        hv_clear(dumper.shadows);
    }

    /* End emitting and destroy the emitter object */
    yaml_stream_end_event_initialize(&event_stream_end);
    yaml_emitter_emit(&dumper.emitter, &event_stream_end);
    yaml_emitter_delete(&dumper.emitter);

    /* Put the YAML stream scalar on the XS output stack */
    if (yaml) {
        SvUTF8_off(yaml);
        XPUSHs(yaml);
    }
    PUTBACK;
}

Hrm. Maybe back to the Perl.

Looking at YAML.pm, YAML::Dumper.pm and YAML::Dumper::Base.pm make it seem like you could make a master class on how modules go together. Previously, when I've dived in to figure out issues, yeah, there's lots of confusing parts but I could find my way to the broken thing. perl_libyaml.c is looking much more approachable now.