From Vladimir.Marek@oracle.com

I took the NativeCall sample from here​:

https://perl6advent.wordpress.com/2015/12/21/day-21-nativecall-backs-and-beyond-c/

use NativeCall;

sub XML_SetElementHandler(OpaquePointer $parser,
  &start (OpaquePointer, Str, CArray[Str]),
  &end (OpaquePointer, Str))
  is native('expat') { ... }

sub XML_ParserCreate(Str --> OpaquePointer) is native('expat') { ... }
sub XML_ParserFree(OpaquePointer) is native('expat') { ... }
sub XML_Parse(OpaquePointer, Buf, int32, int32 --> int32) is native('expat') { ... }

my $xml = q​:to/XML/;
  <calendar>
  <advent day="21">
  <topic title="NativeCall Bits and Pieces"/>
  </advent>
  </calendar>
  XML

my $depth = 0;

sub start-element($, $elem, $attr)
{
  say "open $elem".indent($depth * 4);
  ++$depth;
}

sub end-element($, $elem)
{
  --$depth;
  say "close $elem".indent($depth * 4);
}

my $parser = XML_ParserCreate('UTF-8');
XML_SetElementHandler($parser, &start-element, &end-element);

my $buf = $xml.encode('UTF-8');
XML_Parse($parser, $buf, $buf.elems, 1);

XML_ParserFree($parser);

All works fine, unless I try to use the $attr in start-element

sub start-element($, $elem, $attr)
{
  say "open $elem".indent($depth * 4);
  say $attr.elems;
  ++$depth;
}

The program then terminates with​:

MoarVM panic​: Internal error​: Unwound entire stack and missed handler

I have seen that on Solaris, but Linux has the same issue. In both cases
custom build perl6 using 'rakudobrew build moar' so I suppose latest
git revision.

Thank you
--
  Vlad

From @dwarring

Can confirm this is still current behaviour with Rakudo 2019. I have came across exactly the same issue and found this ticket. I've noticed, If I change the example to catch exceptions. It then runs​:

  sub start-element($, $elem, $attr)
  {
  say "open $elem".indent($depth * 4);
  say $attr.elems;
  ++$depth;
  CATCH { default { warn "whoops​: $_" } }
  }

Produces​:

  open calendar
  whoops​: Don't know how many elements a C array returned from a library
  in block at /tmp/tst.p6 line 27
  open advent
  whoops​: Don't know how many elements a C array returned from a library
  in block at /tmp/tst.p6 line 27
...etc

Underlying issue seems to be that any uncaught exception in a NativeCall Perl callback currently results in a unfriendly NativeCall MoarVM panic. The exception itself is lost.

On Tue, 27 Jun 2017 05​:59​:08 -0700, Vladimir.Marek@​oracle.com wrote​:

I took the NativeCall sample from here​:

https://perl6advent.wordpress.com/2015/12/21/day-21-nativecall-backs-
and-beyond-c/

--------------------------------------------------------------------------------
use NativeCall;

sub XML_SetElementHandler(OpaquePointer $parser,
&start (OpaquePointer, Str, CArray[Str]),
&end (OpaquePointer, Str))
is native('expat') { ... }

sub XML_ParserCreate(Str --> OpaquePointer) is
native('expat') { ... }
sub XML_ParserFree(OpaquePointer) is
native('expat') { ... }
sub XML_Parse(OpaquePointer, Buf, int32, int32 --> int32) is
native('expat') { ... }

my $xml = q​:to/XML/;
<calendar>
<advent day="21">
<topic title="NativeCall Bits and Pieces"/>
</advent>
</calendar>
XML

my $depth = 0;

sub start-element($, $elem, $attr)
{
say "open $elem".indent($depth * 4);
++$depth;
}

sub end-element($, $elem)
{
--$depth;
say "close $elem".indent($depth * 4);
}

my $parser = XML_ParserCreate('UTF-8');
XML_SetElementHandler($parser, &start-element, &end-element);

my $buf = $xml.encode('UTF-8');
XML_Parse($parser, $buf, $buf.elems, 1);

XML_ParserFree($parser);
--------------------------------------------------------------------------------

All works fine, unless I try to use the $attr in start-element

sub start-element($, $elem, $attr)
{
say "open $elem".indent($depth * 4);
say $attr.elems;
++$depth;
}

The program then terminates with​:

MoarVM panic​: Internal error​: Unwound entire stack and missed handler

I have seen that on Solaris, but Linux has the same issue. In both
cases
custom build perl6 using 'rakudobrew build moar' so I suppose latest
git revision.

Thank you

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