Truffleruby: [interop] Cannot access Python attribute from Ruby

[fniephaus]

Repro

polyglot --jvm --shell
GraalVM MultiLanguage Shell 19.0.0
Copyright (c) 2013-2019, Oracle and/or its affiliates
  Python version 3.7.0
  Ruby version 2.6.2
js> python>
python> from collections import deque
python> d = deque(maxlen=42)
python> import polyglot
python> polyglot.export_value(d, 'd')
deque([], maxlen=42)
python> js>
js> Polyglot.import('d').maxlen
42
js> ruby>
ruby> Polyglot.import('d')
<foreign>
ruby> Polyglot.import('d').maxlen
Internal error occured: org.graalvm.polyglot.PolyglotException: org.truffleruby.language.control.RaiseException: Message not supported. (UnsupportedMessageException)
    from com.oracle.truffle.api.interop.UnsupportedMessageException.create(UnsupportedMessageException.java:118)
    from com.oracle.graal.python.builtins.objects.PythonAbstractObject$PExecuteNode.doExecute(PythonAbstractObject.java:783)
    from com.oracle.graal.python.builtins.objects.PythonAbstractObjectFactory$PExecuteNodeGen.executeAndSpecialize(PythonAbstractObjectFactory.java:549)
    from com.oracle.graal.python.builtins.objects.PythonAbstractObjectFactory$PExecuteNodeGen.execute(PythonAbstractObjectFactory.java:531)
    from com.oracle.graal.python.builtins.objects.PythonAbstractObject.invokeMember(PythonAbstractObject.java:427)
    from com.oracle.graal.python.builtins.objects.PythonAbstractObjectGen$InteropLibraryExports$Cached.invokeMemberNode_AndSpecialize(PythonAbstractObjectGen.java:833)
    from com.oracle.graal.python.builtins.objects.PythonAbstractObjectGen$InteropLibraryExports$Cached.invokeMember(PythonAbstractObjectGen.java:816)
    from com.oracle.truffle.api.interop.InteropLibraryGen$CachedDispatch.invokeMember(InteropLibraryGen.java:3104)
    from com.oracle.truffle.api.interop.LegacyToLibraryNode.sendInvoke(LegacyToLibraryNode.java:272)
    from com.oracle.truffle.api.interop.ForeignAccess.sendInvoke(ForeignAccess.java:395)
Translated to internal error (RuntimeError)
Run with --verbose to see the full stack trace.
ruby>

After looking at the bottom stack frames, I'm assuming this could be an incompatibility between the old and the new interop, but it might as well just be a general interop issue in TruffleRuby.

[eregon]

A small guess: this is Ruby so Polyglot.import('d').maxlen calls a method maxlen (with no arguments). Polyglot.import('d')[:maxlen] is probably what you want. I tried, and that works.

[fniephaus]

Oh right, so maybe there's only an issue in GraalPython, which should indicate there's no such method instead of failing with an exception.

[chumer]

UnsupportedMessageException should never be rethrown, but always translated into a language specific error. In this case "method maxlen not found". So the problem is in Ruby here.

[eregon]

Indeed, this internal error message is suboptimal, I'll fix it.
In this case, I guess we should throw a NoMethodError since there is no maxlen method (there is a member maxlen but it's not invocable).

[chrisseaton]

I think we are translating to a language-specific error aren't we? A Ruby RuntimeError, which is thrown as a TruffleException subclass, Ruby's RaiseException. The message just includes the original Java class name, which could be improved.

[eregon]

The most obvious fix gives:

ruby> Polyglot.import('d').maxlen
Message not supported. (NoMethodError)
        at <ruby> <top (required)>(<shell>:1:0-26)
        at <ruby> parsing-request(Unknown)

Is that good enough or do we want something that look exactly like a Ruby NoMethodError for the message too, but then doesn't indicate what the receiving language threw?

[chumer]

I'd make the Ruby no method error be the most prominent thing. If the interop error provides a message in addition, then you can put that in braces.

[chumer]

It should immediatly look familiar to Ruby users .

[eregon]

It is, this is standard Ruby error formatting.
Here is the error message in a Ruby-only context:

ruby> 3.foobar
undefined method `foobar' for 3:Integer
Did you mean?  floor (NoMethodError)
        at <ruby> <top (required)>(<shell>:1:0-7)
        at <ruby> parsing-request(Unknown)

[fniephaus]

Not sure how consistent the error format is in Ruby, but maybe the error should look more like the following?

ruby> Polyglot.import('d').maxlen
undefined method `maxlen' for <foreign>
        at <ruby> <top (required)>(<shell>:1:0-26)
        at <ruby> parsing-request(Unknown)

Nonetheless, the solution you mentioned in https://github.com/oracle/truffleruby/issues/1690#issuecomment-493010598 would also be fine.

[eregon]

I merged my proposed fix from https://github.com/oracle/truffleruby/issues/1690#issuecomment-493010598 in 33c2fe41d432d5ee687ec76f4b1a2844f4bbf7cd, it will be in the next release.

We can certainly improve interop exceptions further, but I went for a straightforward fix here as we are doing a large interop refactoring currently.

@fniephaus Thank you for the report!

[fniephaus]

Great...that's perfect, thanks! Looking forward to the refactoring :)