NPE in PythonLanguage#isThreadAccessAllowed

[fniephaus]

When requesting Python code evaluation in a Ruby application, which uses multithreading, a NPE is raised in PythonLanguage. The reason for this seems to be that isWithThread is only set in PythonLanguage#createContext and it looks like this is not being called before the code evaluation.

Line of interest

https://github.com/graalvm/graalpython/blob/52ef933f7f76661b40841f00df056a9980d12871/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/PythonLanguage.java#L547

Example Stack trace

Internal error occurred: org.graalvm.polyglot.PolyglotException: org.truffleruby.language.control.RaiseException: <no message> (NullPointerException)
        from com.oracle.graal.python.PythonLanguage.isThreadAccessAllowed(PythonLanguage.java:547)
        from com.oracle.truffle.api.LanguageAccessor$LanguageImpl.isThreadAccessAllowed(LanguageAccessor.java:233)
        from com.oracle.truffle.polyglot.PolyglotLanguageContext.checkThreadAccess(PolyglotLanguageContext.java:269)
        from com.oracle.truffle.polyglot.PolyglotLanguageContext.ensureCreated(PolyglotLanguageContext.java:452)
        from com.oracle.truffle.polyglot.PolyglotLanguageContext.ensureInitialized(PolyglotLanguageContext.java:531)
        from com.oracle.truffle.polyglot.PolyglotContextImpl.getContextInitialized(PolyglotContextImpl.java:362)
        from com.oracle.truffle.polyglot.EngineAccessor$EngineImpl.parseForLanguage(EngineAccessor.java:194)
        from com.oracle.truffle.api.TruffleLanguage$Env.parsePublic(TruffleLanguage.java:1931)
        from org.truffleruby.interop.PolyglotNodes$EvalFileNode.eval(PolyglotNodes.java:128)
        from org.truffleruby.interop.PolyglotNodes$EvalFileNode.evalFile(PolyglotNodes.java:122)
        from org.truffleruby.interop.PolyglotNodesFactory$EvalFileNodeFactory$EvalFileNodeGen.executeAndSpecialize(PolyglotNodesFactory.java:354)
        from org.truffleruby.interop.PolyglotNodesFactory$EvalFileNodeFactory$EvalFileNodeGen.execute(PolyglotNodesFactory.java:336)
        from org.truffleruby.language.control.SequenceNode.execute(SequenceNode.java:35)
        from org.truffleruby.language.methods.ExceptionTranslatingNode.execute(ExceptionTranslatingNode.java:32)
        from org.truffleruby.language.LazyRubyNode.execute(LazyRubyNode.java:43)
        from org.truffleruby.language.RubyRootNode.execute(RubyRootNode.java:
...

/cc @janehmueller

[msimacek]

Python context doesn't support access from multiple threads. Until we have proper support for multithreading, we cannot allow multithreaded access. We will fix this particular NPE, but your code will most likely still fail, just with a nicer error message.