JDK9 blocker: stops supporting our scheme for MODULE$ initialization

[DarkDimius]

https://bugs.openjdk.java.net/browse/JDK-8159215

the way we currently compile module classes, assigns the static final MODULE$ from the module class constructor <init> which is called by <clinit>. This is prohibited by the spec and will be enforced by JDK9.

./scala-2.11.8/bin/scala
Exception in thread "main" java.lang.IllegalAccessError: Update to static final field scala.tools.nsc.MainGenericRunner$.MODULE$ attempted from a different method (scala.tools.nsc.MainGenericRunner$) than the initializer method <clinit>
        at scala.tools.nsc.MainGenericRunner$.<init>(MainGenericRunner.scala:102)
        at scala.tools.nsc.MainGenericRunner$.<clinit>(MainGenericRunner.scala)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

If they enforce the restriction and we move the initializers to the only allowed place, <clinit>, following code will always throw NPE.

class Super {
  Object.foo
}

object Singleton extends Super {

 def foo = 1

 // <clinit> -> new Singleton
 // <init> MODULE$ = this
}

Reported by Zoltán Majó. We have discussed the issue. The JDK product team seems have decided that this change is to stay. What it means, is that code compiled both by dotty and Scala cannot use bytecode version 53, unless we drop the final flag on all MODULE$.

PING @adriaanm, @lrytz, @retronym. I'll keep you informed on progress.

[DarkDimius]

I have an idea how to fix this without breaking current behavior.
Will need to check when I'm back.

[retronym]

@DarkDimius Is this what you had in mind? scala/scala@2.12.x...retronym:topic/module-init

[DarkDimius]

looks like it :+1

On 4 Aug 2016, at 12:13, Jason Zaugg wrote:

@DarkDimius Is this what you had in mind?
scala/scala@2.12.x...retronym:topic/module-init

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#1441 (comment)

[DarkDimius]

@retronym, there's one piece missing in your change. You need to move the body of constructor to the static initializer. Here's an example that demostrates the need:

scala> class A { def foo = O.bar}; object O extends A {foo; def bar = println("bar")}
defined class A
defined object O

scala> O
bar
res1: O.type = O$@1593948d

By the time user-defined body of constructor of objects starts execution, the MODULE$ should be published.

[smarter]

From the bug report you linked, it looks like they want to enforce this check for older classfiles too:

In the case of final fields, an old class file that assigns to one outside the appropriate initialization method is very much in the wrong, so gating the putfield/putstatic logic by class file version is unnecessary.

Should someone comment on that bug report to let them know that doing this would break every existing Scala classfile?

[DarkDimius]

@smarter they are not enforcing this for old classfiles. I'm in contact with them.

[retronym]

I believe that it's preferable to expose the null that to expose a partially initialised object. That's the status quo, no?

[DarkDimius]

@retronym current behavior publishes a partially initialized object that other classes can use(see the example above). If we prohibit this, it would mean that in the body of the object you cannot use anything that transitively uses the same object. I believe this is a very harsh restriction that is hard to maintain in big codebases.

[DarkDimius]

another example, that does not involve superclasses:

scala> class CC { def foo = O.bar}; object O {new CC().foo; def bar = println(1)};
defined class CC
defined object O

scala> O
1
res7: O.type = O$@36ebc363

[liufengyun]

@smarter Is this issue still relevant, given Won't Fix for https://bugs.openjdk.java.net/browse/JDK-8159215 ?

[smarter]

Is this issue still relevant ?

Yes, the won't fix is about adding the check when the JVM load classfiles where the Java bytecode version corresponds to Java <= 8, but the issue remains for newer Java bytecode versions, we need to implement the same scheme that 2.13 has implemented: scala/scala#7270