I'm developer of Robocode engine. We would like to make Robocode multilingual and Scala seems to be good match. We have Scala plugin prototype here.
The problem: Because users are creative programmers, they may try to win battle different ways. As well robots are downloaded from online database where anyone could upload one. So gap in security may lead to security hole into users computer. Robots written in Java are running in restricted sandbox. Almost everything is prohibited [network, GUI, disk (limited), threads (limited), classloaders and reflection]. The sandbox is similar to browser applet. We use SecurityManager, custom ClassLoader per robot, etc ...
There are two ways how to host Scala runtime in Robocode:
1) load it together with robot inside of sandbox. Pretty safe for us, preferred solution. But it will damage Scala runtime abilities because runtime uses reflection. Maybe generates classes at runtime ? Use threads to do some internal cleanup ? Access to JVM/internals ? (I would not like to limit abilities of language)
2) use Scala runtime as trusted code, outside the box, security on same level as JDK. Visibility to (malicious) robot. Are the Scala runtime APIs safe ? Do methods they have security guards ? Is there any safe mode ? Is there any singleton in Scala runtime, which could be abused to communicate between robots ? Any concurency/threadpool/messaging which could simulate threads ? (Is there any security audit for Scala runtime?)
3) Something in between, some classes of runtime in and some out. Which classes/packages must be visible to robot/which are just private implementation ? (this seems to be future solution)
The question: Is it possible to enumerate and isolate the parts of runtime which must run in trusted scope from the rest ? Specific packages and classes ? Or better idea ?
I'm looking for specific answer, which will lead to secure solution. Random thoughts welcome, but not awarded. There is ongoing discussion at scala email group. No specific answer yet.
I think #1 is your best bet and even that is a moving target. As brought up on the mailing list, structural types use reflection. I don't think structural types are common in the standard library, but I don't think anyone keeps track of where they are.
There's also always the possibility that there are other features using reflection behind the scenes. For example, for a while in the 2.8 branch some array functionality was using reflection. I think that's been changed after benchmarking, but there's always the possibility that there's some problem where someone said "Aha! I will use reflection to solve this."
The Scala standard library is filled with singletons. Most of them are immutable, but I know that the Scheduler object in the actors library could be abused for communication because it is essentially a proxy for an actual scheduler so you can plug your own custom scheduler into it.
At this time I don't think Scala requires using a custom class loader and all of its classes are produced at compile time instead of runtime, but then again that's probably a moving target. Scala generates a lot of class files, and there is always talk of making it generate some of them at runtime when they are needed instead of at compile time.
So, in short, I do not think it's possible (within reasonable constraints on effort) to enumerate and isolate the pieces of Scala that can (and should) be trusted.