scala runtime reflect sometimes throws exception

Question

object Reflects {

  def mirror() = universe.runtimeMirror(getClass.getClassLoader)

  def caseFields(x: AnyRef) = {
    val instanceMirror = mirror().reflect(x)
    instanceMirror.symbol.typeSignature.members.collect {
      case m: MethodSymbol if (m.isCaseAccessor) => m.name.toString -> instanceMirror.reflectMethod(m).apply()
    }
  }

}

I define an object Reflects, when I invole caseFields method within other class

Sometimes this method throws following exception

java.lang.UnsupportedOperationException: tail of empty list
at scala.collection.immutable.Nil$.tail(List.scala:339) ~[scala-library.jar:na]
at scala.collection.immutable.Nil$.tail(List.scala:334) ~[scala-library.jar:na]
at scala.reflect.internal.SymbolTable.popPhase(SymbolTable.scala:172) ~[scala-reflect.jar:na]

And other strange exception.

What's wrong with this method

Answer 1

In 2.10.3 (and probably in 2.10.4, because it doesn't look like we're going to have time to backport the fix from 2.11.0-M7), runtime reflection isn't thread-safe: http://docs.scala-lang.org/overviews/reflection/thread-safety.html. Your stack trace is one of the multitude of possible manifestations of the problem.

Bad news is that in 2.10.x there's no workaround for the thread-unsafety issue apart from putting all reflective operations in a synchronized block. Good news is that in 2.11.0 the problem should be gone.