Check that access to context base is singlethreaded.

ContextBase is not intended to be threadsafe, We now test that
indeed it is not shared by compileUnits calls operating on
different threads.

Author: odersky

src/dotty/tools/dotc/Run.scala

@@ -50,6 +50,7 @@ class Run(comp: Compiler)(implicit ctx: Context) {
     }
 
   protected def compileUnits() = Stats.monitorHeartBeat {
+    ctx.checkSingleThreaded()
     val phases = ctx.squashPhases(ctx.phasePlan,
       ctx.settings.Yskip.value, ctx.settings.YstopBefore.value, ctx.settings.YstopAfter.value, ctx.settings.Ycheck.value)
     ctx.usePhases(phases)

src/dotty/tools/dotc/core/Contexts.scala

@@ -611,6 +611,16 @@ object Contexts {
       superIdOfClass.clear()
       lastSuperId = -1
     }
+
+    // Test that access is single threaded
+
+    /** The thread on which `checkSingleThreaded was invoked last */
+    @sharable private var thread: Thread = null
+
+    /** Check that we are on the same thread as before */
+    def checkSingleThreaded() =
+      if (thread == null) thread = Thread.currentThread()
+      else assert(thread == Thread.currentThread(), "illegal multithreaded access to ContextBase")
   }
 
   object Context {