Fix SandboxSecurityManager when multithreaded

Author: nicolasstucki

compiler/src/dotty/tools/dotc/util/Sandbox.scala

@@ -12,11 +12,8 @@ object Sandbox {
   final val timeout = 3000 // TODO add a flag to allow custom timeouts
 
   def runInSecuredThread[T](thunk: => T): T = {
-    // Implementation assumes single threaded
-    val oldSecurityManager = System.getSecurityManager
     try {
-      val securityManager = new SandboxSecurityManager
-      System.setSecurityManager(securityManager)
+      val securityManager: SandboxSecurityManager = SandboxSecurityManager.init()
       class SandboxThread extends Thread {
         var result: scala.util.Try[T] =
           scala.util.Failure(new Exception("Sandbox failed with a fatal error"))
@@ -35,7 +32,33 @@ object Sandbox {
         throw new InvocationTargetException(new QuoteError(s"Failed to evaluate inlined quote. Caused by timeout ($timeout ms)."))
       } else thread.result.fold[T](throw _, identity)
     } finally {
-      System.setSecurityManager(oldSecurityManager)
+      SandboxSecurityManager.stop()
+    }
+  }
+
+  object SandboxSecurityManager {
+    @volatile private[this] var running: Int = 0
+
+    private[Sandbox] def init(): SandboxSecurityManager = synchronized {
+      running += 1
+      System.getSecurityManager match {
+        case securityManager: SandboxSecurityManager =>
+          assert(running > 1)
+          securityManager
+        case securityManager =>
+          assert(running == 1)
+          assert(securityManager == null)
+          val sandboxSecurityManager = new SandboxSecurityManager
+          System.setSecurityManager(sandboxSecurityManager)
+          sandboxSecurityManager
+      }
+    }
+
+    private[Sandbox] def stop(): Unit = synchronized {
+      running -= 1
+      assert(System.getSecurityManager.isInstanceOf[SandboxSecurityManager])
+      if (running == 0)
+        System.setSecurityManager(null)
     }
   }