Fix thread safety issue in runtime reflection

Disable the cache of the `SubstMap` used in `Type.subst` when
in the runtime reflection universe.

Author: retronym

src/reflect/scala/reflect/internal/Types.scala

@@ -111,12 +111,11 @@ trait Types
   private object substTypeMapCache {
     private[this] var cached: SubstTypeMap = new SubstTypeMap(Nil, Nil)
 
-    def apply(from: List[Symbol], to: List[Type]): SubstTypeMap = {
+    def apply(from: List[Symbol], to: List[Type]): SubstTypeMap = if (isCompilerUniverse) {
       if ((cached.from ne from) || (cached.to ne to))
         cached = new SubstTypeMap(from, to)
-
       cached
-    }
+    } else new SubstTypeMap(from, to)
   }
 
   /** The current skolemization level, needed for the algorithms

test/junit/scala/reflect/runtime/ThreadSafetyTest.scala

@@ -0,0 +1,103 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.reflect.runtime
+
+import java.util.concurrent.{Callable, Executors}
+
+import org.junit.Test
+
+class ThreadSafetyTest {
+  import scala.reflect.runtime.universe._
+  sealed abstract class Instance(tp: Type)
+  final case class ListInstance(tp: Type, elemInstance: Instance) extends Instance(appliedType(symbolOf[List[_]], tp :: Nil))
+
+  final case class DoubleInstance() extends Instance(typeOf[Double])
+
+  final case class StringInstance() extends Instance(typeOf[String])
+
+  final case class LowPriorityInstance(tpe: Type) extends Instance(tpe)
+
+  def classKeyOf[T: TypeTag]: String = {
+    classKeyOf(typeOf[T])
+  }
+  def classKeyOf(tpe: Type): String = {
+    tpe.typeSymbol.fullName
+  }
+
+  class Lazy[T](thunk: () => T) {
+    lazy val force: T = thunk()
+  }
+
+  class Registry {
+    private val cache = new java.util.concurrent.ConcurrentHashMap[Type, Lazy[Instance]]()
+    def instance[T: TypeTag]: Lazy[Instance] = {
+      instance(typeOf[T])
+    }
+    def instance(tpe: Type): Lazy[Instance] = {
+      val value = keyOf(tpe)
+      assert(value =:= tpe, (value, tpe))
+      cache.computeIfAbsent(value, create(_))
+    }
+
+    private def create(tpe: Type): Lazy[Instance] = {
+      val key = classKeyOf(tpe)
+      if (key == "scala.collection.immutable.List") {
+        new Lazy(() => {val elemTpe = tpe.dealias.typeArgs.head; ListInstance(tpe.dealias, instance(elemTpe).force)})
+      } else if (key == "scala.Double") {
+        new Lazy(() => DoubleInstance())
+      } else if (key == "java.lang.String") {
+        new Lazy(() => StringInstance())
+      } else {
+        new Lazy(() => LowPriorityInstance(tpe))
+      }
+    }
+    private def keyOf(tp: Type): universe.Type = {
+      tp.map(_.dealias)
+    }
+  }
+
+  @Test
+  def test(): Unit = {
+    val executor = Executors.newFixedThreadPool(16)
+    for (i <- (0 to 128)) {
+      val registry = new Registry
+      val is = List(
+        (() => typeOf[List[Double]], "ListInstance(List[Double],DoubleInstance())"),
+        (() => typeOf[List[String]], "ListInstance(List[String],StringInstance())"),
+        (() => typeOf[List[List[Double]]], "ListInstance(List[List[Double]],ListInstance(List[Double],DoubleInstance()))"),
+        (() => typeOf[List[List[List[Double]]]], "ListInstance(List[List[List[Double]]],ListInstance(List[List[Double]],ListInstance(List[Double],DoubleInstance())))"),
+        (() => typeOf[List[List[List[Object]]]], "ListInstance(List[List[List[Object]]],ListInstance(List[List[Object]],ListInstance(List[Object],LowPriorityInstance(Object))))")
+      )
+      sealed abstract class Result
+      case object Okay extends Result
+      case class Failed(i: Int, tp: Type, expected: String, instance: String) extends Result
+      def check(i: Int): Result = {
+        val (f, expected) = is(i % is.size)
+        val tp = f()
+        val instance = registry.instance(tp).force
+        if (instance.toString == expected) Okay else Failed(i, tp, expected, "[" + instance + "]")
+      }
+      val par = true
+      val fs: List[Result] = if (par) Array.tabulate(32)(i =>
+        executor.submit(new Callable[Result] {
+          override def call(): Result = {
+            check(i % is.size)
+          }
+        })).map(_.get).toList
+      else is.indices.map(check).toList
+      val fails = fs.filter(_ != Okay)
+      assert(fails.isEmpty, "iteration " + i + ": " + fails.mkString("\n"))
+    }
+    executor.shutdownNow()
+  }
+}