At the end of a run, close macro runtime's classloader

We can only do this on 2.12.x, because URLClassLoader#close
is new in JDK 7.

Tested manually with the REPL and resident compilers.

```
% qscalac sandbox/macro.scala && (for i in 1 2; do echo sandbox/client.scala; done; printf '\n') | qscalac -Xresident -Ylog:all -Ydebug 2>&1 | grep "Closing macro runtime classloader"
[log terminal] Closing macro runtime classloader
[log terminal] Closing macro runtime classloader

% qscalac sandbox/macro.scala && (for i in 1 2; do echo Macro.m; done; printf '\n') | qscala -Ylog:all -Ydebug 2>&1 | grep "Closing macro runtime classloader"; stty echo
[log terminal] Closing macro runtime classloader
[log terminal] Closing macro runtime classloader
```

Note: this doesn't close handles to JAR files held by the
compiler classpath implementation, that will require changes
elsewhere.

1 files changed, 13 insertions, 1 deletions

diff --git a/src/compiler/scala/reflect/macros/runtime/MacroRuntimes.scala b/src/compiler/scala/reflect/macros/runtime/MacroRuntimes.scala
index 5fd9c0db34..d4b8175d28 100644
--- a/src/compiler/scala/reflect/macros/runtime/MacroRuntimes.scala
+++ b/src/compiler/scala/reflect/macros/runtime/MacroRuntimes.scala
@@ -1,8 +1,12 @@
 package scala.reflect.macros
 package runtime
 
+import java.net.URLClassLoader
+
 import scala.reflect.internal.Flags._
+import scala.reflect.internal.util.ScalaClassLoader
 import scala.reflect.runtime.ReflectionUtils
+import scala.reflect.internal.util.AbstractFileClassLoader
 
 trait MacroRuntimes extends JavaReflectionRuntimes {
   self: scala.tools.nsc.typechecker.Analyzer =>
@@ -44,7 +48,15 @@ trait MacroRuntimes extends JavaReflectionRuntimes {
    *  which compiles implementations into a virtual directory (very much like REPL does) and then conjures
    *  a classloader mapped to that virtual directory.
    */
-  lazy val defaultMacroClassloader: ClassLoader = findMacroClassLoader()
+  private lazy val defaultMacroClassloaderCache = {
+    def attemptClose(loader: ClassLoader): Unit = loader match {
+      case u: URLClassLoader => debuglog("Closing macro runtime classloader"); u.close()
+      case afcl: AbstractFileClassLoader => attemptClose(afcl.getParent)
+      case _ => ???
+    }
+    perRunCaches.newGeneric(findMacroClassLoader, attemptClose _)
+  }
+  def defaultMacroClassloader: ClassLoader = defaultMacroClassloaderCache()
 
   /** Abstracts away resolution of macro runtimes.
    */