Close test and run classloaders

It's good practice to call close on a URLClassLoader when we're done
with it.

main/src/main/scala/sbt/Defaults.scala

@@ -8,7 +8,7 @@
 package sbt
 
 import java.io.{ File, PrintWriter }
-import java.net.{ URI, URL }
+import java.net.{ URI, URL, URLClassLoader }
 import java.util.Optional
 import java.util.concurrent.{ Callable, TimeUnit }
 
@@ -787,9 +787,14 @@ object Defaults extends BuildCommon {
     // ((streams in test, loadedTestFrameworks, testLoader, testGrouping in test, testExecution in test, fullClasspath in test, javaHome in test, testForkedParallel, javaOptions in test) flatMap allTestGroupsTask).value,
     testResultLogger in (Test, test) :== TestResultLogger.SilentWhenNoTests, // https://github.com/sbt/sbt/issues/1185
     test := {
+      val loader = testLoader.value match { case u: URLClassLoader => Some(u); case _ => None }
       val trl = (testResultLogger in (Test, test)).value
       val taskName = Project.showContextKey(state.value).show(resolvedScoped.value)
-      trl.run(streams.value.log, executeTests.value, taskName)
+      try {
+        trl.run(streams.value.log, executeTests.value, taskName)
+      } finally {
+        loader.foreach(_.close())
+      }
     },
     testOnly := inputTests(testOnly).evaluated,
     testQuick := inputTests(testQuick).evaluated

run/src/main/scala/sbt/Run.scala

@@ -10,14 +10,15 @@ package sbt
 import java.io.File
 import java.lang.reflect.{ Method, Modifier }
 import Modifier.{ isPublic, isStatic }
+
 import sbt.internal.inc.classpath.ClasspathUtilities
 import sbt.internal.inc.ScalaInstance
 import sbt.internal.util.MessageOnlyException
-
 import sbt.io.Path
-
 import sbt.util.Logger
-import scala.util.{ Try, Success, Failure }
+
+import scala.reflect.internal.util.ScalaClassLoader.URLClassLoader
+import scala.util.{ Failure, Success, Try }
 import scala.util.control.NonFatal
 import scala.sys.process.Process
 
@@ -90,8 +91,15 @@ class Run(newLoader: Seq[File] => ClassLoader, trapExit: Boolean) extends ScalaR
   ): Unit = {
     log.debug("  Classpath:\n\t" + classpath.mkString("\n\t"))
     val loader = newLoader(classpath)
-    val main = getMainMethod(mainClassName, loader)
-    invokeMain(loader, main, options)
+    try {
+      val main = getMainMethod(mainClassName, loader)
+      invokeMain(loader, main, options)
+    } finally {
+      loader match {
+        case u: URLClassLoader => u.close()
+        case _                 =>
+      }
+    }
   }
   private def invokeMain(
       loader: ClassLoader,

sbt/src/sbt-test/project/unified/build.sbt

@@ -48,9 +48,10 @@ Provided by:
 Defined at:
 \t(sbt.Defaults.testTasks) Defaults.scala:670
 Dependencies:
-\tTest / executeTests
-\tTest / test / streams
 \tTest / state
+\tTest / testLoader
+\tTest / test / streams
+\tTest / executeTests
 \tTest / test / testResultLogger
 Delegates:
 \tTest / test