Some more fixes.

main/Defaults.scala

@@ -57,6 +57,7 @@ object Defaults extends BuildCommon
 		onComplete <<= taskTemporaryDirectory { dir => () => IO.delete(dir); IO.createDirectory(dir) },
 		concurrentRestrictions <<= concurrentRestrictions or defaultRestrictions,
 		parallelExecution :== true,
+		parallelTestGroups := 1,
 		sbtVersion <<= appConfiguration { _.provider.id.version },
 		sbtBinaryVersion <<= sbtVersion apply binarySbtVersion,
 		sbtResolver <<= sbtVersion { sbtV => if(sbtV endsWith "-SNAPSHOT") Classpaths.typesafeSnapshots else Classpaths.typesafeReleases },
@@ -117,10 +118,10 @@ object Defaults extends BuildCommon
 		excludeFilter :== (".*"  - ".") || HiddenFileFilter,
 		pomIncludeRepository :== Classpaths.defaultRepositoryFilter
 	))
-	def defaultTestTasks(key: Scoped): Seq[Setting[_]] = Seq(
-		tags in key := Seq(Tags.Test -> 1, Tags.Subprocess -> 1),
-		logBuffered in key := true
-	)
+	def defaultTestTasks(key: Scoped): Seq[Setting[_]] = inTask(key)(Seq(
+		tags := Seq(Tags.Test -> 1),
+		logBuffered := true
+	))
 	def projectCore: Seq[Setting[_]] = Seq(
 		name <<= thisProject(_.id),
 		logManager <<= extraLoggers(LogManager.defaults),
@@ -337,8 +338,9 @@ object Defaults extends BuildCommon
 	}
 
 	def testExecutionTask(task: Scoped): Initialize[Task[Tests.Execution]] =
-			(testOptions in task, parallelExecution in task, fork in task, javaOptions in task, tags in task) map {
-				(opts, par, fork, jvmOpts, ts) => new Tests.Execution(opts, par, if (fork) Tests.Fork(jvmOpts) else Tests.InProcess, ts)
+			(testOptions in task, parallelExecution in task, fork in task, javaOptions in task, tags in task, parallelTestGroups in task) map {
+				(opts, par, fork, jvmOpts, ts, lim) =>
+					new Tests.Execution(opts, par, if (fork) Tests.Fork(jvmOpts) else Tests.InProcess, if (fork) ts :+ (Tags.ForkedTestGroup -> lim) else ts)
 			}
 
 	def testQuickFilter: Initialize[Task[Seq[String] => String => Boolean]] =

main/Keys.scala

@@ -199,6 +199,7 @@ object Keys
 	val testExecution = TaskKey[Tests.Execution]("test-execution", "Settings controlling test execution")
 	val testFilter = TaskKey[Seq[String] => String => Boolean]("test-filter", "Filter controlling whether the test is executed")
 	val testGrouping = TaskKey[Seq[Tests.Group]]("test-grouping", "Groups discovered tests into groups.")
+	val parallelTestGroups = SettingKey[Int]("parallel-test-groups", "Maximum number of test groups that may run at the same time.")
 	val isModule = AttributeKey[Boolean]("is-module", "True if the target is a module.")
 
 	// Classpath/Dependency Management Keys

main/Tags.scala

@@ -18,7 +18,8 @@ object Tags
 	val CPU = Tag("cpu")
 	val Network = Tag("network")
 	val Disk = Tag("disk")
-	val Subprocess = Tag("subprocess")
+
+	val ForkedTestGroup = Tag("forked-test-group")
 
 	/** Describes a restriction on concurrently executing tasks.
 	* A Rule is constructed using one of the Tags.limit* methods. */

testing/agent/src/main/java/sbt/ForkMain.java

@@ -70,7 +70,12 @@ public class ForkMain {
     Socket socket = new Socket(InetAddress.getByName(null), Integer.valueOf(args[0]));
 		final ObjectInputStream is = new ObjectInputStream(socket.getInputStream());
 		final ObjectOutputStream os = new ObjectOutputStream(socket.getOutputStream());
-		new Run().run(is, os);
+		try {
+			new Run().run(is, os);
+		} finally {
+			is.close();
+			os.close();	
+		}
   }
   private static class Run {
     boolean matches(Fingerprint f1, Fingerprint f2) {