Synchronize at a higher level on the Scala provider cache.

Construction of Scala providers was already properly synchronized jvm and machine-wide.
The cache on top of construction was not.  This could cause the same Scala version to
be loaded in multiple class loaders.

launch/Cache.scala

@@ -9,7 +9,7 @@ import java.util.HashMap
 final class Cache[K,X,V](create: (K,X) => V)
 {
 	private[this] val delegate = new HashMap[K,Reference[V]]
-	def apply(k: K, x: X): V = getFromReference(k, x, delegate.get(k))
+	def apply(k: K, x: X): V = synchronized { getFromReference(k, x, delegate.get(k)) }
 	private[this] def getFromReference(k: K, x: X, existingRef: Reference[V]) = if(existingRef eq null) newEntry(k, x) else get(k, x, existingRef.get)
 	private[this] def get(k: K, x: X, existing: V) = if(existing == null) newEntry(k, x) else existing
 	private[this] def newEntry(k: K, x: X): V =

sbt/src/sbt-test/project/scala-loader/project/Build.scala

@@ -0,0 +1,19 @@
+import sbt._
+import Keys._
+
+object Build extends Build {
+
+	lazy val checkLoader = TaskKey[Unit]("check-loaders")
+
+	def checkTask = subs.map(sub => scalaInstance in LocalProject(sub.id)).join.map { sis =>
+		assert(sis.sliding(2).forall{ case Seq(x,y) => x.loader == y.loader }, "Not all ScalaInstances had the same class loader.")
+	}
+
+	override def projects = root +: subs
+	lazy val root = Project("root", file(".")).settings( checkLoader <<= checkTask )
+	lazy val subs = ( for(i <- 1 to 20) yield newProject(i) ).toSeq
+
+	def newProject(i: Int): Project = Project("x" + i.toString, file(i.toString)).settings(
+		scalaVersion := "2.10.2"
+	)
+}

sbt/src/sbt-test/project/scala-loader/test

@@ -0,0 +1,2 @@
+# verify that the class loader for Scala used for each project is the same
+> check-loaders