Load global plugins in their own class loader and replace the base loader with that. Fixes #272.

Also, replace the base classpath with the global classpath.

main/src/main/scala/sbt/Load.scala

@@ -74,15 +74,25 @@ object Load
 	}
 	def buildGlobalSettings(base: File, files: Seq[File], config: LoadBuildConfiguration): ClassLoader => Seq[Setting[_]] =
 	{	
-		val eval = mkEval(data(config.globalPluginClasspath), base, defaultEvalOptions)
+		val eval = mkEval(data(config.classpath), base, defaultEvalOptions)
 		val imports = baseImports ++ importAllRoot(config.globalPluginNames)
 		loader => EvaluateConfigurations(eval, files, imports)(loader).settings
 	}
 	def loadGlobal(state: State, base: File, global: File, config: LoadBuildConfiguration): LoadBuildConfiguration =
-		if(base != global && global.exists)
-			config.copy(globalPlugin = Some(GlobalPlugin.load(global, state, config)))
-		else
+		if(base != global && global.exists) {
+			val gp = GlobalPlugin.load(global, state, config)
+			val pm = setGlobalPluginLoader(gp, config.pluginManagement)
+			val cp = (gp.data.fullClasspath ++ config.classpath).distinct
+			config.copy(globalPlugin = Some(gp), pluginManagement = pm, classpath = cp)
+		} else
 			config
+
+	private[this] def setGlobalPluginLoader(gp: GlobalPlugin, pm: PluginManagement): PluginManagement =
+	{
+		val newLoader = ClasspathUtilities.toLoader(Build.data(gp.data.fullClasspath), pm.initialLoader)
+		pm.copy(initialLoader = newLoader)
+	}
+
 	def defaultDelegates: LoadedBuild => Scope => Seq[Scope] = (lb: LoadedBuild) => {
 		val rootProject = getRootProject(lb.units)
 		def resolveRef(project: Reference): ResolvedReference = Scope.resolveReference(lb.root, rootProject, project)
@@ -441,6 +451,7 @@ object Load
 		expand(auto)
 	}
 
+	@deprecated("No longer used.", "0.13.0")
 	def globalPluginClasspath(globalPlugin: Option[GlobalPlugin]): Seq[Attributed[File]] =
 		globalPlugin match
 		{
@@ -483,7 +494,7 @@ object Load
 		!(dir * -GlobFilter(DefaultTargetName)).get.isEmpty
 	}
 	def noPlugins(dir: File, config: LoadBuildConfiguration): LoadedPlugins =
-		loadPluginDefinition(dir, config, PluginData(config.globalPluginClasspath, None, None))
+		loadPluginDefinition(dir, config, PluginData(config.classpath, None, None))
 	def buildPlugins(dir: File, s: State, config: LoadBuildConfiguration): LoadedPlugins =
 		loadPluginDefinition(dir, config, buildPluginDefinition(dir, s, config))
 
@@ -661,8 +672,11 @@ final case class LoadBuildConfiguration(stagingDirectory: File, classpath: Seq[A
 	pluginManagement: PluginManagement, injectSettings: Load.InjectSettings, globalPlugin: Option[GlobalPlugin], extraBuilds: Seq[URI],
 	log: Logger)
 {
-	lazy val (globalPluginClasspath, globalPluginLoader) = Load.pluginDefinitionLoader(this, Load.globalPluginClasspath(globalPlugin))
-	lazy val globalPluginNames = if(globalPluginClasspath.isEmpty) Nil else Load.getPluginNames(globalPluginClasspath, globalPluginLoader)
+	@deprecated("Use `classpath`.", "0.13.0")
+	lazy val globalPluginClasspath = classpath
+	@deprecated("Use `pluginManagement.initialLoader`.", "0.13.0")
+	lazy val globalPluginLoader = pluginManagement.initialLoader
+	lazy val globalPluginNames = if(classpath.isEmpty) Nil else Load.getPluginNames(classpath, pluginManagement.initialLoader)
 }
 
 final class IncompatiblePluginsException(msg: String, cause: Throwable) extends Exception(msg, cause)

sbt/src/sbt-test/project/global-plugin-src/test

@@ -0,0 +1,4 @@
+# tests that a source file in $sbt.global.base/plugins/ is available to the build definition in project/
+
+# dummy to ensure project gets loaded
+> name

sbt/src/sbt-test/project/global-plugin/changes/Build.scala

@@ -0,0 +1,21 @@
+import sbt._
+import sbt.Keys._
+
+object MyBuild extends Build {
+  lazy val mySettings = Defaults.defaultSettings ++ Seq(
+    name := "my-test-proj",
+    organization := "com.example",
+    check <<= update map checkVersion,
+    version := "0.1.0-SNAPSHOT")
+
+  lazy val proj = Project("my-test-proj", file("."), settings = mySettings)
+
+  lazy val check = taskKey[Unit]("Verifies that the junit dependency has the older version (4.5)")
+
+  def checkVersion(report: UpdateReport) {
+    for(mod <- report.allModules) {
+      if(mod.name == "junit") assert(mod.revision == "4.5", s"JUnit version (${mod.revision}) was not overridden")
+    }
+  }
+}
+

sbt/src/sbt-test/project/global-plugin/changes/global-plugins.sbt

@@ -0,0 +1,3 @@
+// use a small java library instead of a plugin to avoid incompatibilities when upgrading
+// use an old version to check that it will override a newer version in a build definition
+libraryDependencies += "junit" % "junit" % "4.5"

sbt/src/sbt-test/project/global-plugin/changes/plugins.sbt

@@ -0,0 +1,2 @@
+// the version should be overridden by the global plugin
+libraryDependencies += "junit" % "junit" % "4.8"

sbt/src/sbt-test/project/global-plugin/test

@@ -1,4 +1,17 @@
-# tests that a source file in $sbt.global.base/plugins/ is available to the build definition in project/
+$ copy-file changes/Build.scala project/Build.scala
+> reload
 
-# dummy to ensure project gets loaded
-> name
+# ensure that a new global dependency gets picked up
+$ copy-file changes/global-plugins.sbt global/plugins/plugins.sbt
+> reload
+
+# check that the class can be loaded
+> eval Class.forName("org.junit.Test")
+
+# check that it is on the classpath
+> eval (x => ()) : (org.junit.Test => Unit)
+
+# ensure that the global plugin version overrides the local version
+$ copy-file changes/plugins.sbt project/plugins.sbt
+> reload
+> check