Expose Ivy's circular dependency check via UpdateOptions

ivy/src/main/scala/sbt/CircularDependencyLevel.scala

@@ -0,0 +1,27 @@
+package sbt
+
+import org.apache.ivy.plugins.circular.{ CircularDependencyStrategy, WarnCircularDependencyStrategy, IgnoreCircularDependencyStrategy, ErrorCircularDependencyStrategy }
+
+/**
+ * Wrapper around circular dependency strategy.
+ */
+sealed trait CircularDependencyLevel {
+  private[sbt] def ivyStrategy: CircularDependencyStrategy
+  private[sbt] def name: String
+  override def toString: String = name
+}
+
+object CircularDependencyLevel {
+  val Warn: CircularDependencyLevel = new CircularDependencyLevel {
+    def ivyStrategy: CircularDependencyStrategy = WarnCircularDependencyStrategy.getInstance
+    def name: String = "warn"
+  }
+  val Ignore: CircularDependencyLevel = new CircularDependencyLevel {
+    def ivyStrategy: CircularDependencyStrategy = IgnoreCircularDependencyStrategy.getInstance
+    def name: String = "ignore"
+  }
+  val Error: CircularDependencyLevel = new CircularDependencyLevel {
+    def ivyStrategy: CircularDependencyStrategy = ErrorCircularDependencyStrategy.getInstance
+    def name: String = "error"
+  }
+}

ivy/src/main/scala/sbt/Ivy.scala

@@ -69,6 +69,7 @@ final class IvySbt(val configuration: IvyConfiguration) {
     {
       val is = new IvySettings
       is.setBaseDir(baseDirectory)
+      is.setCircularDependencyStrategy(configuration.updateOptions.circularDependencyLevel.ivyStrategy)
       CustomPomParser.registerDefault
       configuration match {
         case e: ExternalIvyConfiguration =>

ivy/src/main/scala/sbt/UpdateOptions.scala

@@ -10,25 +10,33 @@ import java.io.File
  * See also UpdateConfiguration in IvyActions.scala.
  */
 final class UpdateOptions private[sbt] (
+    /** If set to CircularDependencyLevel.Error, halt the dependency resolution. */
+    val circularDependencyLevel: CircularDependencyLevel,
     /** If set to true, check all resolvers for snapshots. */
     val latestSnapshots: Boolean,
     /** If set to true, use consolidated resolution. */
     val consolidatedResolution: Boolean) {
 
+  def withCircularDependencyLevel(circularDependencyLevel: CircularDependencyLevel): UpdateOptions =
+    copy(circularDependencyLevel = circularDependencyLevel)
   def withLatestSnapshots(latestSnapshots: Boolean): UpdateOptions =
     copy(latestSnapshots = latestSnapshots)
   def withConsolidatedResolution(consolidatedResolution: Boolean): UpdateOptions =
     copy(consolidatedResolution = consolidatedResolution)
 
   private[sbt] def copy(
+    circularDependencyLevel: CircularDependencyLevel = this.circularDependencyLevel,
     latestSnapshots: Boolean = this.latestSnapshots,
     consolidatedResolution: Boolean = this.consolidatedResolution): UpdateOptions =
-    new UpdateOptions(latestSnapshots, consolidatedResolution)
+    new UpdateOptions(circularDependencyLevel,
+      latestSnapshots,
+      consolidatedResolution)
 }
 
 object UpdateOptions {
   def apply(): UpdateOptions =
     new UpdateOptions(
+      circularDependencyLevel = CircularDependencyLevel.Warn,
       latestSnapshots = true,
       consolidatedResolution = false)
 }