Detect and recover from infinite task loops

pkg/analyzer/lib/src/task/driver.dart

Index: pkg/analyzer/lib/src/task/driver.dart
diff --git a/pkg/analyzer/lib/src/task/driver.dart b/pkg/analyzer/lib/src/task/driver.dart
index 88df107a3bc479c8aa6046653c6d118e0eff9f6a..f077b3a3f8d07a1279745d4712fb7b1e402938fa 100644
--- a/pkg/analyzer/lib/src/task/driver.dart
+++ b/pkg/analyzer/lib/src/task/driver.dart
@@ -413,8 +413,46 @@ class WorkOrder implements Iterator<WorkItem> {
     while (childItem != null) {
       pendingItems.add(currentItem);
       currentItem = childItem;
+      if (_hasInfiniteTaskLoop()) {
+        currentItem = pendingItems.removeLast();
+        try {
+          throw new InfiniteTaskLoopException(childItem);
+        } on InfiniteTaskLoopException catch (exception, stackTrace) {
+          currentItem.exception = new CaughtException(exception, stackTrace);
+        }
+        return true;
+      }
       childItem = currentItem.gatherInputs(taskManager);
     }
     return true;
   }
+
+  /**
+   * Check to see whether the current work item is attempting to perform the
+   * same task on the same target as any of the pending work items. If it is,
+   * then throw an [InfiniteTaskLoopException].
+   */
+  bool _hasInfiniteTaskLoop() {
+    TaskDescriptor descriptor = currentItem.descriptor;
+    AnalysisTarget target = currentItem.target;
+    for (WorkItem item in pendingItems) {
+      if (item.descriptor == descriptor && item.target == target) {
+        return true;
+      }
+    }
+    return false;
+  }
+}
+
+/**
+ * An exception indicating that an attempt was made to perform a task on a
+ * target while gathering the inputs to perform the same task for the same
+ * target.
+ */
+class InfiniteTaskLoopException extends AnalysisException {
+  /**
+   * Initialize a newly created exception to represent an attempt to perform
+   * the task for the target represented by the given [item].
+   */
+  InfiniteTaskLoopException(WorkItem item) : super('Infinite loop while performing task ${item.descriptor.name} for ${item.target}');
 }