Avoiding a race condition where pending indices count is 0 at start of reindex

Author: masseyke

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/task/ReindexDataStreamTask.java

@@ -55,6 +55,7 @@ public ReindexDataStreamTask(
         this.persistentTaskStartTime = persistentTaskStartTime;
         this.initialTotalIndices = initialTotalIndices;
         this.initialTotalIndicesToBeUpgraded = initialTotalIndicesToBeUpgraded;
+        this.pending.set(initialTotalIndicesToBeUpgraded);
         this.completeTask = new RunOnce(() -> {
             if (exception == null) {
                 markAsCompleted();