Ensure Compaction workload is stored in write-once meta-data files separate from timeline files.

This avoids concurrency issues when compactor(s) and ingestor are running in parallel.
    In the Next PR -> Safety concern regarding Cleaner retaining all meta-data and file-slices for pending compactions will be addressed

hoodie-client/src/main/java/com/uber/hoodie/io/compact/HoodieCompactionInstantWithPlan.java

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (c) 2016 Uber Technologies, Inc. (hoodie-dev-group@uber.com)
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *           http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+package com.uber.hoodie.io.compact;
+
+import com.uber.hoodie.avro.model.HoodieCompactionPlan;
+
+/**
+ * Contains Hoodie Compaction instant along with workload
+ */
+public class HoodieCompactionInstantWithPlan {
+
+  private final String compactionInstantTime;
+  private final HoodieCompactionPlan compactionPlan;
+
+  public HoodieCompactionInstantWithPlan(String compactionInstantTime,
+      HoodieCompactionPlan compactionPlan) {
+    this.compactionInstantTime = compactionInstantTime;
+    this.compactionPlan = compactionPlan;
+  }
+
+  public String getCompactionInstantTime() {
+    return compactionInstantTime;
+  }
+
+  public HoodieCompactionPlan getCompactionPlan() {
+    return compactionPlan;
+  }
+}

hoodie-client/src/test/java/com/uber/hoodie/common/HoodieTestDataGenerator.java

@@ -16,6 +16,7 @@
 
 package com.uber.hoodie.common;
 
+import com.uber.hoodie.avro.model.HoodieCompactionPlan;
 import com.uber.hoodie.common.model.HoodieCommitMetadata;
 import com.uber.hoodie.common.model.HoodieKey;
 import com.uber.hoodie.common.model.HoodiePartitionMetadata;
@@ -23,6 +24,8 @@ import com.uber.hoodie.common.model.HoodieRecord;
 import com.uber.hoodie.common.model.HoodieTestUtils;
 import com.uber.hoodie.common.table.HoodieTableMetaClient;
 import com.uber.hoodie.common.table.HoodieTimeline;
+import com.uber.hoodie.common.table.timeline.HoodieInstant;
+import com.uber.hoodie.common.util.AvroUtils;
 import com.uber.hoodie.common.util.FSUtils;
 import com.uber.hoodie.common.util.HoodieAvroUtils;
 import java.io.IOException;
@@ -135,6 +138,21 @@ public class HoodieTestDataGenerator {
     }
   }
 
+  public static void createCompactionAuxiliaryMetadata(String basePath, HoodieInstant instant,
+      Configuration configuration) throws IOException {
+    Path commitFile = new Path(
+        basePath + "/" + HoodieTableMetaClient.AUXILIARYFOLDER_NAME + "/" + instant.getFileName());
+    FileSystem fs = FSUtils.getFs(basePath, configuration);
+    FSDataOutputStream os = fs.create(commitFile, true);
+    HoodieCompactionPlan workload = new HoodieCompactionPlan();
+    try {
+      // Write empty commit metadata
+      os.writeBytes(new String(AvroUtils.serializeCompactionPlan(workload).get(), StandardCharsets.UTF_8));
+    } finally {
+      os.close();
+    }
+  }
+
   public static void createSavepointFile(String basePath, String commitTime) throws IOException {
     createSavepointFile(basePath, commitTime, HoodieTestUtils.getDefaultHadoopConf());
   }