[SPARK-21987][SQL] fix a compatibility issue of sql event logs

## What changes were proposed in this pull request?

In apache#18600 we removed the `metadata` field from `SparkPlanInfo`. This causes a problem when we replay event logs that are generated by older Spark versions.

## How was this patch tested?

a regression test.

Author: Wenchen Fan <[email protected]>

Closes apache#19237 from cloud-fan/event.

Author: cloud-fan

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlanInfo.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.sql.execution
 
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties
+
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.sql.execution.exchange.ReusedExchangeExec
 import org.apache.spark.sql.execution.metric.SQLMetricInfo
@@ -26,6 +28,7 @@ import org.apache.spark.sql.execution.metric.SQLMetricInfo
  * Stores information about a SQL SparkPlan.
  */
 @DeveloperApi
+@JsonIgnoreProperties(Array("metadata")) // The metadata field was removed in Spark 2.3.
 class SparkPlanInfo(
     val nodeName: String,
     val simpleString: String,

sql/core/src/test/scala/org/apache/spark/sql/execution/SQLJsonProtocolSuite.scala

@@ -0,0 +1,52 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.spark.sql.execution
+
+import org.json4s.jackson.JsonMethods.parse
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.execution.ui.SparkListenerSQLExecutionStart
+import org.apache.spark.util.JsonProtocol
+
+class SQLJsonProtocolSuite extends SparkFunSuite {
+
+  test("SparkPlanGraph backward compatibility: metadata") {
+    val SQLExecutionStartJsonString =
+      """
+        |{
+        |  "Event":"org.apache.spark.sql.execution.ui.SparkListenerSQLExecutionStart",
+        |  "executionId":0,
+        |  "description":"test desc",
+        |  "details":"test detail",
+        |  "physicalPlanDescription":"test plan",
+        |  "sparkPlanInfo": {
+        |    "nodeName":"TestNode",
+        |    "simpleString":"test string",
+        |    "children":[],
+        |    "metadata":{},
+        |    "metrics":[]
+        |  },
+        |  "time":0
+        |}
+      """.stripMargin
+    val reconstructedEvent = JsonProtocol.sparkEventFromJson(parse(SQLExecutionStartJsonString))
+    val expectedEvent = SparkListenerSQLExecutionStart(0, "test desc", "test detail", "test plan",
+      new SparkPlanInfo("TestNode", "test string", Nil, Nil), 0)
+    assert(reconstructedEvent == expectedEvent)
+  }
+}