[SPARK-47177][SQL] Cached SQL plan do not display final AQE plan in explain string

### What changes were proposed in this pull request?

This pr adds lock for ExplainUtils.processPlan to avoid tag race condition.

### Why are the changes needed?

To fix the issue [SPARK-47177](https://issues.apache.org/jira/browse/SPARK-47177)

### Does this PR introduce _any_ user-facing change?

yes, affect plan explain

### How was this patch tested?

add test

### Was this patch authored or co-authored using generative AI tooling?

no

Closes #45282 from ulysses-you/SPARK-47177.

Authored-by: ulysses-you <[email protected]>
Signed-off-by: youxiduo <[email protected]>

Author: ulysses-you

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -1009,18 +1009,19 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]]
     append(str)
     append("\n")
 
-    if (innerChildren.nonEmpty) {
+    val innerChildrenLocal = innerChildren
+    if (innerChildrenLocal.nonEmpty) {
       lastChildren.add(children.isEmpty)
       lastChildren.add(false)
-      innerChildren.init.foreach(_.generateTreeString(
+      innerChildrenLocal.init.foreach(_.generateTreeString(
         depth + 2, lastChildren, append, verbose,
         addSuffix = addSuffix, maxFields = maxFields, printNodeId = printNodeId, indent = indent))
       lastChildren.remove(lastChildren.size() - 1)
       lastChildren.remove(lastChildren.size() - 1)
 
       lastChildren.add(children.isEmpty)
       lastChildren.add(true)
-      innerChildren.last.generateTreeString(
+      innerChildrenLocal.last.generateTreeString(
         depth + 2, lastChildren, append, verbose,
         addSuffix = addSuffix, maxFields = maxFields, printNodeId = printNodeId, indent = indent)
       lastChildren.remove(lastChildren.size() - 1)

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

@@ -75,8 +75,12 @@ object ExplainUtils extends AdaptiveSparkPlanHelper {
    * Given a input physical plan, performs the following tasks.
    *   1. Generates the explain output for the input plan excluding the subquery plans.
    *   2. Generates the explain output for each subquery referenced in the plan.
+   *
+   * Note that, ideally this is a no-op as different explain actions operate on different plan,
+   * instances but cached plan is an exception. The `InMemoryRelation#innerChildren` use a shared
+   * plan instance across multi-queries. Add lock for this method to avoid tag race condition.
    */
-  def processPlan[T <: QueryPlan[T]](plan: T, append: String => Unit): Unit = {
+  def processPlan[T <: QueryPlan[T]](plan: T, append: String => Unit): Unit = synchronized {
     try {
       // Initialize a reference-unique set of Operators to avoid accdiental overwrites and to allow
       // intentional overwriting of IDs generated in previous AQE iteration

sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/InMemoryRelation.scala

@@ -403,17 +403,7 @@ case class InMemoryRelation(
 
   @volatile var statsOfPlanToCache: Statistics = null
 
-
-  override lazy val innerChildren: Seq[SparkPlan] = {
-    // The cachedPlan needs to be cloned here because it does not get cloned when SparkPlan.clone is
-    // called. This is a problem because when the explain output is generated for
-    // a plan it traverses the innerChildren and modifies their TreeNode.tags. If the plan is not
-    // cloned, there is a thread safety issue in the case that two plans with a shared cache
-    // operator have explain called at the same time. The cachedPlan cannot be cloned because
-    // it contains stateful information so we only clone it for the purpose of generating the
-    // explain output.
-    Seq(cachedPlan.clone())
-  }
+  override def innerChildren: Seq[SparkPlan] = Seq(cachedPlan)
 
   override def doCanonicalize(): logical.LogicalPlan =
     copy(output = output.map(QueryPlan.normalizeExpressions(_, output)),

sql/core/src/test/scala/org/apache/spark/sql/execution/columnar/InMemoryRelationSuite.scala

@@ -18,27 +18,42 @@
 package org.apache.spark.sql.execution.columnar
 
 import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.execution.SparkPlan
+import org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper
+import org.apache.spark.sql.functions.expr
 import org.apache.spark.sql.test.SharedSparkSessionBase
 import org.apache.spark.storage.StorageLevel
 
-class InMemoryRelationSuite extends SparkFunSuite with SharedSparkSessionBase {
-  test("SPARK-43157: Clone innerChildren cached plan") {
-    val d = spark.range(1)
-    val relation = InMemoryRelation(StorageLevel.MEMORY_ONLY, d.queryExecution, None)
-    val cloned = relation.clone().asInstanceOf[InMemoryRelation]
-
-    val relationCachedPlan = relation.innerChildren.head
-    val clonedCachedPlan = cloned.innerChildren.head
-
-    // verify the plans are not the same object but are logically equivalent
-    assert(!relationCachedPlan.eq(clonedCachedPlan))
-    assert(relationCachedPlan === clonedCachedPlan)
-  }
+class InMemoryRelationSuite extends SparkFunSuite
+  with SharedSparkSessionBase with AdaptiveSparkPlanHelper {
 
   test("SPARK-46779: InMemoryRelations with the same cached plan are semantically equivalent") {
     val d = spark.range(1)
     val r1 = InMemoryRelation(StorageLevel.MEMORY_ONLY, d.queryExecution, None)
     val r2 = r1.withOutput(r1.output.map(_.newInstance()))
     assert(r1.sameResult(r2))
   }
+
+  test("SPARK-47177: Cached SQL plan do not display final AQE plan in explain string") {
+    def findIMRInnerChild(p: SparkPlan): SparkPlan = {
+      val tableCache = find(p) {
+        case _: InMemoryTableScanExec => true
+        case _ => false
+      }
+      assert(tableCache.isDefined)
+      tableCache.get.asInstanceOf[InMemoryTableScanExec].relation.innerChildren.head
+    }
+
+    val d1 = spark.range(1).withColumn("key", expr("id % 100"))
+      .groupBy("key").agg(Map("key" -> "count"))
+    val cached_d2 = d1.cache()
+    val df = cached_d2.withColumn("key2", expr("key % 10"))
+      .groupBy("key2").agg(Map("key2" -> "count"))
+
+    assert(findIMRInnerChild(df.queryExecution.executedPlan).treeString
+      .contains("AdaptiveSparkPlan isFinalPlan=false"))
+    df.collect()
+    assert(findIMRInnerChild(df.queryExecution.executedPlan).treeString
+      .contains("AdaptiveSparkPlan isFinalPlan=true"))
+  }
 }