[SPARK-57521][ML][CONNECT] Exclude parent from Model.estimatedSize to fix overcounting in ML cache

mllib/src/main/scala/org/apache/spark/ml/Model.scala

@@ -57,5 +57,19 @@ abstract class Model[M <: Model[M]] extends Transformer { self =>
    * 4, For 3-rd extension, if external languages are used, it is recommended to override
    * this method and return a proper size.
    */
-  private[spark] def estimatedSize: Long = SizeEstimator.estimate(self)
+  private[spark] def estimatedSize: Long = {
+    // SPARK-57521: Temporarily clear the parent reference during size estimation.
+    // After fit(), the parent estimator may retain indirect references to the SparkSession
+    // (via closures or query plan state from DataFrame operations executed during fit).
+    // SizeEstimator traverses the entire reachable object graph, causing it to count
+    // shared SparkSession state as part of every model's size.
+    // The parent is @transient (not persisted) and is not needed for transform() or save().
+    val savedParent = parent
+    parent = null
+    try {
+      SizeEstimator.estimate(self)
+    } finally {
+      parent = savedParent
+    }
+  }
 }

mllib/src/test/scala/org/apache/spark/ml/ModelSuite.scala

@@ -0,0 +1,72 @@
+/*
+ * 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.ml
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.ml.feature.{MinMaxScaler, StringIndexer}
+import org.apache.spark.ml.linalg.Vectors
+import org.apache.spark.mllib.util.MLlibTestSparkContext
+
+class ModelSuite extends SparkFunSuite with MLlibTestSparkContext {
+
+  test("SPARK-57521: estimatedSize should not include parent's reachable object graph") {
+    val df = spark.createDataFrame(Seq(
+      Tuple1("a"), Tuple1("b"), Tuple1("c")
+    )).toDF("label")
+
+    val model = new StringIndexer()
+      .setInputCol("label").setOutputCol("idx")
+      .fit(df)
+
+    assert(model.hasParent, "model should have parent after fit()")
+    val size = model.estimatedSize
+
+    // Model data is 3 string labels + overhead, well under 50KB.
+    // Without the fix, SizeEstimator traverses model.parent -> estimator ->
+    // SparkSession, counting hundreds of KB (local) to hundreds of MB
+    // (production cluster) of shared session state per model.
+    assert(size < 50 * 1024,
+      s"estimatedSize ($size bytes) should reflect model data only, " +
+      s"not the parent estimator's reachable object graph (SparkSession)")
+
+    // Parent must be preserved — it is only excluded during estimation
+    assert(model.hasParent, "parent should be preserved after estimatedSize call")
+  }
+
+  test("SPARK-57521: estimatedSize excludes parent for multiple estimator types") {
+    // The issue affects all estimators that execute DataFrame operations during fit().
+    // Test with two different estimator types to verify the fix is in Model, not
+    // specific to any one estimator.
+    val df = spark.createDataFrame(Seq(
+      Tuple1(Vectors.dense(1.0, 2.0)),
+      Tuple1(Vectors.dense(3.0, 4.0))
+    )).toDF("features")
+
+    val model = new MinMaxScaler()
+      .setInputCol("features").setOutputCol("scaled")
+      .fit(df)
+
+    assert(model.hasParent)
+    val size = model.estimatedSize
+
+    assert(size < 50 * 1024,
+      s"MinMaxScalerModel estimatedSize ($size bytes) should not include " +
+      s"parent's reachable object graph")
+    assert(model.hasParent, "parent should be preserved after estimatedSize call")
+  }
+}