[FLINK-39094][table-planner] Avoid creating duplicate function instances in code generation

flink-table/flink-table-planner/src/main/scala/org/apache/flink/table/planner/codegen/CodeGeneratorContext.scala

@@ -129,6 +129,9 @@ class CodeGeneratorContext(
   // string_constant -> reused_term
   private val reusableStringConstants: mutable.Map[String, String] = mutable.Map[String, String]()
 
+  // set of function instance term that will be added only once
+  private val reusableFunctionTerms: mutable.HashSet[String] = mutable.HashSet[String]()
+
   // map of type serializer that will be added only once
   // LogicalType -> reused_term
   private val reusableTypeSerializers: mutable.Map[LogicalType, String] =
@@ -835,10 +838,13 @@ class CodeGeneratorContext(
       function: UserDefinedFunction,
       functionContextClass: Class[_ <: FunctionContext] = classOf[FunctionContext],
       contextArgs: Seq[String] = null): String = {
-    val classQualifier = function.getClass.getName
     val fieldTerm = CodeGenUtils.udfFieldName(function)
-
-    addReusableObjectInternal(function, fieldTerm, classQualifier)
+    // check if function has been added before to avoid duplicate function instances
+    if (!reusableFunctionTerms.contains(fieldTerm)) {
+      reusableFunctionTerms += fieldTerm
+      val classQualifier = function.getClass.getName
+      addReusableObjectInternal(function, fieldTerm, classQualifier)
+    }
 
     val openFunction = if (contextArgs != null) {
       s"""

flink-table/flink-table-planner/src/test/scala/org/apache/flink/table/planner/codegen/CodeGeneratorContextTest.scala

@@ -0,0 +1,91 @@
+/*
+ * 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.flink.table.planner.codegen
+
+import org.apache.flink.configuration.Configuration
+import org.apache.flink.table.functions.ScalarFunction
+
+import org.junit.jupiter.api.Assertions.{assertEquals, assertNotEquals}
+import org.junit.jupiter.api.Test
+
+/** Tests for [[CodeGeneratorContext]]. */
+class CodeGeneratorContextTest {
+
+  private val classLoader = Thread.currentThread().getContextClassLoader
+
+  @Test
+  def testAddSameStatelessFunctionInstanceDedup(): Unit = {
+    val ctx = new CodeGeneratorContext(new Configuration, classLoader)
+    val udf = new StatelessTestFunction()
+
+    val term1 = ctx.addReusableFunction(udf)
+    val term2 = ctx.addReusableFunction(udf)
+
+    assertEquals(term1, term2)
+    assertEquals(1, ctx.references.size)
+  }
+
+  @Test
+  def testAddSameStatefulFunctionInstancesDedup(): Unit = {
+    val ctx = new CodeGeneratorContext(new Configuration, classLoader)
+    val udf1 = new StatefulTestFunction(42)
+    val udf2 = new StatefulTestFunction(42)
+
+    val term1 = ctx.addReusableFunction(udf1)
+    val term2 = ctx.addReusableFunction(udf2)
+
+    assertEquals(term1, term2)
+    assertEquals(1, ctx.references.size)
+  }
+
+  @Test
+  def testAddDifferentStatelessFunctionInstancesDedup(): Unit = {
+    val ctx = new CodeGeneratorContext(new Configuration, classLoader)
+    val udf1 = new StatelessTestFunction()
+    val udf2 = new StatelessTestFunction()
+
+    val term1 = ctx.addReusableFunction(udf1)
+    val term2 = ctx.addReusableFunction(udf2)
+
+    assertEquals(term1, term2)
+    assertEquals(1, ctx.references.size)
+  }
+
+  @Test
+  def testAddDifferentStatefulFunctionInstances(): Unit = {
+    val ctx = new CodeGeneratorContext(new Configuration, classLoader)
+    val udf1 = new StatefulTestFunction(1)
+    val udf2 = new StatefulTestFunction(2)
+
+    val term1 = ctx.addReusableFunction(udf1)
+    val term2 = ctx.addReusableFunction(udf2)
+
+    assertNotEquals(term1, term2)
+    assertEquals(2, ctx.references.size)
+  }
+}
+
+@SerialVersionUID(1L)
+class StatelessTestFunction extends ScalarFunction {
+  def eval(a: Long): Long = a
+}
+
+@SerialVersionUID(1L)
+class StatefulTestFunction(private val mode: Int) extends ScalarFunction {
+  def eval(a: Long): Long = a + mode
+}