fix: Prevent native write when input is not Arrow format

common/src/main/scala/org/apache/spark/sql/comet/util/Utils.scala

@@ -269,8 +269,14 @@ object Utils extends CometTypeShim {
 
         case c =>
           throw new SparkException(
-            "Comet execution only takes Arrow Arrays, but got " +
-              s"${c.getClass}")
+            s"Comet execution only takes Arrow Arrays, but got ${c.getClass}. " +
+              "This typically happens when a Comet scan falls back to Spark due to unsupported " +
+              "data types (e.g., complex types like structs, arrays, or maps with native_comet). " +
+              "To resolve this, you can: " +
+              "(1) enable spark.comet.scan.allowIncompatible=true to use a compatible native " +
+              "scan variant, or " +
+              "(2) enable spark.comet.convert.parquet.enabled=true to convert Spark Parquet " +
+              "data to Arrow format automatically.")
       }
     }
     (fieldVectors, provider)

spark/src/main/scala/org/apache/comet/rules/CometExecRule.scala

@@ -207,7 +207,20 @@ case class CometExecRule(session: SparkSession) extends Rule[SparkPlan] {
         w.child
 
       case op: DataWritingCommandExec =>
-        convertToComet(op, CometDataWritingCommand).getOrElse(op)
+        // Get the actual data source child that will feed data to the native writer
+        val dataSourceChild = op.child match {
+          case writeFiles: WriteFilesExec => writeFiles.child
+          case other => other
+        }
+        // Only convert to native write if the data source produces Arrow data.
+        // If it's a Spark scan (not CometNativeExec), the native writer will fail at runtime
+        // because it expects Arrow arrays but will receive OnHeapColumnVector.
+        if (dataSourceChild.isInstanceOf[CometNativeExec]) {
+          convertToComet(op, CometDataWritingCommand).getOrElse(op)
+        } else {
+          withInfo(op, "Cannot perform native write because input is not in Arrow format")
+          op
+        }
 
       // For AQE broadcast stage on a Comet broadcast exchange
       case s @ BroadcastQueryStageExec(_, _: CometBroadcastExchangeExec, _) =>

spark/src/test/scala/org/apache/comet/parquet/CometParquetWriterSuite.scala

@@ -75,16 +75,15 @@ class CometParquetWriterSuite extends CometTestBase {
 
           withSQLConf(CometConf.COMET_NATIVE_SCAN_IMPL.key -> "native_datafusion") {
             val capturedPlan = writeWithCometNativeWriteExec(inputPath, outputPath)
-            capturedPlan.foreach { qe =>
-              val executedPlan = qe.executedPlan
-              val hasNativeScan = executedPlan.exists {
+            capturedPlan.foreach { plan =>
+              val hasNativeScan = plan.exists {
                 case _: CometNativeScanExec => true
                 case _ => false
               }
 
               assert(
                 hasNativeScan,
-                s"Expected CometNativeScanExec in the plan, but got:\n${executedPlan.treeString}")
+                s"Expected CometNativeScanExec in the plan, but got:\n${plan.treeString}")
             }
 
             verifyWrittenFile(outputPath)
@@ -311,6 +310,54 @@ class CometParquetWriterSuite extends CometTestBase {
     }
   }
 
+  test("native write falls back when scan produces non-Arrow data") {
+    // This test verifies that when a native scan (like native_comet) doesn't support
+    // certain data types (complex types), the native write correctly falls back to Spark
+    // instead of failing at runtime with "Comet execution only takes Arrow Arrays" error.
+    withTempPath { dir =>
+      val inputPath = new File(dir, "input.parquet").getAbsolutePath
+      val outputPath = new File(dir, "output.parquet").getAbsolutePath
+
+      // Create data with complex types and write without Comet
+      withSQLConf(CometConf.COMET_ENABLED.key -> "false") {
+        val df = Seq((1, Seq(1, 2, 3)), (2, Seq(4, 5)), (3, Seq(6, 7, 8, 9)))
+          .toDF("id", "values")
+        df.write.parquet(inputPath)
+      }
+
+      // With native Parquet write enabled but using native_comet scan which doesn't
+      // support complex types, the scan falls back to Spark. The native write should
+      // detect this and also fall back to Spark instead of failing at runtime.
+      withSQLConf(
+        CometConf.COMET_NATIVE_PARQUET_WRITE_ENABLED.key -> "true",
+        CometConf.COMET_EXEC_ENABLED.key -> "true",
+        CometConf.getOperatorAllowIncompatConfigKey(classOf[DataWritingCommandExec]) -> "true",
+        // Use native_comet which doesn't support complex types
+        CometConf.COMET_NATIVE_SCAN_IMPL.key -> "native_comet") {
+
+        val plan =
+          captureWritePlan(path => spark.read.parquet(inputPath).write.parquet(path), outputPath)
+
+        // Verify NO CometNativeWriteExec in the plan (should have fallen back to Spark)
+        val hasNativeWrite = plan.exists {
+          case _: CometNativeWriteExec => true
+          case d: DataWritingCommandExec =>
+            d.child.exists(_.isInstanceOf[CometNativeWriteExec])
+          case _ => false
+        }
+
+        assert(
+          !hasNativeWrite,
+          "Expected fallback to Spark write (no CometNativeWriteExec), but found native write " +
+            s"in plan:\n${plan.treeString}")
+
+        // Verify the data was written correctly
+        val result = spark.read.parquet(outputPath).collect()
+        assert(result.length == 3, "Expected 3 rows to be written")
+      }
+    }
+  }
+
   test("parquet write complex types fuzz test") {
     withTempPath { dir =>
       val outputPath = new File(dir, "output.parquet").getAbsolutePath
@@ -347,18 +394,21 @@ class CometParquetWriterSuite extends CometTestBase {
     inputPath
   }
 
-  private def writeWithCometNativeWriteExec(
-      inputPath: String,
-      outputPath: String,
-      num_partitions: Option[Int] = None): Option[QueryExecution] = {
-    val df = spark.read.parquet(inputPath)
-
-    // Use a listener to capture the execution plan during write
+  /**
+   * Captures the execution plan during a write operation.
+   *
+   * @param writeOp
+   *   The write operation to execute (takes output path as parameter)
+   * @param outputPath
+   *   The path to write to
+   * @return
+   *   The captured execution plan
+   */
+  private def captureWritePlan(writeOp: String => Unit, outputPath: String): SparkPlan = {
     var capturedPlan: Option[QueryExecution] = None
 
     val listener = new org.apache.spark.sql.util.QueryExecutionListener {
       override def onSuccess(funcName: String, qe: QueryExecution, durationNs: Long): Unit = {
-        // Capture plans from write operations
         if (funcName == "save" || funcName.contains("command")) {
           capturedPlan = Some(qe)
         }
@@ -373,8 +423,7 @@ class CometParquetWriterSuite extends CometTestBase {
     spark.listenerManager.register(listener)
 
     try {
-      // Perform native write with optional partitioning
-      num_partitions.fold(df)(n => df.repartition(n)).write.parquet(outputPath)
+      writeOp(outputPath)
 
       // Wait for listener to be called with timeout
       val maxWaitTimeMs = 15000
@@ -387,36 +436,45 @@ class CometParquetWriterSuite extends CometTestBase {
         iterations += 1
       }
 
-      // Verify that CometNativeWriteExec was used
       assert(
         capturedPlan.isDefined,
         s"Listener was not called within ${maxWaitTimeMs}ms - no execution plan captured")
 
-      capturedPlan.foreach { qe =>
-        val executedPlan = stripAQEPlan(qe.executedPlan)
+      stripAQEPlan(capturedPlan.get.executedPlan)
+    } finally {
+      spark.listenerManager.unregister(listener)
+    }
+  }
 
-        // Count CometNativeWriteExec instances in the plan
-        var nativeWriteCount = 0
-        executedPlan.foreach {
+  private def writeWithCometNativeWriteExec(
+      inputPath: String,
+      outputPath: String,
+      num_partitions: Option[Int] = None): Option[SparkPlan] = {
+    val df = spark.read.parquet(inputPath)
+
+    val plan = captureWritePlan(
+      path => num_partitions.fold(df)(n => df.repartition(n)).write.parquet(path),
+      outputPath)
+
+    // Count CometNativeWriteExec instances in the plan
+    var nativeWriteCount = 0
+    plan.foreach {
+      case _: CometNativeWriteExec =>
+        nativeWriteCount += 1
+      case d: DataWritingCommandExec =>
+        d.child.foreach {
           case _: CometNativeWriteExec =>
             nativeWriteCount += 1
-          case d: DataWritingCommandExec =>
-            d.child.foreach {
-              case _: CometNativeWriteExec =>
-                nativeWriteCount += 1
-              case _ =>
-            }
           case _ =>
         }
-
-        assert(
-          nativeWriteCount == 1,
-          s"Expected exactly one CometNativeWriteExec in the plan, but found $nativeWriteCount:\n${executedPlan.treeString}")
-      }
-    } finally {
-      spark.listenerManager.unregister(listener)
+      case _ =>
     }
-    capturedPlan
+
+    assert(
+      nativeWriteCount == 1,
+      s"Expected exactly one CometNativeWriteExec in the plan, but found $nativeWriteCount:\n${plan.treeString}")
+
+    Some(plan)
   }
 
   private def verifyWrittenFile(outputPath: String): Unit = {