viirya commented on code in PR #6130:
URL: https://github.com/apache/datafusion-comet/pull/6130#discussion_r4227466265


##########
native/core/src/execution/operators/arrow_python_udf.rs:
##########
@@ -0,0 +1,441 @@
+// 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.
+
+use std::fmt::Formatter;
+use std::sync::Arc;
+
+use arrow::array::{ArrayRef, BinaryArray, RecordBatch, StringArray};
+use arrow::datatypes::{DataType, Field, Schema, SchemaRef, TimeUnit};
+use datafusion::common::tree_node::TreeNodeRecursion;
+use datafusion::common::{exec_err, Result};
+use datafusion::execution::TaskContext;
+use datafusion::physical_expr::{EquivalenceProperties, PhysicalExpr};
+use datafusion::physical_plan::execution_plan::EmissionType;
+use datafusion::physical_plan::stream::RecordBatchStreamAdapter;
+use datafusion::physical_plan::{
+    apply_expression_roots, DisplayAs, DisplayFormatType, ExecutionPlan, 
ExecutionPlanProperties,
+    PlanProperties, SendableRecordBatchStream,
+};
+use futures::stream;
+use futures::StreamExt;
+
+use crate::execution::python_udf::ArrowPythonUdf;
+
+#[derive(Debug, Clone)]
+pub struct ArrowPythonUdfSpec {
+    pub command: Vec<u8>,
+    pub args: Vec<Arc<dyn PhysicalExpr>>,
+    pub arg_names: Vec<String>,
+    pub return_type: DataType,
+    pub return_name: String,
+    pub python_version: String,
+}
+
+/// Evaluates scalar PyArrow UDFs inside the native pipeline. Workers are
+/// instantiated in `execute`, once per partition. Python module state remains
+/// shared by every task in the executor's embedded interpreter.
+#[derive(Debug)]
+pub struct ArrowPythonUdfExec {

Review Comment:
   Added in `794b2767f`. `ArrowPythonUdfExec` records `BaselineMetrics` 
(`output_rows` and `elapsed_compute`) and a `python_time` subset timer around 
each Python call. `CometArrowEvalPythonExec` declares the matching SQL metrics. 
The multi-row-group Parquet test checks `output_rows`, checks that 
`python_time` is non-zero, and checks that it stays within `elapsed_compute`.



##########
native/core/src/execution/python_udf.rs:
##########
@@ -0,0 +1,453 @@
+// 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.
+
+//! In-process bridge for Spark 4.1+ scalar Arrow UDFs. Each instance owns one
+//! unpickled Python callable and must be created for one Spark task/partition.
+//! The public API deliberately deals in Arrow arrays; the physical operator is
+//! responsible for evaluating Catalyst arguments and preserving input columns.
+
+use arrow::array::{make_array, Array, ArrayRef};
+use arrow::datatypes::DataType;
+use arrow::error::{ArrowError, Result};
+use arrow::ffi::{from_ffi, FFI_ArrowArray, FFI_ArrowSchema};
+use pyo3::ffi::Py_uintptr_t;
+use pyo3::prelude::*;
+use pyo3::types::{PyBytes, PyTuple};
+
+fn initialize_python() -> Result<()> {
+    use std::ffi::CStr;
+    use std::sync::OnceLock;
+
+    static RESULT: OnceLock<std::result::Result<(), String>> = OnceLock::new();
+    RESULT
+        .get_or_init(|| {
+            // Spark sets PYTHONHASHSEED=0 on its Python workers by default.
+            // Match that seed before any Python object is created in the 
embedded
+            // interpreter, without changing the JVM process environment.
+            // SAFETY: OnceLock serializes initialization by Comet. No other 
Comet
+            // code accesses the Python C API before this function returns.
+            unsafe {
+                if pyo3::ffi::Py_IsInitialized() != 0 {
+                    return Ok(());
+                }
+                let mut config = 
std::mem::MaybeUninit::<pyo3::ffi::PyConfig>::uninit();
+                pyo3::ffi::PyConfig_InitPythonConfig(config.as_mut_ptr());
+                let mut config = config.assume_init();
+                config.install_signal_handlers = 0;
+                config.use_hash_seed = 1;
+                config.hash_seed = 0;
+                let status = pyo3::ffi::Py_InitializeFromConfig(&config);
+                let error = if pyo3::ffi::PyStatus_Exception(status) != 0 {
+                    if status.err_msg.is_null() {
+                        "Python interpreter initialization failed".to_string()
+                    } else {
+                        CStr::from_ptr(status.err_msg)
+                            .to_string_lossy()
+                            .into_owned()
+                    }
+                } else {
+                    String::new()
+                };
+                pyo3::ffi::PyConfig_Clear(&mut config);
+                if !error.is_empty() {
+                    return Err(error);
+                }
+                pyo3::ffi::PyEval_SaveThread();
+                Ok(())
+            }
+        })
+        .clone()
+        .map_err(ArrowError::ComputeError)
+}
+
+#[cfg(target_os = "linux")]
+fn make_python_symbols_global() -> Result<()> {
+    use std::ffi::CStr;
+    use std::sync::OnceLock;
+
+    static RESULT: OnceLock<std::result::Result<(), String>> = OnceLock::new();
+    RESULT
+        .get_or_init(|| {
+            // The JVM loads libcomet with RTLD_LOCAL. Its libpython 
dependency is
+            // local too, but CPython extension modules resolve Python C API
+            // symbols from the global namespace when they are imported.
+            let mut info = std::mem::MaybeUninit::<libc::Dl_info>::uninit();
+            // SAFETY: Py_Initialize is a linked function address and info is
+            // writable storage for dladdr's result.
+            if unsafe {
+                libc::dladdr(
+                    pyo3::ffi::Py_Initialize as *const () as *const 
libc::c_void,
+                    info.as_mut_ptr(),
+                )
+            } == 0
+            {
+                return Err("cannot locate the linked Python 
library".to_string());
+            }
+            // SAFETY: dladdr initialized info on success and dli_fname is a
+            // null-terminated path valid for the duration of this call.
+            let info = unsafe { info.assume_init() };
+            if info.dli_fname.is_null() {
+                return Err("linked Python library has no path".to_string());
+            }
+            let path = unsafe { CStr::from_ptr(info.dli_fname) };
+            // RTLD_NOLOAD promotes the already-loaded libpython rather than
+            // loading a second copy with separate interpreter state. Keep the
+            // handle for the executor lifetime so its symbols remain global.
+            // SAFETY: path points to a valid C string returned by dladdr.
+            if unsafe {
+                libc::dlopen(
+                    path.as_ptr(),
+                    libc::RTLD_NOW | libc::RTLD_GLOBAL | libc::RTLD_NOLOAD,
+                )
+            }
+            .is_null()
+            {
+                // SAFETY: dlerror returns a null-terminated message, if any.
+                let error = unsafe { libc::dlerror() };
+                let detail = if error.is_null() {
+                    "unknown dynamic loader error".to_string()
+                } else {
+                    unsafe { CStr::from_ptr(error) }
+                        .to_string_lossy()
+                        .into_owned()
+                };
+                return Err(format!("cannot expose Python C API symbols: 
{detail}"));
+            }
+            Ok(())
+        })
+        .clone()
+        .map_err(ArrowError::ComputeError)
+}
+
+#[cfg(not(target_os = "linux"))]
+fn make_python_symbols_global() -> Result<()> {
+    Ok(())
+}
+
+/// A scalar Arrow UDF loaded from Spark's pickled `(function, returnType)` 
command.
+/// Spark serializes the return type for its worker; Comet uses the separately
+/// serialized Arrow type from the physical plan instead.
+pub struct ArrowPythonUdf {
+    callable: Py<PyAny>,
+    return_type: DataType,
+    allow_cast: bool,
+    safe_cast: bool,
+}
+
+impl ArrowPythonUdf {
+    pub fn from_command(
+        command: &[u8],
+        return_type: DataType,
+        allow_cast: bool,
+        safe_cast: bool,
+        python_version: &str,
+    ) -> Result<Self> {
+        make_python_symbols_global()?;
+        initialize_python()?;
+        Python::attach(|py| {
+            if !python_version.is_empty() {
+                let info = py
+                    .import("sys")
+                    .map_err(python_error)?
+                    .getattr("version_info")
+                    .map_err(python_error)?;
+                let major: u8 = info
+                    .get_item(0)
+                    .map_err(python_error)?
+                    .extract()
+                    .map_err(python_error)?;
+                let minor: u8 = info
+                    .get_item(1)
+                    .map_err(python_error)?
+                    .extract()
+                    .map_err(python_error)?;
+                let actual = format!("{major}.{minor}");
+                if actual != python_version {
+                    return Err(ArrowError::ComputeError(format!(
+                        "Arrow UDF requires Python {python_version}, embedded 
interpreter is {actual}"
+                    )));
+                }
+            }
+            let pickle = py.import("pickle").map_err(python_error)?;
+            let loaded = pickle
+                .call_method1("loads", (PyBytes::new(py, command),))
+                .map_err(python_error)?;
+            let tuple = loaded.cast::<PyTuple>().map_err(python_error)?;
+            if tuple.len() != 2 {
+                return Err(ArrowError::ComputeError(format!(
+                    "Arrow UDF command must contain (function, returnType), 
got {} items",
+                    tuple.len()
+                )));
+            }
+            let callable = tuple.get_item(0).map_err(python_error)?;
+            if !callable.is_callable() {
+                return Err(ArrowError::ComputeError(
+                    "Arrow UDF command does not contain a 
callable".to_string(),
+                ));
+            }
+            Ok(Self {
+                callable: callable.unbind(),
+                return_type,
+                allow_cast,
+                safe_cast,
+            })
+        })
+    }
+
+    /// Evaluate one Arrow batch, with the same row count for every argument.
+    /// Python receives and returns `pyarrow.Array` objects via the Arrow C 
Data
+    /// interface; no row conversion or Arrow IPC serialization occurs here.
+    pub fn evaluate(&self, args: &[ArrayRef], num_rows: usize) -> 
Result<ArrayRef> {
+        let names = vec![String::new(); args.len()];
+        self.evaluate_named(args, &names, num_rows)
+    }
+
+    pub fn evaluate_named(
+        &self,
+        args: &[ArrayRef],
+        names: &[String],
+        num_rows: usize,
+    ) -> Result<ArrayRef> {
+        if args.len() != names.len() {
+            return Err(ArrowError::ComputeError(
+                "Arrow UDF argument names are not aligned with 
arguments".to_string(),
+            ));
+        }
+        for (index, arg) in args.iter().enumerate() {
+            if arg.len() != num_rows {
+                return Err(ArrowError::ComputeError(format!(
+                    "Arrow UDF argument {index} has {} rows, expected 
{num_rows}",
+                    arg.len()
+                )));
+            }
+        }
+
+        Python::attach(|py| {
+            let pa = py.import("pyarrow").map_err(python_error)?;
+            let array_class = pa.getattr("Array").map_err(python_error)?;
+            let mut py_args = Vec::with_capacity(args.len());
+            for arg in args {
+                let data = arg.to_data();
+                // PyArrow takes ownership of these C Data structs and clears 
their
+                // release callbacks, so the pointed-to storage must be 
writable.
+                let mut ffi_array = FFI_ArrowArray::new(&data);
+                let mut ffi_schema = 
FFI_ArrowSchema::try_from(data.data_type())?;
+                let py_arg = array_class
+                    .call_method1(
+                        "_import_from_c",
+                        (
+                            &raw mut ffi_array as Py_uintptr_t,
+                            &raw mut ffi_schema as Py_uintptr_t,
+                        ),
+                    )
+                    .map_err(python_error)?;
+                py_args.push(py_arg);
+            }
+
+            let kwargs = pyo3::types::PyDict::new(py);
+            let mut positional = Vec::new();
+            for (arg, name) in py_args.into_iter().zip(names) {
+                if name.is_empty() {
+                    positional.push(arg);
+                } else {
+                    kwargs.set_item(name, arg).map_err(python_error)?;
+                }
+            }
+            let result = self
+                .callable
+                .bind(py)
+                .call(
+                    PyTuple::new(py, positional).map_err(python_error)?,
+                    Some(&kwargs),
+                )
+                .map_err(python_error)?;
+            if !result.is_instance(&array_class).map_err(python_error)? {
+                return Err(ArrowError::ComputeError(
+                    "Arrow UDF must return a pyarrow.Array".to_string(),
+                ));
+            }
+            let result_len = result.len().map_err(python_error)?;
+            if result_len != num_rows {
+                return Err(ArrowError::ComputeError(format!(
+                    "Arrow UDF returned {result_len} rows, expected {num_rows}"
+                )));
+            }
+
+            let mut ffi_return_type = 
FFI_ArrowSchema::try_from(&self.return_type)?;
+            let expected_type = pa
+                .getattr("DataType")
+                .map_err(python_error)?
+                .call_method1(
+                    "_import_from_c",
+                    (&raw mut ffi_return_type as Py_uintptr_t,),
+                )
+                .map_err(python_error)?;
+            let actual_type = result.getattr("type").map_err(python_error)?;
+            let typed_result = if 
actual_type.eq(&expected_type).map_err(python_error)? {
+                result
+            } else if self.allow_cast {
+                let kwargs = pyo3::types::PyDict::new(py);
+                kwargs
+                    .set_item("safe", self.safe_cast)
+                    .map_err(python_error)?;
+                result
+                    .call_method("cast", (expected_type,), Some(&kwargs))
+                    .map_err(python_error)?
+            } else {
+                return Err(ArrowError::ComputeError(format!(
+                    "Arrow UDF returned type {}, expected {}",
+                    actual_type.str().map_err(python_error)?,
+                    expected_type.str().map_err(python_error)?
+                )));
+            };
+
+            let mut out_array = FFI_ArrowArray::empty();
+            let mut out_schema = FFI_ArrowSchema::empty();
+            typed_result
+                .call_method1(
+                    "_export_to_c",
+                    (
+                        &raw mut out_array as Py_uintptr_t,
+                        &raw mut out_schema as Py_uintptr_t,
+                    ),
+                )
+                .map_err(python_error)?;
+            // SAFETY: PyArrow filled both C Data structs and transferred 
ownership
+            // of the array to `out_array`; Arrow validates the schema and 
buffers.
+            let data = unsafe { from_ffi(out_array, &out_schema) }?;
+            if data.data_type() != &self.return_type {
+                return Err(ArrowError::ComputeError(format!(
+                    "Arrow UDF returned type {}, expected {}",
+                    data.data_type(),
+                    self.return_type
+                )));
+            }
+            Ok(make_array(data))
+        })
+    }
+}
+
+fn python_error(error: impl std::fmt::Display) -> ArrowError {

Review Comment:
   The traceback is in `794b2767f`: an exception from the callable is formatted 
with `traceback.format_exception`, so the error has the file, line, and 
exception type. A Rust test and a Scala end-to-end test check for the traceback.
   
   I didn't change the exception class. PySpark's `convert_exception` turns a 
`org.apache.spark.api.python.PythonException` cause into its Python 
`PythonException` only when one of the cause's stack frames is in 
`org.apache.spark.sql.execution.python`. This error is thrown from 
`CometExecIterator`, so a handler catching `pyspark.errors.PythonException` 
would still get an `UnknownException`. Matching it would mean constructing the 
exception from a helper in that package just to pass the frame check. If you 
think that's worth doing, I'd keep it as a follow-up. The docs now say that the 
traceback comes in a Comet native error.



##########
spark/src/main/spark-4.1+/org/apache/spark/sql/comet/CometArrowEvalPythonExec.scala:
##########
@@ -0,0 +1,215 @@
+/*
+ * 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.comet
+
+import java.nio.charset.StandardCharsets
+
+import scala.jdk.CollectionConverters._
+
+import org.apache.spark.api.python.PythonEvalType
+import org.apache.spark.sql.SparkSession
+import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeSet, 
Expression, NamedArgumentExpression, NamedExpression, PythonUDF}
+import org.apache.spark.sql.execution.{PartitioningPreservingUnaryExecNode, 
SparkPlan}
+import org.apache.spark.sql.execution.python.ArrowEvalPythonExec
+import org.apache.spark.sql.types.{BinaryType, BooleanType, ByteType, 
DataType, DateType, DecimalType, DoubleType, FloatType, IntegerType, LongType, 
ShortType, StringType, TimestampNTZType}
+
+import com.google.common.base.Objects
+import com.google.protobuf.ByteString
+
+import org.apache.comet.{CometConf, ConfigEntry, NativeBase}
+import org.apache.comet.CometSparkSessionExtensions.withFallbackReason
+import org.apache.comet.serde.{CometOperatorSerde, Compatible, 
OperatorOuterClass, QueryPlanSerde, SupportLevel, Unsupported}
+import org.apache.comet.serde.OperatorOuterClass.Operator
+
+/** Native execution for Spark 4.1+ scalar `@arrow_udf` functions. */
+object CometArrowEvalPythonExec extends 
CometOperatorSerde[ArrowEvalPythonExec] {
+
+  // SparkContext adds this entry even when the user has not configured a 
Python
+  // environment. Keep other overrides on Spark's worker path.
+  private def hasUnsupportedEnvironment(env: java.util.Map[String, String]): 
Boolean =
+    env != null && env.asScala.exists { case (key, value) =>
+      key != "PYTHONHASHSEED" || value != "0"
+    }
+
+  // PySpark's Accumulator.__reduce__ serializes a reference to
+  // pyspark.accumulators._deserialize_accumulator. Spark's worker forwards its
+  // task-local updates to the JVM when the task finishes; embedded Python does
+  // not have that worker protocol. A match may also come from a harmless 
string
+  // in the pickle, in which case Spark's worker path is the safe choice.
+  private def hasSerializedAccumulator(command: Seq[Byte]): Boolean =
+    new String(command.toArray, 
StandardCharsets.ISO_8859_1).contains("pyspark.accumulators")
+
+  private def hasCompatibleArrowSchema(dataType: DataType): Boolean = dataType 
match {
+    case _: BooleanType | _: ByteType | _: ShortType | _: IntegerType | _: 
LongType |
+        _: FloatType | _: DoubleType | _: BinaryType | _: DateType | _: 
DecimalType |
+        _: TimestampNTZType =>
+      true
+    // Spark's Arrow conversion accepts plain strings. Collated and 
constrained strings
+    // may carry semantics that are not represented by Comet's Utf8 Arrow type.
+    case s: StringType if s == StringType => true
+    case _ => false
+  }
+
+  override def enabledConfig: Option[ConfigEntry[Boolean]] =

Review Comment:
   Fixed in `794b2767f`. `enabledConfig` is now `None`, and `getSupportLevel` 
checks the eval type first, then the `python-udf` feature, then the remaining 
conditions. It reports the config only when the UDF would otherwise run 
natively. A test checks that a scalar pandas UDF reports "Only scalar 
@arrow_udf is supported" without the config hint, and that a supported Arrow 
UDF with the config off still gets the hint.



##########
docs/source/user-guide/latest/tuning/memory.md:
##########
@@ -142,13 +142,15 @@ one line every 10 seconds for the whole executor:
 Comet native memory usage: allocated 5412.3 MiB, reserved 3890.0 MiB (16 
native plans, 8 memory pools); JVM Arrow allocated 310.4 MiB, 96.2 MiB of it 
imported from native
 ```
 
-- `allocated` is the memory that Comet's native code has allocated and not yet 
freed, whether or not
-  a pool tracks it.
-- `reserved` is the part that Comet's memory pools have reserved from Spark's 
off-heap memory. It is
-  charged against `spark.memory.offHeap.size`, so the container already has 
room for it. A pool
-  sometimes has to track memory that Spark could not grant, such as a spilled 
batch read back from
-  disk while the off-heap memory is full. `reserved` leaves that memory out, 
since nothing charges it
-  against `spark.memory.offHeap.size`.
+- `allocated` is the memory allocated through Comet's Rust global allocator 
and not yet freed,
+  whether or not a pool tracks it. It excludes allocations made by native 
libraries outside that
+  allocator, including the embedded Python interpreter and PyArrow when native 
Arrow UDFs are
+  enabled.
+- `reserved` is the part that Comet's memory pools track. It is charged against

Review Comment:
   Fixed in `794b2767f`: the original "reserved from Spark's off-heap memory" 
wording is back. The "Who allocates what" table has a row for the embedded 
interpreter and PyArrow, which nothing bounds and Spark cannot see. The config 
doc now says Spark 4.1+.



##########
spark/src/test/spark-4.1+/org/apache/spark/sql/comet/CometArrowPythonUdfSuite.scala:
##########
@@ -0,0 +1,333 @@
+/*
+ * 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.comet
+
+import java.util.{Base64, Collections}
+
+import scala.sys.process._
+
+import org.apache.spark.api.python.{PythonEvalType, SimplePythonFunction}
+import org.apache.spark.sql.{CometTestBase, Row}
+import org.apache.spark.sql.execution.python.UserDefinedPythonFunction
+import org.apache.spark.sql.functions.{array, expr, lit, map, struct, when}
+import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.types.{ArrayType, BinaryType, BooleanType, 
ByteType, CalendarIntervalType, DataType, DateType, DecimalType, DoubleType, 
FloatType, IntegerType, LongType, MapType, ShortType, StringType, StructField, 
StructType, TimestampNTZType, TimestampType, TimeType, VariantType, 
YearMonthIntervalType}
+
+import org.apache.comet.{CometConf, NativeBase}
+
+class CometArrowPythonUdfSuite extends CometTestBase {
+
+  test("scalar Arrow UDF falls back when the native feature is unavailable") {
+    assume(!NativeBase.supportsPythonUdf())
+
+    val function = SimplePythonFunction(
+      Array.emptyByteArray,
+      Collections.emptyMap[String, String](),
+      Collections.emptyList[String](),
+      "python3",
+      "3.13",
+      Collections.emptyList(),
+      null)
+    val udf = UserDefinedPythonFunction(
+      "arrow_udf",
+      function,
+      LongType,
+      PythonEvalType.SQL_SCALAR_ARROW_UDF,
+      udfDeterministic = true)
+
+    withSQLConf(CometConf.COMET_NATIVE_ARROW_PYTHON_UDF_ENABLED.key -> "true") 
{
+      val source = spark.range(1, 2)
+      val plan = 
source.select(udf(source.col("id"))).queryExecution.executedPlan
+      assert(plan.collect { case _: CometArrowEvalPythonExec => true }.isEmpty)
+    }
+  }
+
+  test("scalar Arrow UDF executes in the native pipeline") {
+    assume(NativeBase.supportsPythonUdf(), "native library was built without 
python-udf")
+
+    val python = sys.env.getOrElse("PYSPARK_PYTHON", "python3")
+    val code =
+      "import base64, pyspark.cloudpickle as cloudpickle, pyarrow.compute as 
pc; " +
+        "from pyspark.sql.types import LongType; " +
+        "print(base64.b64encode(cloudpickle.dumps((pc.negate, 
LongType()))).decode())"
+    val command = Base64.getDecoder.decode(Seq(python, "-c", code).!!.trim)
+    val pythonVersion =
+      Seq(python, "-c", "import sys; print('%d.%d' % 
sys.version_info[:2])").!!.trim
+    val function = SimplePythonFunction(
+      command,
+      Collections.emptyMap[String, String](),
+      Collections.emptyList[String](),
+      python,
+      pythonVersion,
+      Collections.emptyList(),
+      null)
+    val udf = UserDefinedPythonFunction(
+      "negate_arrow",
+      function,
+      LongType,
+      PythonEvalType.SQL_SCALAR_ARROW_UDF,
+      udfDeterministic = true)
+
+    withSQLConf(
+      CometConf.COMET_NATIVE_ARROW_PYTHON_UDF_ENABLED.key -> "true",
+      SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "false") {
+      val source = spark.range(1, 5)
+      val df = source.select(udf(source.col("id")))
+      assert(df.queryExecution.executedPlan.collect { case _: 
CometArrowEvalPythonExec =>
+        true
+      }.nonEmpty)
+      checkAnswer(df, Seq(Row(-1L), Row(-2L), Row(-3L), Row(-4L)))
+
+      val twoResults =
+        source.select(udf(source.col("id")).as("first"), udf(source.col("id") 
+ 1L).as("second"))
+      val nativeUdfs = twoResults.queryExecution.executedPlan.collect {
+        case op: CometArrowEvalPythonExec => op
+      }
+      assert(nativeUdfs.exists(_.nativeOp.getArrowPythonUdf.getFunctionsCount 
== 2))
+      checkAnswer(twoResults, Seq(Row(-1L, -2L), Row(-2L, -3L), Row(-3L, -4L), 
Row(-4L, -5L)))
+
+      val withSubquery = spark.range(4).select(udf(expr("id + (SELECT max(id) 
FROM range(8))")))
+      val subqueryPlan = withSubquery.queryExecution.executedPlan
+      assert(subqueryPlan.collect { case _: CometArrowEvalPythonExec => true 
}.nonEmpty)
+      checkAnswer(withSubquery, Seq(Row(-7L), Row(-8L), Row(-9L), Row(-10L)))
+    }
+
+    withSQLConf(CometConf.COMET_NATIVE_ARROW_PYTHON_UDF_ENABLED.key -> 
"false") {
+      val source = spark.range(1, 2)
+      val plan = 
source.select(udf(source.col("id"))).queryExecution.executedPlan
+      assert(plan.collect { case _: CometArrowEvalPythonExec => true }.isEmpty)
+    }
+
+    withSQLConf(
+      CometConf.COMET_NATIVE_ARROW_PYTHON_UDF_ENABLED.key -> "true",
+      SQLConf.ARROW_EXECUTION_USE_LARGE_VAR_TYPES.key -> "true") {
+      val source = spark.range(1, 2)
+      val plan = 
source.select(udf(source.col("id"))).queryExecution.executedPlan
+      assert(plan.collect { case _: CometArrowEvalPythonExec => true }.isEmpty)
+    }
+  }
+
+  test("native Arrow UDF plan hides commands and compares UDF identity without 
plan IDs") {
+    assume(NativeBase.supportsPythonUdf(), "native library was built without 
python-udf")
+
+    val secret = "private_arrow_udf_command"
+    val function = SimplePythonFunction(
+      secret.getBytes(java.nio.charset.StandardCharsets.UTF_8),
+      Collections.emptyMap[String, String](),
+      Collections.emptyList[String](),
+      "python3",
+      "3.13",
+      Collections.emptyList(),
+      null)
+    val udf = UserDefinedPythonFunction(
+      "secret_arrow",
+      function,
+      LongType,
+      PythonEvalType.SQL_SCALAR_ARROW_UDF,
+      udfDeterministic = true)
+
+    withSQLConf(
+      CometConf.COMET_NATIVE_ARROW_PYTHON_UDF_ENABLED.key -> "true",
+      SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "false") {
+      val source = spark.range(1, 2)
+      val plan = 
source.select(udf(source.col("id"))).queryExecution.executedPlan
+      val native = plan.collectFirst { case op: CometArrowEvalPythonExec => op 
}.get
+      assert(!plan.treeString.contains(secret))
+      assert(!native.toString.contains(secret))
+
+      val differentPlanId = native.copy(nativeOp =
+        native.nativeOp.toBuilder.setPlanId(native.nativeOp.getPlanId + 
1).build())
+      assert(native == differentPlanId)
+      assert(native.hashCode() == differentPlanId.hashCode())
+
+      val otherFunction = SimplePythonFunction(
+        
"different_arrow_udf_command".getBytes(java.nio.charset.StandardCharsets.UTF_8),
+        Collections.emptyMap[String, String](),
+        Collections.emptyList[String](),
+        "python3",
+        "3.13",
+        Collections.emptyList(),
+        null)
+      val otherUdf = UserDefinedPythonFunction(
+        "secret_arrow",
+        otherFunction,
+        LongType,
+        PythonEvalType.SQL_SCALAR_ARROW_UDF,
+        udfDeterministic = true)
+      val otherPlan = 
source.select(otherUdf(source.col("id"))).queryExecution.executedPlan
+      val otherNative = otherPlan.collectFirst { case op: 
CometArrowEvalPythonExec => op }.get
+      assert(native.copy(udfs = otherNative.udfs) != native)
+
+      val differentBatchSize = native.nativeOp.toBuilder
+      differentBatchSize.getArrowPythonUdfBuilder.setMaxRecordsPerBatch(
+        native.nativeOp.getArrowPythonUdf.getMaxRecordsPerBatch + 1)
+      assert(native.copy(nativeOp = differentBatchSize.build()) != native)
+
+      val sameFunctionPlan = 
source.select(udf(source.col("id"))).queryExecution.executedPlan
+      val sameFunctionNative =
+        sameFunctionPlan.collectFirst { case op: CometArrowEvalPythonExec => 
op }.get
+      assert(native.udfs != sameFunctionNative.udfs)
+      assert(native.sameResult(sameFunctionNative))
+    }
+  }
+
+  test("native Arrow UDF preserves every accepted scalar type and nulls") {

Review Comment:
   Updated in `794b2767f`. The type test now runs eight rows with 
`maxRecordsPerBatch=3` and nulls at rows 1, 4, and 7. Each type therefore 
reaches PyArrow at offsets 3 and 6 (non-byte-aligned for booleans), and 
identity results return to the JVM sliced. A new test writes a Parquet file 
with `parquet.block.size=512`, asserts from the footer that it has more than 
one row group, and reads it through the native scan with a 777-row cap, 
comparing the results with Spark.



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