viirya commented on code in PR #58978:
URL: https://github.com/apache/spark/pull/58978#discussion_r4101609452


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/python/InProcessArrowBridge.scala:
##########
@@ -0,0 +1,116 @@
+/*
+ * 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.execution.python
+
+import org.apache.arrow.c.{ArrowArray, ArrowSchema, Data}
+import org.apache.arrow.memory.util.MemoryUtil
+import org.apache.arrow.vector.FieldVector
+import org.apache.arrow.vector.types.pojo.Field
+
+import org.apache.spark.sql.util.ArrowUtils
+import org.apache.spark.sql.vectorized.ArrowColumnVector
+import org.apache.spark.util.Utils
+
+/**
+ * Bridges JVM Arrow column buffers with Python PyArrow arrays for in-process 
UDF execution.
+ *
+ * Both input and output paths use the Arrow C Data Interface (CDI) for 
zero-copy transfer.
+ *
+ * Input path (JVM to Python, zero-copy via CDI):
+ *   JVM pre-allocates [[ArrowArray]] and [[ArrowSchema]] C structs and 
exports each input
+ *   [[FieldVector]] into them via [[Data.exportVector]]. The native addresses 
are passed to
+ *   Python. Python calls ``pa.Array._import_from_c(array_ptr, schema_ptr)`` 
to wrap the
+ *   same Arrow buffers as a PyArrow array -- no memcpy. When Python GCs the 
array, the CDI
+ *   release callback decrements the buffer reference counts; the JVM 
[[FieldVector]] retains
+ *   its own reference. Each batch uses new vectors; closing the old vectors 
releases only the
+ *   JVM's references, leaving any arrays retained by Python valid and 
unchanged.
+ *
+ * Output path (Python to JVM, zero-copy via CDI):
+ *   JVM pre-allocates [[ArrowArray]] and [[ArrowSchema]] C structs. Python 
calls
+ *   ``arr._export_to_c(array_ptr, schema_ptr)`` to fill those structs 
in-place. The JVM
+ *   calls [[Data.importIntoVector]] to reconstruct the [[FieldVector]] 
without copying. When the
+ *   imported [[FieldVector]] is closed, Arrow Java invokes PyArrow's CDI 
release callback,
+ *   decrementing the Python array refcount and allowing garbage collection.
+ *
+ * The runtime validates the returned schema before ArrowColumnVector reads 
the buffers.
+ */
+private[python] object InProcessArrowBridge {
+
+  /**
+   * Export a [[FieldVector]] to pre-allocated Arrow C Data Interface structs.
+   *
+   * Fills ``outArray`` and ``outSchema`` with the CDI representation of 
``vector``.
+   * The export is zero-copy: ``outArray``'s buffer pointers reference the 
same off-heap
+   * memory as ``vector``. The CDI release callback (invoked when the 
Python-side imported
+   * array is GC'd) decrements the buffer reference counts; the 
[[FieldVector]] continues
+   * to hold its own reference.
+   *
+   * Caller must release any unconsumed exports and close both structs on 
every exit path.
+   */
+  def exportColumn(vector: FieldVector, outArray: ArrowArray, outSchema: 
ArrowSchema): Unit =
+    Data.exportVector(ArrowUtils.rootAllocator, vector, null, outArray, 
outSchema)
+
+  /**
+   * Reconstruct an [[ArrowColumnVector]] from JVM-allocated Arrow C Data 
Interface structs.
+   *
+   * The JVM pre-allocates [[ArrowArray]] and [[ArrowSchema]] before invoking 
Python.
+   * Python fills them via ``arr._export_to_c(array_ptr, schema_ptr)``. This 
method
+   * calls [[Data.importIntoVector]] to wrap Python's Arrow buffers 
(zero-copy).
+   *
+   * Lifecycle:
+   *  - [[Data.importIntoVector]] internally calls 
``ArrayImporter.importArray()``, which
+   *    moves the struct snapshot through a non-owning wrapper, leaving the 
caller's struct
+   *    storage alive for cleanup, and wraps the data buffers via
+   *    ``ReferenceCountedArrowArray`` (ForeignAllocation, zero-copy).
+   *  - Data.importField releases and closes a non-owning schema wrapper too.
+   *    The caller closes the original struct storage.
+   *  - When the returned [[ArrowColumnVector]] is closed, the reference count 
drops to
+   *    zero, PyArrow's C ``release`` callback is invoked, and the Python 
array is GC'd.
+   */
+  private def checkOffsets(array: ArrowArray): Unit = {
+    val snapshot = array.snapshot()
+    require(snapshot.offset == 0L, "In-process UDF returned an unsupported 
Arrow CDI offset")
+    (0L until snapshot.n_children).foreach { i =>
+      checkOffsets(ArrowArray.wrap(MemoryUtil.getLong(snapshot.children + i * 
8L)))
+    }
+    if (snapshot.dictionary != 0L) 
checkOffsets(ArrowArray.wrap(snapshot.dictionary))
+  }
+
+  def cdiToColumn(
+      arrowArray: ArrowArray,
+      arrowSchema: ArrowSchema,
+      expected: Option[Field] = None): ArrowColumnVector = {
+    checkOffsets(arrowArray)
+    val field = Data.importField(
+      ArrowUtils.rootAllocator, ArrowSchema.wrap(arrowSchema.memoryAddress()), 
null)
+    expected.foreach { declared =>
+      require(field.getType == declared.getType && field.getChildren == 
declared.getChildren &&

Review Comment:
   Confirmed with a struct field carrying comment metadata. Replaced raw 
metadata equality with recursive Arrow layout validation, then import into a 
vector created from the declared field. This preserves Spark metadata without 
depending on JSON formatting. Added an identity-UDF regression test.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/python/InProcessPythonRuntime.scala:
##########
@@ -0,0 +1,258 @@
+/*
+ * 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.execution.python
+
+import java.nio.ByteBuffer
+import java.util.concurrent.{Callable, ExecutionException, TimeoutException, 
TimeUnit}
+
+import scala.jdk.CollectionConverters._
+
+import jep.{JepException, SharedInterpreter}
+
+import org.apache.spark.{TaskContext, TaskKilledException}
+import org.apache.spark.api.python.PythonException
+import org.apache.spark.internal.Logging
+import org.apache.spark.util.{ThreadUtils, Utils}
+
+/** Owns one interpreter generation per executor plugin lifecycle. */
+private[python] object InProcessPythonRuntime extends Logging {
+  val SITE_PACKAGES_CONFIG = "spark.inprocess.python.sitePackages"
+  private val TRACEBACK_SENTINEL = "__INPROCESS_UDF_TRACEBACK__:"
+  private var active: InterpreterSession = _
+
+  def initialize(sitePackages: Seq[String] = Seq.empty): Unit = synchronized {
+    if (active != null && !active.isTerminated) {
+      require(active.isRunning && active.sitePackages == sitePackages,
+        "In-process Python is stopping or already initialized with different 
sitePackages")
+    } else {
+      val candidate = new InterpreterSession(sitePackages)
+      try {
+        candidate.initialize()
+        active = candidate
+      } catch {
+        case t: Throwable => Utils.tryWithSafeFinally { throw t } { 
candidate.shutdown() }
+      }
+    }
+  }
+
+  def currentSession: InterpreterSession = synchronized {
+    checkState(active != null && active.isRunning)
+    active
+  }
+
+  def shutdown(): Unit = {
+    val session = synchronized { active }
+    if (session != null) session.shutdown()
+  }
+
+  private def checkState(running: Boolean): Unit = {
+    checkState(running, "In-process Python is not running; initialize the 
executor plugin first")
+  }
+
+  private def checkState(running: Boolean, message: String): Unit = {
+    if (!running) throw new IllegalStateException(message)
+  }
+
+  /**
+   * Tasks retain this generation, so stale tasks cannot enter a later 
SparkContext's interpreter.
+   * Lifecycle operations only hold the monitor while enqueueing work, never 
while running Python.
+   */
+  private[python] class InterpreterSession(val sitePackages: Seq[String] = 
Seq.empty) {
+    private val executor = 
ThreadUtils.newDaemonSingleThreadExecutor("inprocess-python")
+    @volatile private var running = true
+    // Accessed only on the owning thread.
+    private var interp: SharedInterpreter = _
+
+    def isRunning: Boolean = running
+    def isTerminated: Boolean = executor.isTerminated
+
+    private[python] def onInterpreterThread[T](body: => T): T = {
+      val context = Option(TaskContext.get())
+      context.foreach(_.killTaskIfInterrupted())
+      val gate = new Object
+      var started = false
+      var cancelled = false
+      val future = synchronized {
+        checkState(running)
+        executor.submit(new Callable[T] {
+          override def call(): T = {
+            gate.synchronized {
+              if (cancelled) throw new TaskKilledException("Cancelled before 
Python invocation")
+              started = true
+            }
+            body
+          }
+        })
+      }
+      var interrupted = false
+      try {
+        while (true) {
+          val taskCancelled = context.exists(_.isInterrupted())
+          if (interrupted || taskCancelled) {
+            val cancelledBeforeStart = gate.synchronized {
+              if (started) false else {
+                cancelled = true
+                future.cancel(false)
+                true
+              }
+            }
+            if (cancelledBeforeStart) {
+              context.foreach(_.killTaskIfInterrupted())
+              throw new InterruptedException("Cancelled before Python 
invocation")
+            }
+          }
+          try {
+            val result = future.get(100, TimeUnit.MILLISECONDS)
+            context.foreach(_.killTaskIfInterrupted())
+            return result
+          } catch {
+            case _: TimeoutException =>
+            case _: InterruptedException => interrupted = true
+            case e: ExecutionException => throw e.getCause
+          }
+        }
+        throw new IllegalStateException("Unreachable")
+      } finally {
+        // Once native work starts, wait for it even after cancellation: the 
caller still owns
+        // CDI structs that Python may use. Pending work, however, is safe to 
cancel immediately.
+        if (interrupted) Thread.currentThread().interrupt()
+      }
+    }
+
+    def initialize(): Unit = onInterpreterThread {
+      val candidate = new SharedInterpreter()

Review Comment:
   JEP 4.3.2 supports this through PyConfig. The runtime now sets hash seed 0 
before the first interpreter initialization, matching the default worker seed. 
Added a test comparing embedded results with a separate CPython process using 
PYTHONHASHSEED=0. The docs also clarify that a custom worker seed does not 
override this process-wide setting.



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/PythonUDF.scala:
##########
@@ -48,7 +48,8 @@ object PythonUDF {
     PythonEvalType.SQL_SCALAR_PANDAS_UDF,
     PythonEvalType.SQL_SCALAR_PANDAS_ITER_UDF,
     PythonEvalType.SQL_SCALAR_ARROW_UDF,
-    PythonEvalType.SQL_SCALAR_ARROW_ITER_UDF
+    PythonEvalType.SQL_SCALAR_ARROW_ITER_UDF,
+    PythonEvalType.SQL_SCALAR_ARROW_INPROCESS_UDF

Review Comment:
   Agreed. Connect now rejects this eval type at the shared Python-function 
conversion boundary, before constructing the function. This covers both inline 
expressions and SQL registration. The Python Connect registration API also 
rejects InProcessUDFWrapper explicitly. Added server planning tests and a 
client registration test.



##########
python/pyspark/inprocess/runtime.py:
##########
@@ -0,0 +1,196 @@
+#
+# 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.
+#
+
+
+"""Arrow CDI entry points called on the executor's dedicated JEP interpreter 
thread.
+
+Functions are registered once per task and released when that task finishes. 
Calls
+pass only a handle and CDI addresses, so large closures are not copied per 
batch.
+"""
+
+import sys
+import traceback as _traceback
+from typing import Any, Callable, Iterable, Optional, Sequence
+
+import pyarrow as pa
+import pyarrow.compute as pc
+
+from pyspark import cloudpickle
+from pyspark.errors import PySparkRuntimeError
+from pyspark.sql.pandas.types import to_arrow_type
+from pyspark.sql.types import _parse_datatype_json_string
+
+_UDF_TRACEBACK_SENTINEL = "__INPROCESS_UDF_TRACEBACK__:"
+_udfs: dict[str, tuple[Callable[..., pa.Array], pa.DataType]] = {}
+
+
+def _inprocess_register(
+    handle: str,
+    serialized_udf: Any,
+    return_type_json: str,
+    timezone: str,
+    python_version: str,
+    large_var_types: bool = False,
+) -> None:
+    try:
+        embedded_version = "%d.%d" % sys.version_info[:2]
+        if python_version != embedded_version:
+            raise PySparkRuntimeError(
+                errorClass="PYTHON_VERSION_MISMATCH",
+                messageParameters={
+                    "worker_version": embedded_version,
+                    "driver_version": python_version,
+                },
+            )
+        # JEP exposes direct ByteBuffers through the buffer protocol. Unpickle 
a separate
+        # function per task without iterating over a PyJArray one JNI call per 
byte.
+        func = cloudpickle.loads(memoryview(serialized_udf))
+        expected_type = to_arrow_type(

Review Comment:
   Confirmed. Updated the shared Python to_arrow_type mapping so the binary 
children of Variant, Geometry, and Geography honor prefers_large_types. Added 
mapping checks and identity-UDF integration coverage with large types both 
enabled and disabled.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/python/InProcessArrowEvalPythonEvaluatorFactory.scala:
##########
@@ -0,0 +1,212 @@
+/*
+ * 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.execution.python
+
+import java.util.UUID
+
+import scala.collection.mutable.ArrayBuffer
+import scala.jdk.CollectionConverters._
+
+import org.apache.arrow.c.{ArrowArray, ArrowSchema}
+import org.apache.arrow.util.AutoCloseables
+import org.apache.arrow.vector.VectorSchemaRoot
+
+import org.apache.spark.TaskContext
+import org.apache.spark.api.python.ChainedPythonFunctions
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.{Attribute, PythonUDF}
+import org.apache.spark.sql.execution.arrow.ArrowWriter
+import org.apache.spark.sql.execution.metric.SQLMetric
+import org.apache.spark.sql.execution.python.EvalPythonExec.ArgumentMetadata
+import org.apache.spark.sql.types.StructType
+import org.apache.spark.sql.util.ArrowUtils
+import org.apache.spark.sql.vectorized.{ArrowColumnVector, ColumnarBatch, 
ColumnVector}
+import org.apache.spark.util.Utils
+
+/**
+ * Evaluates scalar Python UDFs using Arrow CDI in the executor process. Only 
UDF arguments
+ * are converted to Arrow. Original rows are buffered in a spillable queue and 
joined with
+ * the results. Each batch owns its Arrow buffers so Python can safely retain 
input arrays.
+ */
+class InProcessArrowEvalPythonEvaluatorFactory(
+    childOutput: Seq[Attribute],
+    udfs: Seq[PythonUDF],
+    output: Seq[Attribute],
+    batchSize: Int,
+    maxBytes: Long,
+    timeZoneId: String,
+    largeVarTypes: Boolean,
+    metrics: Map[String, SQLMetric])
+  extends EvalPythonEvaluatorFactory(childOutput, udfs, output) {
+
+  private val returnTypes = udfs.map(_.dataType.json)
+
+  override protected def evaluate(
+      funcs: Seq[(ChainedPythonFunctions, Long)],
+      argMetas: Array[Array[ArgumentMetadata]],
+      rows: Iterator[InternalRow],
+      inputSchema: StructType,
+      context: TaskContext): Iterator[InternalRow] = {
+    ArrowUtils.failDuplicatedFieldNames(inputSchema)
+    val functions = funcs.map { case (chain, _) =>
+      require(chain.funcs.size == 1, "In-process UDF chains must use separate 
evaluation nodes")
+      chain.funcs.head
+    }
+    val inputOrdinals = argMetas.map(_.map(_.offset))
+    def checkCancellation(): Unit = context.killTaskIfInterrupted()
+
+    val arrowSchema = ArrowUtils.toArrowSchema(inputSchema, timeZoneId, 
largeVarTypes)
+    var runtime: InProcessPythonRuntime.InterpreterSession = null
+    val handles = functions.map(_ => UUID.randomUUID().toString)
+    var registered = false
+    var writer: ArrowWriter = null
+    val results = ArrayBuffer.empty[ArrowColumnVector]
+    var closed = false
+    val startedAt = System.nanoTime()
+
+    def closeBatch(): Unit = {
+      val resources = ArrayBuffer.empty[AutoCloseable]
+      resources ++= results
+      results.clear()
+      if (writer != null) {
+        resources += writer.root
+        writer = null
+      }
+      AutoCloseables.close(resources.asJava)
+    }
+
+    def close(): Unit = {
+      if (!closed) {
+        closed = true
+        metrics("pythonTotalTime") += (System.nanoTime() - startedAt) / 1000000
+        Utils.tryWithSafeFinally {
+          closeBatch()
+        } {
+          if (registered) runtime.release(handles)
+        }
+      }
+    }
+
+    context.addTaskCompletionListener[Unit](_ => close())
+
+    new Iterator[InternalRow] {
+      private var batchIter: Iterator[InternalRow] = Iterator.empty
+
+      override def hasNext: Boolean = {
+        checkCancellation()
+        val available = !closed && (batchIter.hasNext || rows.hasNext)
+        if (!available) close()
+        available
+      }
+
+      override def next(): InternalRow = {
+        if (!hasNext) throw new NoSuchElementException("End of in-process UDF 
input")
+        try {
+          if (!batchIter.hasNext) {
+            closeBatch()
+            if (!registered) {
+              runtime = InProcessPythonRuntime.currentSession
+              // Mark before registering so failure after any registration 
still cleans up.
+              registered = true
+              val start = System.nanoTime()
+              functions.indices.foreach { i =>
+                val func = functions(i)
+                runtime.register(handles(i), func.command.toArray, 
returnTypes(i),
+                  timeZoneId, func.pythonVer, largeVarTypes)
+              }
+              metrics("pythonInitTime") += (System.nanoTime() - start) / 
1000000
+            }
+            val root = VectorSchemaRoot.create(arrowSchema, 
ArrowUtils.rootAllocator)
+            writer = try {
+              ArrowWriter.create(root)
+            } catch {
+              case t: Throwable => Utils.tryWithSafeFinally { throw t } { 
root.close() }
+            }
+            var count = 0
+            while (rows.hasNext && (batchSize <= 0 || count < batchSize) &&
+                (count == 0 || maxBytes <= 0 || writer.sizeInBytes() < 
maxBytes)) {
+              checkCancellation()
+              writer.write(rows.next())
+              count += 1
+            }
+            writer.finish()
+            metrics("pythonDataSent") += writer.sizeInBytes()
+
+            handles.indices.foreach { udfIndex =>
+              val handle = handles(udfIndex)
+              val ordinals = inputOrdinals(udfIndex)
+              checkCancellation()
+              // Register each acquired resource immediately, including 
partially exported
+              // inputs and results of earlier UDFs if a later UDF throws.
+              val structs = ArrayBuffer.empty[AutoCloseable]
+              def array(): ArrowArray = {
+                val value = ArrowArray.allocateNew(ArrowUtils.rootAllocator)
+                structs += new AutoCloseable {
+                  override def close(): Unit =
+                    Utils.tryWithSafeFinally {
+                      if (value.snapshot().release != 0L) value.release()
+                    } { value.close() }
+                }
+                value
+              }
+              def schema(): ArrowSchema = {
+                val value = ArrowSchema.allocateNew(ArrowUtils.rootAllocator)
+                structs += new AutoCloseable {
+                  override def close(): Unit =
+                    Utils.tryWithSafeFinally {
+                      if (value.snapshot().release != 0L) value.release()
+                    } { value.close() }
+                }
+                value
+              }
+              Utils.tryWithSafeFinally {
+                val inArrays = ordinals.map(_ => array())
+                val inSchemas = ordinals.map(_ => schema())
+                val outArray = array()
+                val outSchema = schema()
+                ordinals.indices.foreach { i =>
+                  InProcessArrowBridge.exportColumn(
+                    writer.root.getVector(ordinals(i)), inArrays(i), 
inSchemas(i))
+                }
+                metrics("pythonProcessingTime") += runtime.invoke(
+                  handle,
+                  inArrays.map(_.memoryAddress()).toArray,
+                  inSchemas.map(_.memoryAddress()).toArray,
+                  outArray.memoryAddress(), outSchema.memoryAddress(),
+                  count, argMetas(udfIndex).map(_.name.getOrElse("")))
+                val expected = ArrowUtils.toArrowField(

Review Comment:
   Fixed together with the bridge validation change: after checking the 
physical layout, the importer uses the declared field, retaining precision 
metadata that Array CDI cannot carry. Added top-level and nested tests for 
TIME(3), TIMESTAMP_NTZ(7), and TIMESTAMP_LTZ(8). Expected fields are now 
computed once per evaluator invocation, outside the batch loop.



##########
python/benchmarks/bench_inprocess_udf.py:
##########
@@ -0,0 +1,137 @@
+#
+# 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.
+#
+
+"""End-to-end in-process, worker Arrow, and pandas UDF benchmarks.
+
+See README.md for the required Spark build and JEP launch environment. These
+measure steady-state queries, including JVM row/Arrow conversion and Python
+execution. Worker Arrow UDFs are the primary baseline and use the same Arrow
+operations as in-process UDFs. The supplementary pandas baseline also includes
+pandas conversion costs; neither comparison isolates IPC overhead alone.
+Historical standalone-script timings are a separate baseline.
+"""
+
+from importlib.util import find_spec
+
+
+class InProcessUDFTimeBench:
+    # One query per sample, with explicit full-query warmup in setup.
+    number = 1
+    rounds = 1
+    repeat = 5
+    warmup_time = 0
+    timeout = 300
+    params = [
+        ["arrow", "inprocess", "pandas"],
+        [
+            ("narrow", 100_000),
+            ("narrow", 1_000_000),
+            ("narrow", 5_000_000),
+            ("wide", 1_000_000),
+            ("wide", 5_000_000),
+            ("wide", 10_000_000),
+            ("short_string", 1_000_000),
+            ("short_string", 5_000_000),
+            ("short_string", 10_000_000),
+            ("long_string", 500_000),
+            ("long_string", 1_000_000),
+            ("long_string", 2_000_000),
+        ],
+    ]
+    param_names = ["udf_type", "workload"]
+
+    def setup(self, udf_type, workload):
+        # JEP cannot be imported from standalone CPython. Check availability
+        # without loading it; broken native/JVM setup must fail, not be 
skipped.
+        if udf_type == "inprocess" and find_spec("jep") is None:
+            raise NotImplementedError("Install JEP and configure its JVM 
launch paths")
+
+        import pyarrow.compute as pc
+        from pyspark.sql import SparkSession
+        from pyspark.sql.functions import arrow_udf, col, lpad, pandas_udf
+        from pyspark.sql.types import LongType, StringType
+
+        use_arrow = udf_type != "pandas"
+        scenario, n_rows = workload
+        n_cols = 10 if scenario == "wide" else 1
+        batch_size = {"narrow": 10_000, "wide": 1_000_000}.get(scenario, 
100_000)
+        self.spark = (
+            SparkSession.builder.master("local[1]")

Review Comment:
   Fixed. The benchmark now registers InProcessPythonPlugin when building the 
in-process session, before warmup. Verified setup, warmup, and query execution 
for integer and string workloads in all three modes, and updated the README.



##########
docs/sql-pyspark-inprocess-udf.md:
##########
@@ -0,0 +1,596 @@
+---
+layout: global
+title: In-Process Python UDFs
+displayTitle: In-Process Python UDFs
+license: |
+  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.
+---
+
+* Table of contents
+{:toc}
+
+## Runtime and result contract
+
+Each executor owns a dedicated interpreter thread. The plugin initializes the
+interpreter on that thread, and task calls and shutdown are dispatched to the
+same thread. Calls from concurrent tasks are queued on the interpreter thread.
+One task per executor is recommended for throughput, but is not a correctness 
requirement.
+Application-level Python parallelism comes from multiple executor JVMs.
+
+Task cancellation cannot safely stop arbitrary native Python code. An 
interrupted
+caller waits for the current invocation to finish before freeing the Arrow CDI
+structures, then restores its interrupt status. A UDF that never returns can
+therefore prevent its task from completing cancellation. Plugin shutdown stops 
accepting
+new calls and waits up to five seconds for the interpreter thread. If a call is
+still running, cleanup stays queued behind it; its memory remains live until 
the
+call returns or the process exits. Shutdown does not forcibly interrupt native
+code. A new interpreter cannot start until the previous one has fully stopped.
+
+A scalar UDF must return a `pyarrow.Array` with exactly one element per input 
row.
+The runtime checks the result type against the declared Spark type, including
+nested fields, decimal scale, and timestamp unit/timezone. Value types must 
match
+exactly: use an explicit PyArrow cast in the UDF for numeric or other 
conversions.
+Nested field nullability may differ if the actual values satisfy the declared 
nullability. Sliced results, including nested
+child slices, are copied to remove offsets that Arrow Java's CDI importer 
cannot
+read. Compatible results retain zero-copy transfer.
+
+The API produces a regular `PythonUDF` expression with an in-process evaluation
+type. Spark's existing `ArrowEvalPython` planning rules handle aggregation,
+nested calls, nondeterminism, and filter/limit pushdown. `ArrowEvalPythonExec` 
selects
+an in-process evaluator factory for this evaluation type, reusing the 
projection,
+row queue, result join, and partition-evaluator path. Ordinary Python UDFs 
continue
+to use Python workers.
+
+`maxRecordsPerBatch <= 0` means no row-count limit. The independent
+`spark.sql.execution.arrow.maxBytesPerBatch` limit still applies when positive.
+Only UDF arguments are converted to Arrow. Other columns stay in Spark rows,
+buffered in a spillable queue until the results are joined back. Duplicate 
nested
+field names in UDF arguments or declared results are rejected before Arrow Java
+reads their buffers.
+
+Each batch uses fresh input buffers. A Python function may retain an input 
array;
+later batches do not overwrite it. Retained arrays keep native memory alive, so
+functions should release them when no longer needed. JVM input vectors and 
result
+vectors are released on task completion, early termination and failure.
+
+UDF deserialization uses PySpark's bundled cloudpickle. Each task registers its
+own function instance once and passes a small handle for subsequent batches.
+Task completion queues release of the registered function and its closure 
state. Imported
+Python modules still share executor-wide state. Extra site-packages paths are
+processed with `site.addsitedir` before loading the runtime bridge, including 
`.pth`
+files. Configured directories and newly discovered `.pth` paths precede system
+paths. Already imported modules cannot be replaced by changing the search path.
+
+Spark broadcasts, accumulators, `SparkContext.addPyFile`, and Python 
`TaskContext`
+are not supported by this embedded runtime. Captured broadcast and accumulator
+objects are rejected during serialization; functions must not access them 
through
+imported modules either. Install modules on executors before startup, 
optionally
+using `spark.inprocess.python.sitePackages`. SQL registration through
+`spark.udf.register` is not supported and is rejected at registration time.
+Functions must receive at least one input column (a literal also works) to 
determine
+the batch length. Positional and keyword arguments are supported. Functions are
+serialized on first use, so globals can be defined or rebound after decoration
+and before that first call. The driver's Python major.minor
+version must match the embedded interpreter; registration checks this before
+unpickling. Python exceptions, including `SystemExit` during deserialization or
+execution, are converted into task failures. Native process termination remains
+outside this exception handling.
+
+## Overview
+
+In-process Python UDFs embed CPython directly into the Spark executor JVM using
+[jep (Java Embedded Python)](https://github.com/ninia/jep), eliminating the 
IPC overhead of
+standard Python UDFs and pandas UDFs. Data is passed to Python as
+[PyArrow](https://arrow.apache.org/docs/python/) arrays via the
+[Arrow C Data 
Interface](https://arrow.apache.org/docs/format/CDataInterface.html) — zero-copy
+for compatible input and output buffers. Row-to-Arrow conversion and 
normalization
+of sliced results still copy data.
+
+**Use `inprocess_udf` when:**
+- You are already using `pandas_udf` for vectorized transformations and want 
lower latency.
+- Your UDF operates on Arrow/PyArrow arrays (e.g. using `pyarrow.compute`).
+- You can deploy enough executor JVMs for Python parallelism (see 
[Requirements](#requirements)).
+
+**Stick with `pandas_udf` or `udf` when:**
+- You need pandas Series semantics in your UDF logic.
+- You need concurrent Python invocations within a single executor.
+- You are not able to install jep on executors.
+
+---
+
+## Quick Start
+
+### 1. Install dependencies
+
+```bash
+pip install "jep>=4.3.2" pyarrow cloudpickle
+```
+
+JEP and `org.apache.arrow:arrow-c-data` are provided dependencies and are not
+bundled with Spark. Supply their JARs on the driver/executor classpaths before
+starting Spark, and make the JEP native library available. Use an 
`arrow-c-data`
+version matching Spark's Arrow Java version. Installing the Python packages 
alone
+does not supply the Arrow Java CDI JAR.
+
+Building JEP from source requires a JDK, a C compiler, and development headers 
for
+the Python version being embedded (for example, `python3.12-dev` on Ubuntu with
+Python 3.12). These headers are build dependencies; running a prebuilt 
compatible
+JEP installation does not require the development package. The corresponding
+Python shared library must remain available at runtime.
+
+### 2. Register the plugin
+
+```python
+spark = SparkSession.builder \
+    .config("spark.plugins",
+            "org.apache.spark.sql.execution.python.InProcessPythonPlugin") \
+    .config("spark.executor.cores", "1") \
+    .config("spark.task.cpus", "1") \
+    .getOrCreate()
+```
+
+### 3. Write and call a UDF
+
+```python
+import pyarrow.compute as pc
+from pyspark.inprocess.udf import inprocess_udf
+from pyspark.sql.types import LongType
+
+@inprocess_udf(return_type=LongType())
+def double(x):
+    return pc.multiply(x, 2)
+
+df = spark.range(10)
+df.select(double(df["id"])).show()
+```
+
+The function receives a `pa.Array` for each input column and must return a 
`pa.Array`.
+
+---
+
+## Examples
+
+### String transformation
+
+```python
+import pyarrow.compute as pc
+from pyspark.inprocess.udf import inprocess_udf
+from pyspark.sql.types import StringType
+
+@inprocess_udf(return_type=StringType())
+def upper(s):
+    return pc.utf8_upper(s)
+
+df = spark.createDataFrame([("hello",), ("world",)], ["text"])
+df.select(upper(df["text"])).show()
+# +------------+
+# |upper(text) |
+# +------------+
+# |HELLO       |
+# |WORLD       |
+# +------------+
+```
+
+### Multi-column UDF
+
+A UDF receives one `pa.Array` argument per input column:
+
+```python
+import pyarrow.compute as pc
+from pyspark.inprocess.udf import inprocess_udf
+from pyspark.sql.types import DoubleType
+
+@inprocess_udf(return_type=DoubleType())
+def weighted_sum(x, y):
+    return pc.add(pc.multiply(x, 0.6), pc.multiply(y, 0.4))
+
+df = spark.createDataFrame([(1.0, 2.0), (3.0, 4.0)], ["x", "y"])
+df.select(weighted_sum(df["x"], df["y"])).show()
+```
+
+### Closure capture
+
+Free variables are captured by cloudpickle and frozen into the serialized UDF. 
The captured
+value is evaluated once at UDF definition time and shipped with the function 
to every executor:
+
+```python
+import pyarrow.compute as pc
+from pyspark.inprocess.udf import inprocess_udf
+from pyspark.sql.types import DoubleType
+
+SCALE_FACTOR = 100.0
+
+@inprocess_udf(return_type=DoubleType())
+def scale(x):
+    return pc.multiply(x, SCALE_FACTOR)
+```
+
+### Non-deterministic UDF
+
+Pass `deterministic=False` when the UDF produces different results for the 
same input (e.g.
+random sampling). This prevents the optimizer from deduplicating or reordering 
calls:
+
+```python
+import random
+import pyarrow as pa
+import pyarrow.compute as pc
+from pyspark.inprocess.udf import inprocess_udf
+from pyspark.sql.types import DoubleType
+
+@inprocess_udf(return_type=DoubleType(), deterministic=False)
+def add_noise(x):
+    noise = pa.array([random.gauss(0.0, 0.01) for _ in range(len(x))])
+    return pc.add(x, noise)
+```
+
+---
+
+## Requirements
+
+| Requirement | Detail |
+|---|---|
+| Python | 3.11+; driver and embedded major.minor versions must match |
+| jep | 4.3.2+ (`pip install jep`) |
+| `arrow-c-data` JAR | Provided separately; match Spark's Arrow Java version |
+| PyArrow | 18.0.0+ |
+| cloudpickle | Bundled with PySpark |
+| Python concurrency | One invocation at a time per executor (see below) |
+
+### Executor concurrency
+
+In-process UDFs use one `SharedInterpreter` on a dedicated thread per executor.
+Multiple Spark tasks can share an executor, including with fractional
+`spark.task.cpus`, but their Python invocations are serialized. `local[*]` 
therefore
+works but does not provide parallel embedded Python execution.
+
+For throughput, consider `spark.executor.cores=1, spark.task.cpus=1` and 
multiple
+executors. More executors also mean more JVM overhead; compare with 
worker-based
+Arrow UDFs under the same total CPU and memory budget.
+
+---
+
+## Deployment and Distribution
+
+### Local development
+
+For local development (e.g. `SparkSession.builder.master("local[*]")`), 
install jep and the
+required Python packages into the virtual environment you run PySpark from. 
The venv's
+site-packages are already on `sys.path`, so no extra configuration is needed.
+
+```bash
+python3 -m venv .venv
+.venv/bin/pip install "jep>=4.3.2" pyarrow cloudpickle pyspark
+source .venv/bin/activate
+```
+
+You must also make the jep native library discoverable by the JVM:
+
+```bash
+# macOS
+export DYLD_LIBRARY_PATH="$(python3 -c 'import jep; import os; 
print(os.path.dirname(jep.__file__))')"
+
+# Linux
+export LD_LIBRARY_PATH="$(python3 -c 'import jep; import os; 
print(os.path.dirname(jep.__file__))')"
+```
+
+### Cluster deployment — prerequisite: build and zip the venv
+
+Both YARN and Kubernetes support distributing a virtual environment via 
`--archives`. Build the
+venv on a machine that matches the executor OS and Python version:
+
+```bash
+python3 -m venv myvenv
+myvenv/bin/pip install "jep>=4.3.2" pyarrow cloudpickle my-custom-lib
+(cd myvenv && zip -r ../myvenv.zip .)
+```
+
+Adjust `python3.11` in the paths below to match the Python version in your 
venv.
+
+---
+
+### YARN
+
+Spark extracts `--archives` to a relative path (`./myvenv/`) on each YARN 
container at task
+launch time. The key extra config compared to local development is
+`spark.executorEnv.PYSPARK_PYTHON`, which tells PySpark's Python worker to use 
the venv's
+Python executable (ensuring a consistent Python version between the 
JVM-embedded interpreter
+and any out-of-process fallbacks).
+
+```bash

Review Comment:
   Updated the examples to put JEP and Arrow CDI on the system classpath and 
use find_spec without importing JEP. The Kubernetes archive example also calls 
out that the JARs must be available before JVM startup, since classpath 
wildcards expand before Spark extracts the archive. Local execution is 
verified; the cluster examples still need deployment validation.



##########
python/pyspark/inprocess/runtime.py:
##########
@@ -0,0 +1,196 @@
+#
+# 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.
+#
+
+
+"""Arrow CDI entry points called on the executor's dedicated JEP interpreter 
thread.
+
+Functions are registered once per task and released when that task finishes. 
Calls
+pass only a handle and CDI addresses, so large closures are not copied per 
batch.
+"""
+
+import sys
+import traceback as _traceback
+from typing import Any, Callable, Iterable, Optional, Sequence
+
+import pyarrow as pa
+import pyarrow.compute as pc
+
+from pyspark import cloudpickle
+from pyspark.errors import PySparkRuntimeError
+from pyspark.sql.pandas.types import to_arrow_type
+from pyspark.sql.types import _parse_datatype_json_string
+
+_UDF_TRACEBACK_SENTINEL = "__INPROCESS_UDF_TRACEBACK__:"
+_udfs: dict[str, tuple[Callable[..., pa.Array], pa.DataType]] = {}
+
+
+def _inprocess_register(
+    handle: str,
+    serialized_udf: Any,
+    return_type_json: str,
+    timezone: str,
+    python_version: str,
+    large_var_types: bool = False,
+) -> None:
+    try:
+        embedded_version = "%d.%d" % sys.version_info[:2]
+        if python_version != embedded_version:
+            raise PySparkRuntimeError(
+                errorClass="PYTHON_VERSION_MISMATCH",
+                messageParameters={
+                    "worker_version": embedded_version,
+                    "driver_version": python_version,
+                },
+            )
+        # JEP exposes direct ByteBuffers through the buffer protocol. Unpickle 
a separate
+        # function per task without iterating over a PyJArray one JNI call per 
byte.
+        func = cloudpickle.loads(memoryview(serialized_udf))
+        expected_type = to_arrow_type(
+            _parse_datatype_json_string(return_type_json),
+            timezone=timezone,
+            prefers_large_types=large_var_types,
+            error_on_duplicated_field_names_in_struct=True,
+        )
+        _udfs[handle] = (func, expected_type)
+    except BaseException:
+        # In JEP, an uncaught SystemExit can terminate the entire executor JVM.
+        raise RuntimeError(_UDF_TRACEBACK_SENTINEL + _traceback.format_exc()) 
from None
+
+
+def _inprocess_release(handles: Iterable[str]) -> None:
+    for handle in handles:
+        _udfs.pop(handle, None)
+
+
+def _nullable_type(data_type: pa.DataType) -> pa.DataType:
+    def nullable_field(field: pa.Field) -> pa.Field:
+        return pa.field(field.name, _nullable_type(field.type), nullable=True)
+
+    if pa.types.is_struct(data_type):
+        return pa.struct([nullable_field(field) for field in data_type])
+    if pa.types.is_list(data_type):
+        return pa.list_(nullable_field(data_type.value_field))
+    if pa.types.is_large_list(data_type):
+        return pa.large_list(nullable_field(data_type.value_field))
+    if pa.types.is_map(data_type):
+        return pa.map_(
+            _nullable_type(data_type.key_type),
+            nullable_field(data_type.item_field),
+            keys_sorted=data_type.keys_sorted,
+        )
+    return data_type
+
+
+def _check_nested_nulls(array: pa.Array, expected_type: pa.DataType) -> None:
+    def check_field(values: pa.Array, field: pa.Field) -> None:
+        if not field.nullable and values.null_count:
+            raise ValueError(f"In-process UDF returned nulls in non-nullable 
field {field.name}")
+        _check_nested_nulls(values, field.type)
+
+    if pa.types.is_struct(expected_type):
+        # Children under a null parent do not contribute values to the result.
+        visible = pc.filter(array, pc.is_valid(array))
+        for i, field in enumerate(expected_type):
+            check_field(visible.field(i), field)
+    elif pa.types.is_list(expected_type) or 
pa.types.is_large_list(expected_type):
+        check_field(pc.list_flatten(array), expected_type.value_field)
+    elif pa.types.is_map(expected_type):
+        visible = pa.concat_arrays([pc.filter(array, pc.is_valid(array))])
+        check_field(visible.keys, expected_type.key_field)
+        check_field(visible.items, expected_type.item_field)
+
+
+def _has_offset(array: pa.Array) -> bool:
+    if array.offset:
+        return True
+    if pa.types.is_struct(array.type):
+        return any(_has_offset(array.field(i)) for i in 
range(array.type.num_fields))
+    if pa.types.is_list(array.type) or pa.types.is_large_list(array.type):
+        return _has_offset(array.values)
+    if pa.types.is_map(array.type):
+        return _has_offset(array.values)
+    return False
+
+
+def _with_schema(array: pa.Array, expected_type: pa.DataType) -> pa.Array:
+    # Rebind buffers after validating logical nullability. Arrow cast checks 
hidden child
+    # slots too, rejecting null children underneath null parents. from_buffers 
preserves
+    # those masks and applies the declared names, metadata and nullability 
without casting.
+    children = None
+    if pa.types.is_struct(expected_type):
+        children = [_with_schema(array.field(i), f.type) for i, f in 
enumerate(expected_type)]
+    elif pa.types.is_list(expected_type) or 
pa.types.is_large_list(expected_type):
+        children = [_with_schema(array.values, expected_type.value_type)]
+    elif pa.types.is_map(expected_type):
+        entries_type = pa.struct([expected_type.key_field, 
expected_type.item_field])
+        children = [_with_schema(array.values, entries_type)]
+    return pa.Array.from_buffers(
+        expected_type,
+        len(array),
+        array.buffers()[: array.type.num_buffers],
+        null_count=array.null_count,
+        children=children,
+    )
+
+
+def _validate_result(result: pa.Array, expected_rows: int, expected_type: 
pa.DataType) -> pa.Array:
+    if not isinstance(result, pa.Array):
+        raise TypeError(f"In-process UDF must return a pyarrow.Array, got 
{type(result).__name__}")
+    if len(result) != expected_rows:
+        raise ValueError(f"In-process UDF returned {len(result)} rows; 
expected {expected_rows}")
+    if _nullable_type(result.type) != _nullable_type(expected_type):
+        raise TypeError(f"In-process UDF returned {result.type}; expected 
{expected_type}")
+    result.validate()
+    _check_nested_nulls(result, expected_type)
+    # Arrow Java's CDI importer does not honor ArrowArray.offset, including 
child offsets.
+    # Concatenation materializes the logical slice, preserving validity and 
nested values.
+    if _has_offset(result):
+        result = pa.concat_arrays([result])
+    return _with_schema(result, expected_type)
+
+
+def _inprocess_invoke(
+    handle: str,
+    input_array_ptrs: Sequence[int],
+    input_schema_ptrs: Sequence[int],
+    output_array_ptr: int,
+    output_schema_ptr: int,
+    expected_rows: int,
+    argument_names: Optional[Sequence[str]] = None,
+) -> None:
+    """Consume input CDI structs and export a validated, row-preserving result.
+
+    The caller owns the struct memory and releases unconsumed exports on 
failure.
+    Each batch owns its buffers; retained Python inputs are never overwritten.
+    """
+    try:
+        udf_func, expected_type = _udfs[handle]
+        if len(input_array_ptrs) != len(input_schema_ptrs):
+            raise ValueError("Mismatched input ArrowArray and ArrowSchema 
pointer counts")
+        input_arrays = [
+            pa.Array._import_from_c(int(ap), int(sp))
+            for ap, sp in zip(input_array_ptrs, input_schema_ptrs)
+        ]
+        names = argument_names if argument_names is not None else [""] * 
len(input_arrays)
+        if len(names) != len(input_arrays):
+            raise ValueError("Mismatched input argument names")
+        args = [value for name, value in zip(names, input_arrays) if not name]
+        kwargs = {str(name): value for name, value in zip(names, input_arrays) 
if name}
+        result = _validate_result(udf_func(*args, **kwargs), 
int(expected_rows), expected_type)
+        result._export_to_c(int(output_array_ptr), int(output_schema_ptr))
+    except BaseException:
+        raise RuntimeError(_UDF_TRACEBACK_SENTINEL + _traceback.format_exc()) 
from None

Review Comment:
   Both settings now travel with each UDF registration and apply to 
registration and invocation errors, without changing process-wide environment 
variables. Simplification uses the existing worker helper. The JVM exception no 
longer retains a separate JepException cause that could expose the unfiltered 
traceback. Added coverage for all four setting combinations.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/python/InProcessPythonRuntime.scala:
##########
@@ -0,0 +1,258 @@
+/*
+ * 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.execution.python
+
+import java.nio.ByteBuffer
+import java.util.concurrent.{Callable, ExecutionException, TimeoutException, 
TimeUnit}
+
+import scala.jdk.CollectionConverters._
+
+import jep.{JepException, SharedInterpreter}
+
+import org.apache.spark.{TaskContext, TaskKilledException}
+import org.apache.spark.api.python.PythonException
+import org.apache.spark.internal.Logging
+import org.apache.spark.util.{ThreadUtils, Utils}
+
+/** Owns one interpreter generation per executor plugin lifecycle. */
+private[python] object InProcessPythonRuntime extends Logging {
+  val SITE_PACKAGES_CONFIG = "spark.inprocess.python.sitePackages"
+  private val TRACEBACK_SENTINEL = "__INPROCESS_UDF_TRACEBACK__:"
+  private var active: InterpreterSession = _
+
+  def initialize(sitePackages: Seq[String] = Seq.empty): Unit = synchronized {
+    if (active != null && !active.isTerminated) {
+      require(active.isRunning && active.sitePackages == sitePackages,
+        "In-process Python is stopping or already initialized with different 
sitePackages")
+    } else {
+      val candidate = new InterpreterSession(sitePackages)
+      try {
+        candidate.initialize()
+        active = candidate
+      } catch {
+        case t: Throwable => Utils.tryWithSafeFinally { throw t } { 
candidate.shutdown() }
+      }
+    }
+  }
+
+  def currentSession: InterpreterSession = synchronized {
+    checkState(active != null && active.isRunning)
+    active
+  }
+
+  def shutdown(): Unit = {
+    val session = synchronized { active }
+    if (session != null) session.shutdown()
+  }
+
+  private def checkState(running: Boolean): Unit = {
+    checkState(running, "In-process Python is not running; initialize the 
executor plugin first")
+  }
+
+  private def checkState(running: Boolean, message: String): Unit = {
+    if (!running) throw new IllegalStateException(message)
+  }
+
+  /**
+   * Tasks retain this generation, so stale tasks cannot enter a later 
SparkContext's interpreter.
+   * Lifecycle operations only hold the monitor while enqueueing work, never 
while running Python.
+   */
+  private[python] class InterpreterSession(val sitePackages: Seq[String] = 
Seq.empty) {
+    private val executor = 
ThreadUtils.newDaemonSingleThreadExecutor("inprocess-python")
+    @volatile private var running = true
+    // Accessed only on the owning thread.
+    private var interp: SharedInterpreter = _
+
+    def isRunning: Boolean = running
+    def isTerminated: Boolean = executor.isTerminated
+
+    private[python] def onInterpreterThread[T](body: => T): T = {
+      val context = Option(TaskContext.get())
+      context.foreach(_.killTaskIfInterrupted())
+      val gate = new Object
+      var started = false
+      var cancelled = false
+      val future = synchronized {
+        checkState(running)
+        executor.submit(new Callable[T] {
+          override def call(): T = {
+            gate.synchronized {
+              if (cancelled) throw new TaskKilledException("Cancelled before 
Python invocation")
+              started = true
+            }
+            body
+          }
+        })
+      }
+      var interrupted = false
+      try {
+        while (true) {
+          val taskCancelled = context.exists(_.isInterrupted())
+          if (interrupted || taskCancelled) {
+            val cancelledBeforeStart = gate.synchronized {
+              if (started) false else {

Review Comment:
   Documented the executor-wide impact explicitly, including subsequent calls 
from other tasks, jobs, and sessions. The bounded shutdown wait does not 
recover a permanently hung interpreter; recovery requires replacing the 
executor process. This change does not add a watchdog or forcibly interrupt 
native execution.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/python/InProcessPythonRuntime.scala:
##########
@@ -0,0 +1,258 @@
+/*
+ * 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.execution.python
+
+import java.nio.ByteBuffer
+import java.util.concurrent.{Callable, ExecutionException, TimeoutException, 
TimeUnit}
+
+import scala.jdk.CollectionConverters._
+
+import jep.{JepException, SharedInterpreter}
+
+import org.apache.spark.{TaskContext, TaskKilledException}
+import org.apache.spark.api.python.PythonException
+import org.apache.spark.internal.Logging
+import org.apache.spark.util.{ThreadUtils, Utils}
+
+/** Owns one interpreter generation per executor plugin lifecycle. */
+private[python] object InProcessPythonRuntime extends Logging {
+  val SITE_PACKAGES_CONFIG = "spark.inprocess.python.sitePackages"
+  private val TRACEBACK_SENTINEL = "__INPROCESS_UDF_TRACEBACK__:"
+  private var active: InterpreterSession = _
+
+  def initialize(sitePackages: Seq[String] = Seq.empty): Unit = synchronized {
+    if (active != null && !active.isTerminated) {
+      require(active.isRunning && active.sitePackages == sitePackages,

Review Comment:
   Added distinct lifecycle errors for a still-stopping interpreter and a 
sitePackages configuration mismatch. The plugin now logs those directly instead 
of suggesting installation troubleshooting. The stopping message explains that 
outstanding native work must finish, or the executor process must be replaced. 
Added tests for both cases.



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