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+// 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.
+
+//! Python-aware extension codecs.
+//!
+//! Datafusion-python plans can carry references to Python-defined
+//! objects that the upstream protobuf codecs do not know how to
+//! serialize: pure-Python scalar / aggregate / window UDFs, Python
+//! query-planning extensions, and so on. Their state lives inside
+//! `Py<PyAny>` callables and closures rather than being recoverable
+//! from a name in the receiver's function registry. To ship a plan
+//! across a process boundary (pickle, `multiprocessing`, Ray actor,
+//! `datafusion-distributed`, etc.) those payloads have to be encoded
+//! into the proto wire format itself.
+//!
+//! [`PythonLogicalCodec`] is the [`LogicalExtensionCodec`] that
+//! datafusion-python parks on every `SessionContext`. It wraps a
+//! user-supplied (or default) inner codec and adds Python-aware
+//! in-band encoding on top: when the encoder sees a Python-defined
+//! UDF, the codec cloudpickles the callable + signature into the
+//! `fun_definition` proto field; when the decoder sees a payload it
+//! produced, it reconstructs the UDF from the bytes alone — no
+//! pre-registration on the receiver. UDFs the codec does not
+//! recognise are delegated to `inner`, which is typically
+//! `DefaultLogicalExtensionCodec` but may be a downstream-supplied
+//! FFI codec installed via
+//! `SessionContext.with_logical_extension_codec(...)`.
+//!
+//! [`PythonPhysicalCodec`] is the symmetric wrapper around
+//! [`PhysicalExtensionCodec`]. Logical and physical layers each have
+//! a `try_encode_udf` / `try_decode_udf` pair, so a `ScalarUDF`
+//! referenced inside a `LogicalPlan`, an `ExecutionPlan`, or a
+//! `PhysicalExpr` must encode identically through either layer for
+//! plans to survive a serialization round-trip. Both codecs share
+//! the same payload framing for that reason.
+//!
+//! Payloads emitted by these codecs are tagged with an 8-byte magic
+//! prefix so the decoder can distinguish them from arbitrary bytes
+//! (empty `fun_definition` from the default codec, user FFI payloads
+//! that picked a non-colliding prefix). Dispatch precedence on
+//! decode: **Python-inline payload (magic prefix match) → `inner`
+//! codec → caller's `FunctionRegistry` fallback.**
+//!
+//! ## Wire-format magic prefix registry
+//!
+//! | Layer + kind                  | Magic prefix |
+//! | ----------------------------- | ------------ |
+//! | `PythonLogicalCodec` scalar   | `DFPYUDF1`   |
+//! | `PythonLogicalCodec` agg      | `DFPYUDA1`   |
+//! | `PythonLogicalCodec` window   | `DFPYUDW1`   |
+//! | `PythonPhysicalCodec` scalar  | `DFPYUDF1`   |
+//! | `PythonPhysicalCodec` agg     | `DFPYUDA1`   |
+//! | `PythonPhysicalCodec` window  | `DFPYUDW1`   |
+//! | `PythonPhysicalCodec` expr    | `DFPYPE1`    |
+//! | User FFI extension codec      | user-chosen  |
+//! | Default codec                 | (none)       |
+//!
+//! Downstream FFI codecs should pick non-colliding prefixes (use a
+//! `DF` namespace plus a crate-specific suffix). The codec
+//! implementations in this module currently delegate every method to
+//! `inner`; the encoder/decoder hooks for each kind are added as the
+//! corresponding Python-side type becomes serializable.
+
+use std::sync::Arc;
+
+use arrow::datatypes::SchemaRef;
+use datafusion::common::{Result, TableReference};
+use datafusion::datasource::TableProvider;
+use datafusion::datasource::file_format::FileFormatFactory;
+use datafusion::execution::TaskContext;
+use datafusion::logical_expr::{AggregateUDF, Extension, LogicalPlan, ScalarUDF, WindowUDF};
+use datafusion::physical_expr::PhysicalExpr;
+use datafusion::physical_plan::ExecutionPlan;
+use datafusion_proto::logical_plan::{DefaultLogicalExtensionCodec, LogicalExtensionCodec};
+use datafusion_proto::physical_plan::{DefaultPhysicalExtensionCodec, PhysicalExtensionCodec};
+
+/// Wire-format prefix that tags a `fun_definition` payload as an
+/// inlined Python scalar UDF (cloudpickled tuple of name, callable,
+/// input schema, return field, volatility). Defined once here so
+/// the encoder and decoder cannot drift.
+#[allow(dead_code)]
+pub(crate) const PY_SCALAR_UDF_MAGIC: &[u8] = b"DFPYUDF1";
+
+/// `LogicalExtensionCodec` parked on every `SessionContext`. Holds
+/// the Python-aware encoding hooks for logical-layer types
+/// (`LogicalPlan`, `Expr`) and delegates everything it does not
+/// handle to the composable `inner` codec — typically
+/// `DefaultLogicalExtensionCodec`, or a downstream FFI codec
+/// installed via `SessionContext.with_logical_extension_codec(...)`.
+///
+/// Sitting at the top of the session's logical codec stack means
+/// every serializer that reads `session.logical_codec()` automatically
+/// picks up Python-aware encoding for free.
+#[derive(Debug)]
+pub struct PythonLogicalCodec {
+    inner: Arc<dyn LogicalExtensionCodec>,
+}
+
+impl PythonLogicalCodec {
+    pub fn new(inner: Arc<dyn LogicalExtensionCodec>) -> Self {
+        Self { inner }
+    }
+
+    pub fn inner(&self) -> &Arc<dyn LogicalExtensionCodec> {
+        &self.inner
+    }
+}
+
+impl Default for PythonLogicalCodec {
+    fn default() -> Self {
+        Self::new(Arc::new(DefaultLogicalExtensionCodec {}))
+    }
+}
+
+impl LogicalExtensionCodec for PythonLogicalCodec {
+    fn try_decode(
+        &self,
+        buf: &[u8],
+        inputs: &[LogicalPlan],
+        ctx: &TaskContext,
+    ) -> Result<Extension> {
+        self.inner.try_decode(buf, inputs, ctx)
+    }
+
+    fn try_encode(&self, node: &Extension, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode(node, buf)
+    }
+
+    fn try_decode_table_provider(
+        &self,
+        buf: &[u8],
+        table_ref: &TableReference,
+        schema: SchemaRef,
+        ctx: &TaskContext,
+    ) -> Result<Arc<dyn TableProvider>> {
+        self.inner
+            .try_decode_table_provider(buf, table_ref, schema, ctx)
+    }
+
+    fn try_encode_table_provider(
+        &self,
+        table_ref: &TableReference,
+        node: Arc<dyn TableProvider>,
+        buf: &mut Vec<u8>,
+    ) -> Result<()> {
+        self.inner.try_encode_table_provider(table_ref, node, buf)
+    }
+
+    fn try_decode_file_format(
+        &self,
+        buf: &[u8],
+        ctx: &TaskContext,
+    ) -> Result<Arc<dyn FileFormatFactory>> {
+        self.inner.try_decode_file_format(buf, ctx)
+    }
+
+    fn try_encode_file_format(
+        &self,
+        buf: &mut Vec<u8>,
+        node: Arc<dyn FileFormatFactory>,
+    ) -> Result<()> {
+        self.inner.try_encode_file_format(buf, node)
+    }
+
+    fn try_encode_udf(&self, node: &ScalarUDF, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_udf(node, buf)
+    }
+
+    fn try_decode_udf(&self, name: &str, buf: &[u8]) -> Result<Arc<ScalarUDF>> {
+        self.inner.try_decode_udf(name, buf)
+    }
+
+    fn try_encode_udaf(&self, node: &AggregateUDF, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_udaf(node, buf)
+    }
+
+    fn try_decode_udaf(&self, name: &str, buf: &[u8]) -> Result<Arc<AggregateUDF>> {
+        self.inner.try_decode_udaf(name, buf)
+    }
+
+    fn try_encode_udwf(&self, node: &WindowUDF, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_udwf(node, buf)
+    }
+
+    fn try_decode_udwf(&self, name: &str, buf: &[u8]) -> Result<Arc<WindowUDF>> {
+        self.inner.try_decode_udwf(name, buf)
+    }
+}
+
+/// `PhysicalExtensionCodec` mirror of [`PythonLogicalCodec`] parked
+/// on the same `SessionContext`. Carries the Python-aware encoding
+/// hooks for physical-layer types (`ExecutionPlan`, `PhysicalExpr`)
+/// and delegates the rest to `inner`.
+///
+/// The `PhysicalExtensionCodec` trait has its own `try_encode_udf`
+/// / `try_decode_udf` pair distinct from the logical one, so a
+/// `ScalarUDF` referenced inside a physical plan needs Python-aware
+/// encoding on this layer too — otherwise a plan with a Python UDF
+/// would round-trip at the logical level but break at the physical
+/// level. Both layers reuse the shared payload framing
+/// ([`PY_SCALAR_UDF_MAGIC`] et al.) so the wire format is identical.
+#[derive(Debug)]
+pub struct PythonPhysicalCodec {
+    inner: Arc<dyn PhysicalExtensionCodec>,
+}
+
+impl PythonPhysicalCodec {
+    pub fn new(inner: Arc<dyn PhysicalExtensionCodec>) -> Self {
+        Self { inner }
+    }
+
+    pub fn inner(&self) -> &Arc<dyn PhysicalExtensionCodec> {
+        &self.inner
+    }
+}
+
+impl Default for PythonPhysicalCodec {
+    fn default() -> Self {
+        Self::new(Arc::new(DefaultPhysicalExtensionCodec {}))
+    }
+}
+
+impl PhysicalExtensionCodec for PythonPhysicalCodec {
+    fn try_decode(
+        &self,
+        buf: &[u8],
+        inputs: &[Arc<dyn ExecutionPlan>],
+        ctx: &TaskContext,
+    ) -> Result<Arc<dyn ExecutionPlan>> {
+        self.inner.try_decode(buf, inputs, ctx)
+    }
+
+    fn try_encode(&self, node: Arc<dyn ExecutionPlan>, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode(node, buf)
+    }
+
+    fn try_encode_udf(&self, node: &ScalarUDF, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_udf(node, buf)
+    }
+
+    fn try_decode_udf(&self, name: &str, buf: &[u8]) -> Result<Arc<ScalarUDF>> {
+        self.inner.try_decode_udf(name, buf)
+    }
+
+    fn try_encode_expr(&self, node: &Arc<dyn PhysicalExpr>, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_expr(node, buf)
+    }
+
+    fn try_decode_expr(
+        &self,
+        buf: &[u8],
+        inputs: &[Arc<dyn PhysicalExpr>],
+    ) -> Result<Arc<dyn PhysicalExpr>> {
+        self.inner.try_decode_expr(buf, inputs)
+    }
+
+    fn try_encode_udaf(&self, node: &AggregateUDF, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_udaf(node, buf)
+    }
+
+    fn try_decode_udaf(&self, name: &str, buf: &[u8]) -> Result<Arc<AggregateUDF>> {
+        self.inner.try_decode_udaf(name, buf)
+    }
+
+    fn try_encode_udwf(&self, node: &WindowUDF, buf: &mut Vec<u8>) -> Result<()> {
+        self.inner.try_encode_udwf(node, buf)
+    }
+
+    fn try_decode_udwf(&self, name: &str, buf: &[u8]) -> Result<Arc<WindowUDF>> {
+        self.inner.try_decode_udwf(name, buf)
+    }
+}